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Quantum entanglement - Wikipedia
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class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Concept subsection</span> </button> <ul id="toc-Concept-sublist" class="vector-toc-list"> <li id="toc-Meaning_of_entanglement" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Meaning_of_entanglement"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.1</span> <span>Meaning of entanglement</span> </div> </a> <ul id="toc-Meaning_of_entanglement-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Paradox" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Paradox"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.2</span> <span>Paradox</span> </div> </a> <ul id="toc-Paradox-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Hidden-variables_theory" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Hidden-variables_theory"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.3</span> <span>Hidden-variables theory</span> </div> </a> <ul id="toc-Hidden-variables_theory-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Violations_of_Bell's_inequality" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Violations_of_Bell's_inequality"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.4</span> <span>Violations of Bell's inequality</span> </div> </a> <ul id="toc-Violations_of_Bell's_inequality-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Quantum_teleportation_and_entanglement_swapping" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Quantum_teleportation_and_entanglement_swapping"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.5</span> <span>Quantum teleportation and entanglement swapping</span> </div> </a> <ul id="toc-Quantum_teleportation_and_entanglement_swapping-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Emergent_time" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Emergent_time"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.6</span> <span>Emergent time</span> </div> </a> <ul id="toc-Emergent_time-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Emergent_gravity" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Emergent_gravity"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.7</span> <span>Emergent gravity</span> </div> </a> <ul id="toc-Emergent_gravity-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Non-locality_and_entanglement" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Non-locality_and_entanglement"> <div class="vector-toc-text"> <span class="vector-toc-numb">3</span> <span>Non-locality and entanglement</span> </div> </a> <ul id="toc-Non-locality_and_entanglement-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Quantum-mechanical_framework" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Quantum-mechanical_framework"> <div class="vector-toc-text"> <span class="vector-toc-numb">4</span> <span>Quantum-mechanical framework</span> </div> </a> <button aria-controls="toc-Quantum-mechanical_framework-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Quantum-mechanical framework subsection</span> </button> <ul id="toc-Quantum-mechanical_framework-sublist" class="vector-toc-list"> <li id="toc-Pure_states" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Pure_states"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.1</span> <span>Pure states</span> </div> </a> <ul id="toc-Pure_states-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Ensembles" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Ensembles"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.2</span> <span>Ensembles</span> </div> </a> <ul id="toc-Ensembles-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Reduced_density_matrices" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Reduced_density_matrices"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.3</span> <span>Reduced density matrices</span> </div> </a> <ul id="toc-Reduced_density_matrices-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Two_applications_that_use_them" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Two_applications_that_use_them"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.4</span> <span>Two applications that use them</span> </div> </a> <ul id="toc-Two_applications_that_use_them-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Entanglement_as_a_resource" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Entanglement_as_a_resource"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.5</span> <span>Entanglement as a resource</span> </div> </a> <ul id="toc-Entanglement_as_a_resource-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Classification_of_entanglement" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Classification_of_entanglement"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.6</span> <span>Classification of entanglement</span> </div> </a> <ul id="toc-Classification_of_entanglement-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Entropy" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Entropy"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.7</span> <span>Entropy</span> </div> </a> <ul id="toc-Entropy-sublist" class="vector-toc-list"> <li id="toc-Definition" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Definition"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.7.1</span> <span>Definition</span> </div> </a> <ul id="toc-Definition-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-As_a_measure_of_entanglement" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#As_a_measure_of_entanglement"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.7.2</span> <span>As a measure of entanglement</span> </div> </a> <ul id="toc-As_a_measure_of_entanglement-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Entanglement_measures" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Entanglement_measures"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.8</span> <span>Entanglement measures</span> </div> </a> <ul id="toc-Entanglement_measures-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Quantum_field_theory" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Quantum_field_theory"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.9</span> <span>Quantum field theory</span> </div> </a> <ul id="toc-Quantum_field_theory-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Applications" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Applications"> <div class="vector-toc-text"> <span class="vector-toc-numb">5</span> <span>Applications</span> </div> </a> <button aria-controls="toc-Applications-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Applications subsection</span> </button> <ul id="toc-Applications-sublist" class="vector-toc-list"> <li id="toc-Entangled_states" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Entangled_states"> <div class="vector-toc-text"> <span class="vector-toc-numb">5.1</span> <span>Entangled states</span> </div> </a> <ul id="toc-Entangled_states-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Methods_of_creating_entanglement" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Methods_of_creating_entanglement"> <div class="vector-toc-text"> <span class="vector-toc-numb">5.2</span> <span>Methods of creating entanglement</span> </div> </a> <ul id="toc-Methods_of_creating_entanglement-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Testing_a_system_for_entanglement" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Testing_a_system_for_entanglement"> <div class="vector-toc-text"> <span class="vector-toc-numb">5.3</span> <span>Testing a system for entanglement</span> </div> </a> <ul id="toc-Testing_a_system_for_entanglement-sublist" class="vector-toc-list"> <li id="toc-Entanglement_of_top_quarks" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Entanglement_of_top_quarks"> <div class="vector-toc-text"> <span class="vector-toc-numb">5.3.1</span> <span>Entanglement of top quarks</span> </div> </a> <ul id="toc-Entanglement_of_top_quarks-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> </ul> </li> <li id="toc-Entanglement_of_macroscopic_objects" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Entanglement_of_macroscopic_objects"> <div class="vector-toc-text"> <span class="vector-toc-numb">6</span> <span>Entanglement of macroscopic objects</span> </div> </a> <button aria-controls="toc-Entanglement_of_macroscopic_objects-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Entanglement of macroscopic objects subsection</span> </button> <ul id="toc-Entanglement_of_macroscopic_objects-sublist" class="vector-toc-list"> <li id="toc-Entanglement_of_elements_of_living_systems" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Entanglement_of_elements_of_living_systems"> <div class="vector-toc-text"> <span class="vector-toc-numb">6.1</span> <span>Entanglement of elements of living systems</span> </div> </a> <ul id="toc-Entanglement_of_elements_of_living_systems-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-See_also" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#See_also"> <div class="vector-toc-text"> <span class="vector-toc-numb">7</span> <span>See also</span> </div> </a> <ul id="toc-See_also-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-References" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#References"> <div class="vector-toc-text"> <span class="vector-toc-numb">8</span> <span>References</span> </div> </a> <ul id="toc-References-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Further_reading" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#Further_reading"> <div class="vector-toc-text"> <span class="vector-toc-numb">9</span> <span>Further reading</span> </div> </a> <ul id="toc-Further_reading-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-External_links" class="vector-toc-list-item vector-toc-level-1"> <a class="vector-toc-link" href="#External_links"> <div class="vector-toc-text"> <span class="vector-toc-numb">10</span> <span>External links</span> </div> </a> <ul id="toc-External_links-sublist" class="vector-toc-list"> </ul> </li> </ul> </div> </div> </nav> </div> </div> <div class="mw-content-container"> <main id="content" class="mw-body"> <header class="mw-body-header vector-page-titlebar"> <nav aria-label="Contents" class="vector-toc-landmark"> <div id="vector-page-titlebar-toc" class="vector-dropdown vector-page-titlebar-toc vector-button-flush-left" > <input type="checkbox" id="vector-page-titlebar-toc-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-vector-page-titlebar-toc" class="vector-dropdown-checkbox " aria-label="Toggle the table of contents" > <label id="vector-page-titlebar-toc-label" for="vector-page-titlebar-toc-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--icon-only " aria-hidden="true" ><span class="vector-icon mw-ui-icon-listBullet mw-ui-icon-wikimedia-listBullet"></span> <span class="vector-dropdown-label-text">Toggle the table of contents</span> </label> <div class="vector-dropdown-content"> <div id="vector-page-titlebar-toc-unpinned-container" class="vector-unpinned-container"> </div> </div> </div> </nav> <h1 id="firstHeading" class="firstHeading mw-first-heading"><span class="mw-page-title-main">Quantum entanglement</span></h1> <div id="p-lang-btn" class="vector-dropdown mw-portlet mw-portlet-lang" > <input type="checkbox" id="p-lang-btn-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-p-lang-btn" class="vector-dropdown-checkbox mw-interlanguage-selector" aria-label="Go to an article in another language. Available in 63 languages" > <label id="p-lang-btn-label" for="p-lang-btn-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--action-progressive mw-portlet-lang-heading-63" aria-hidden="true" ><span class="vector-icon mw-ui-icon-language-progressive mw-ui-icon-wikimedia-language-progressive"></span> <span class="vector-dropdown-label-text">63 languages</span> </label> <div class="vector-dropdown-content"> <div class="vector-menu-content"> <ul class="vector-menu-content-list"> <li class="interlanguage-link interwiki-als mw-list-item"><a href="https://als.wikipedia.org/wiki/Quantenverschr%C3%A4nkung" title="Quantenverschränkung – Alemannic" lang="gsw" hreflang="gsw" data-title="Quantenverschränkung" data-language-autonym="Alemannisch" data-language-local-name="Alemannic" class="interlanguage-link-target"><span>Alemannisch</span></a></li><li class="interlanguage-link interwiki-ar mw-list-item"><a href="https://ar.wikipedia.org/wiki/%D8%AA%D8%B4%D8%A7%D8%A8%D9%83_%D9%83%D9%85%D9%8A" title="تشابك كمي – Arabic" lang="ar" hreflang="ar" data-title="تشابك كمي" data-language-autonym="العربية" data-language-local-name="Arabic" class="interlanguage-link-target"><span>العربية</span></a></li><li class="interlanguage-link interwiki-ast mw-list-item"><a href="https://ast.wikipedia.org/wiki/Entellazamientu_cu%C3%A1nticu" title="Entellazamientu cuánticu – Asturian" lang="ast" hreflang="ast" data-title="Entellazamientu cuánticu" data-language-autonym="Asturianu" data-language-local-name="Asturian" class="interlanguage-link-target"><span>Asturianu</span></a></li><li class="interlanguage-link interwiki-bn mw-list-item"><a href="https://bn.wikipedia.org/wiki/%E0%A6%95%E0%A7%8B%E0%A6%AF%E0%A6%BC%E0%A6%BE%E0%A6%A8%E0%A7%8D%E0%A6%9F%E0%A6%BE%E0%A6%AE_%E0%A6%AC%E0%A6%BF%E0%A6%9C%E0%A6%A1%E0%A6%BC%E0%A6%A8" title="কোয়ান্টাম বিজড়ন – Bangla" lang="bn" hreflang="bn" data-title="কোয়ান্টাম বিজড়ন" data-language-autonym="বাংলা" data-language-local-name="Bangla" class="interlanguage-link-target"><span>বাংলা</span></a></li><li class="interlanguage-link interwiki-zh-min-nan mw-list-item"><a href="https://zh-min-nan.wikipedia.org/wiki/Li%C5%8Dng-ch%C3%AD_tak-t%C3%AE%E2%81%BF" title="Liōng-chí tak-tîⁿ – Minnan" lang="nan" hreflang="nan" data-title="Liōng-chí tak-tîⁿ" data-language-autonym="閩南語 / Bân-lâm-gú" data-language-local-name="Minnan" class="interlanguage-link-target"><span>閩南語 / Bân-lâm-gú</span></a></li><li class="interlanguage-link interwiki-bg mw-list-item"><a href="https://bg.wikipedia.org/wiki/%D0%9A%D0%B2%D0%B0%D0%BD%D1%82%D0%BE%D0%B2%D0%BE_%D0%B7%D0%B0%D0%BF%D0%BB%D0%B8%D1%82%D0%B0%D0%BD%D0%B5" title="Квантово заплитане – Bulgarian" lang="bg" hreflang="bg" data-title="Квантово заплитане" data-language-autonym="Български" data-language-local-name="Bulgarian" class="interlanguage-link-target"><span>Български</span></a></li><li class="interlanguage-link interwiki-bs mw-list-item"><a href="https://bs.wikipedia.org/wiki/Kvantna_zapletenost" title="Kvantna zapletenost – Bosnian" lang="bs" hreflang="bs" data-title="Kvantna zapletenost" data-language-autonym="Bosanski" data-language-local-name="Bosnian" class="interlanguage-link-target"><span>Bosanski</span></a></li><li class="interlanguage-link interwiki-ca mw-list-item"><a href="https://ca.wikipedia.org/wiki/Entrella%C3%A7ament_qu%C3%A0ntic" title="Entrellaçament quàntic – Catalan" lang="ca" hreflang="ca" data-title="Entrellaçament quàntic" data-language-autonym="Català" data-language-local-name="Catalan" class="interlanguage-link-target"><span>Català</span></a></li><li class="interlanguage-link interwiki-cs mw-list-item"><a href="https://cs.wikipedia.org/wiki/Kvantov%C3%A9_prov%C3%A1z%C3%A1n%C3%AD" title="Kvantové provázání – Czech" lang="cs" hreflang="cs" data-title="Kvantové provázání" data-language-autonym="Čeština" data-language-local-name="Czech" class="interlanguage-link-target"><span>Čeština</span></a></li><li class="interlanguage-link interwiki-da mw-list-item"><a href="https://da.wikipedia.org/wiki/Kvantemekanisk_sammenfiltring" title="Kvantemekanisk sammenfiltring – Danish" lang="da" hreflang="da" data-title="Kvantemekanisk sammenfiltring" data-language-autonym="Dansk" data-language-local-name="Danish" class="interlanguage-link-target"><span>Dansk</span></a></li><li class="interlanguage-link interwiki-ary mw-list-item"><a href="https://ary.wikipedia.org/wiki/%D8%AA%D8%AE%D8%A8%D8%A7%D9%84_%D9%83%D9%88%D8%A7%D9%86%D8%AA%D9%8A" title="تخبال كوانتي – Moroccan Arabic" lang="ary" hreflang="ary" data-title="تخبال كوانتي" data-language-autonym="الدارجة" data-language-local-name="Moroccan Arabic" class="interlanguage-link-target"><span>الدارجة</span></a></li><li class="interlanguage-link interwiki-de mw-list-item"><a href="https://de.wikipedia.org/wiki/Quantenverschr%C3%A4nkung" title="Quantenverschränkung – German" lang="de" hreflang="de" data-title="Quantenverschränkung" data-language-autonym="Deutsch" data-language-local-name="German" class="interlanguage-link-target"><span>Deutsch</span></a></li><li class="interlanguage-link interwiki-et mw-list-item"><a href="https://et.wikipedia.org/wiki/Kvantp%C3%B5imumine" title="Kvantpõimumine – Estonian" lang="et" hreflang="et" data-title="Kvantpõimumine" data-language-autonym="Eesti" data-language-local-name="Estonian" class="interlanguage-link-target"><span>Eesti</span></a></li><li class="interlanguage-link interwiki-el mw-list-item"><a href="https://el.wikipedia.org/wiki/%CE%9A%CE%B2%CE%B1%CE%BD%CF%84%CE%B9%CE%BA%CE%AE_%CE%B4%CE%B9%CE%B5%CE%BC%CF%80%CE%BB%CE%BF%CE%BA%CE%AE" title="Κβαντική διεμπλοκή – Greek" lang="el" hreflang="el" data-title="Κβαντική διεμπλοκή" data-language-autonym="Ελληνικά" data-language-local-name="Greek" class="interlanguage-link-target"><span>Ελληνικά</span></a></li><li class="interlanguage-link interwiki-es mw-list-item"><a href="https://es.wikipedia.org/wiki/Entrelazamiento_cu%C3%A1ntico" title="Entrelazamiento cuántico – Spanish" lang="es" hreflang="es" data-title="Entrelazamiento cuántico" data-language-autonym="Español" data-language-local-name="Spanish" class="interlanguage-link-target"><span>Español</span></a></li><li class="interlanguage-link interwiki-eu mw-list-item"><a href="https://eu.wikipedia.org/wiki/Korapilatze_kuantiko" title="Korapilatze kuantiko – Basque" lang="eu" hreflang="eu" data-title="Korapilatze kuantiko" data-language-autonym="Euskara" data-language-local-name="Basque" class="interlanguage-link-target"><span>Euskara</span></a></li><li class="interlanguage-link interwiki-fa mw-list-item"><a href="https://fa.wikipedia.org/wiki/%D8%AF%D8%B1%D9%87%D9%85%E2%80%8C%D8%AA%D9%86%DB%8C%D8%AF%DA%AF%DB%8C_%DA%A9%D9%88%D8%A7%D9%86%D8%AA%D9%88%D9%85%DB%8C" title="درهمتنیدگی کوانتومی – Persian" lang="fa" hreflang="fa" data-title="درهمتنیدگی کوانتومی" data-language-autonym="فارسی" data-language-local-name="Persian" class="interlanguage-link-target"><span>فارسی</span></a></li><li class="interlanguage-link interwiki-fr mw-list-item"><a href="https://fr.wikipedia.org/wiki/Intrication_quantique" title="Intrication quantique – French" lang="fr" hreflang="fr" data-title="Intrication quantique" data-language-autonym="Français" data-language-local-name="French" class="interlanguage-link-target"><span>Français</span></a></li><li class="interlanguage-link interwiki-ga mw-list-item"><a href="https://ga.wikipedia.org/wiki/Fost%C3%BA_candamach" title="Fostú candamach – Irish" lang="ga" hreflang="ga" data-title="Fostú candamach" data-language-autonym="Gaeilge" data-language-local-name="Irish" class="interlanguage-link-target"><span>Gaeilge</span></a></li><li class="interlanguage-link interwiki-gl mw-list-item"><a href="https://gl.wikipedia.org/wiki/Entrelazamento_cu%C3%A1ntico" title="Entrelazamento cuántico – Galician" lang="gl" hreflang="gl" data-title="Entrelazamento cuántico" data-language-autonym="Galego" data-language-local-name="Galician" class="interlanguage-link-target"><span>Galego</span></a></li><li class="interlanguage-link interwiki-ko mw-list-item"><a href="https://ko.wikipedia.org/wiki/%EC%96%91%EC%9E%90_%EC%96%BD%ED%9E%98" title="양자 얽힘 – Korean" lang="ko" hreflang="ko" data-title="양자 얽힘" data-language-autonym="한국어" data-language-local-name="Korean" class="interlanguage-link-target"><span>한국어</span></a></li><li class="interlanguage-link interwiki-hy mw-list-item"><a href="https://hy.wikipedia.org/wiki/%D5%94%D5%BE%D5%A1%D5%B6%D5%BF%D5%A1%D5%B5%D5%AB%D5%B6_%D5%AD%D5%B3%D5%B3%D5%BE%D5%A1%D5%AE%D5%B8%D6%82%D5%A9%D5%B5%D5%B8%D6%82%D5%B6" title="Քվանտային խճճվածություն – Armenian" lang="hy" hreflang="hy" data-title="Քվանտային խճճվածություն" data-language-autonym="Հայերեն" data-language-local-name="Armenian" class="interlanguage-link-target"><span>Հայերեն</span></a></li><li class="interlanguage-link interwiki-hi mw-list-item"><a href="https://hi.wikipedia.org/wiki/%E0%A4%95%E0%A5%8D%E0%A4%B5%E0%A4%BE%E0%A4%A3%E0%A5%8D%E0%A4%9F%E0%A4%AE_%E0%A4%89%E0%A4%B2%E0%A4%9D%E0%A4%BE%E0%A4%B5" title="क्वाण्टम उलझाव – Hindi" lang="hi" hreflang="hi" data-title="क्वाण्टम उलझाव" data-language-autonym="हिन्दी" data-language-local-name="Hindi" class="interlanguage-link-target"><span>हिन्दी</span></a></li><li class="interlanguage-link interwiki-hr mw-list-item"><a href="https://hr.wikipedia.org/wiki/Kvantno_sprezanje" title="Kvantno sprezanje – Croatian" lang="hr" hreflang="hr" data-title="Kvantno sprezanje" data-language-autonym="Hrvatski" data-language-local-name="Croatian" class="interlanguage-link-target"><span>Hrvatski</span></a></li><li class="interlanguage-link interwiki-id mw-list-item"><a href="https://id.wikipedia.org/wiki/Keterkaitan_kuantum" title="Keterkaitan kuantum – Indonesian" lang="id" hreflang="id" data-title="Keterkaitan kuantum" data-language-autonym="Bahasa Indonesia" data-language-local-name="Indonesian" class="interlanguage-link-target"><span>Bahasa Indonesia</span></a></li><li class="interlanguage-link interwiki-zu mw-list-item"><a href="https://zu.wikipedia.org/wiki/Ukuphixana_kohoyana" title="Ukuphixana kohoyana – Zulu" lang="zu" hreflang="zu" data-title="Ukuphixana kohoyana" data-language-autonym="IsiZulu" data-language-local-name="Zulu" class="interlanguage-link-target"><span>IsiZulu</span></a></li><li class="interlanguage-link interwiki-it mw-list-item"><a href="https://it.wikipedia.org/wiki/Entanglement_quantistico" title="Entanglement quantistico – Italian" lang="it" hreflang="it" data-title="Entanglement quantistico" data-language-autonym="Italiano" data-language-local-name="Italian" class="interlanguage-link-target"><span>Italiano</span></a></li><li class="interlanguage-link interwiki-he mw-list-item"><a href="https://he.wikipedia.org/wiki/%D7%A9%D7%96%D7%99%D7%A8%D7%94_%D7%A7%D7%95%D7%95%D7%A0%D7%98%D7%99%D7%AA" title="שזירה קוונטית – Hebrew" lang="he" hreflang="he" data-title="שזירה קוונטית" data-language-autonym="עברית" data-language-local-name="Hebrew" class="interlanguage-link-target"><span>עברית</span></a></li><li class="interlanguage-link interwiki-ka mw-list-item"><a href="https://ka.wikipedia.org/wiki/%E1%83%99%E1%83%95%E1%83%90%E1%83%9C%E1%83%A2%E1%83%A3%E1%83%A0%E1%83%98_%E1%83%92%E1%83%90%E1%83%93%E1%83%90%E1%83%AE%E1%83%9A%E1%83%90%E1%83%A0%E1%83%97%E1%83%A3%E1%83%9A%E1%83%9D%E1%83%91%E1%83%90" title="კვანტური გადახლართულობა – Georgian" lang="ka" hreflang="ka" data-title="კვანტური გადახლართულობა" data-language-autonym="ქართული" data-language-local-name="Georgian" class="interlanguage-link-target"><span>ქართული</span></a></li><li class="interlanguage-link interwiki-lt mw-list-item"><a href="https://lt.wikipedia.org/wiki/Kvantin%C4%97_sietis" title="Kvantinė sietis – Lithuanian" lang="lt" hreflang="lt" data-title="Kvantinė sietis" data-language-autonym="Lietuvių" data-language-local-name="Lithuanian" class="interlanguage-link-target"><span>Lietuvių</span></a></li><li class="interlanguage-link interwiki-lmo mw-list-item"><a href="https://lmo.wikipedia.org/wiki/Entanglement" title="Entanglement – Lombard" lang="lmo" hreflang="lmo" data-title="Entanglement" data-language-autonym="Lombard" data-language-local-name="Lombard" class="interlanguage-link-target"><span>Lombard</span></a></li><li class="interlanguage-link interwiki-hu mw-list-item"><a href="https://hu.wikipedia.org/wiki/Kvantum-%C3%B6sszefon%C3%B3d%C3%A1s" title="Kvantum-összefonódás – Hungarian" lang="hu" hreflang="hu" data-title="Kvantum-összefonódás" data-language-autonym="Magyar" data-language-local-name="Hungarian" class="interlanguage-link-target"><span>Magyar</span></a></li><li class="interlanguage-link interwiki-ml mw-list-item"><a href="https://ml.wikipedia.org/wiki/%E0%B4%95%E0%B5%8D%E0%B4%B5%E0%B4%BE%E0%B4%A3%E0%B5%8D%E0%B4%9F%E0%B4%82_%E0%B4%8E%E0%B5%BB%E2%80%8C%E0%B4%9F%E0%B4%BE%E0%B4%82%E0%B4%97%E0%B4%BF%E0%B5%BE%E0%B4%AE%E0%B5%86%E0%B4%A8%E0%B5%8D%E0%B4%B1%E0%B5%8D" title="ക്വാണ്ടം എൻടാംഗിൾമെന്റ് – Malayalam" lang="ml" hreflang="ml" data-title="ക്വാണ്ടം എൻടാംഗിൾമെന്റ്" data-language-autonym="മലയാളം" data-language-local-name="Malayalam" class="interlanguage-link-target"><span>മലയാളം</span></a></li><li class="interlanguage-link interwiki-ms mw-list-item"><a href="https://ms.wikipedia.org/wiki/Keterlibatan_kuantum" title="Keterlibatan kuantum – Malay" lang="ms" hreflang="ms" data-title="Keterlibatan kuantum" data-language-autonym="Bahasa Melayu" data-language-local-name="Malay" class="interlanguage-link-target"><span>Bahasa Melayu</span></a></li><li class="interlanguage-link interwiki-nl mw-list-item"><a href="https://nl.wikipedia.org/wiki/Kwantumverstrengeling" title="Kwantumverstrengeling – Dutch" lang="nl" hreflang="nl" data-title="Kwantumverstrengeling" data-language-autonym="Nederlands" data-language-local-name="Dutch" class="interlanguage-link-target"><span>Nederlands</span></a></li><li class="interlanguage-link interwiki-ne mw-list-item"><a href="https://ne.wikipedia.org/wiki/%E0%A4%AA%E0%A5%8D%E0%A4%B0%E0%A4%AE%E0%A4%BE%E0%A4%A4%E0%A5%8D%E0%A4%B0%E0%A4%BE_%E0%A4%B5%E0%A5%8D%E0%A4%AF%E0%A4%A4%E0%A4%BF%E0%A4%B7%E0%A4%99%E0%A5%8D%E0%A4%97" title="प्रमात्रा व्यतिषङ्ग – Nepali" lang="ne" hreflang="ne" data-title="प्रमात्रा व्यतिषङ्ग" data-language-autonym="नेपाली" data-language-local-name="Nepali" class="interlanguage-link-target"><span>नेपाली</span></a></li><li class="interlanguage-link interwiki-ja mw-list-item"><a href="https://ja.wikipedia.org/wiki/%E9%87%8F%E5%AD%90%E3%82%82%E3%81%A4%E3%82%8C" title="量子もつれ – Japanese" lang="ja" hreflang="ja" data-title="量子もつれ" data-language-autonym="日本語" data-language-local-name="Japanese" class="interlanguage-link-target"><span>日本語</span></a></li><li class="interlanguage-link interwiki-no mw-list-item"><a href="https://no.wikipedia.org/wiki/Kvantesammenfiltring" title="Kvantesammenfiltring – Norwegian Bokmål" lang="nb" hreflang="nb" data-title="Kvantesammenfiltring" data-language-autonym="Norsk bokmål" data-language-local-name="Norwegian Bokmål" class="interlanguage-link-target"><span>Norsk bokmål</span></a></li><li class="interlanguage-link interwiki-nn mw-list-item"><a href="https://nn.wikipedia.org/wiki/Kvantesamanfiltring" title="Kvantesamanfiltring – Norwegian Nynorsk" lang="nn" hreflang="nn" data-title="Kvantesamanfiltring" data-language-autonym="Norsk nynorsk" data-language-local-name="Norwegian Nynorsk" class="interlanguage-link-target"><span>Norsk nynorsk</span></a></li><li class="interlanguage-link interwiki-pa mw-list-item"><a href="https://pa.wikipedia.org/wiki/%E0%A8%95%E0%A9%81%E0%A8%86%E0%A8%82%E0%A8%9F%E0%A8%AE_%E0%A8%87%E0%A9%B0%E0%A8%9F%E0%A9%88%E0%A8%82%E0%A8%97%E0%A8%B2%E0%A8%AE%E0%A9%88%E0%A8%82%E0%A8%9F" title="ਕੁਆਂਟਮ ਇੰਟੈਂਗਲਮੈਂਟ – Punjabi" lang="pa" hreflang="pa" data-title="ਕੁਆਂਟਮ ਇੰਟੈਂਗਲਮੈਂਟ" data-language-autonym="ਪੰਜਾਬੀ" data-language-local-name="Punjabi" class="interlanguage-link-target"><span>ਪੰਜਾਬੀ</span></a></li><li class="interlanguage-link interwiki-pwn mw-list-item"><a href="https://pwn.wikipedia.org/wiki/mapareumalj_a_quantum" title="mapareumalj a quantum – Paiwan" lang="pwn" hreflang="pwn" data-title="mapareumalj a quantum" data-language-autonym="Pinayuanan" data-language-local-name="Paiwan" class="interlanguage-link-target"><span>Pinayuanan</span></a></li><li class="interlanguage-link interwiki-pl mw-list-item"><a href="https://pl.wikipedia.org/wiki/Stan_spl%C4%85tany" title="Stan splątany – Polish" lang="pl" hreflang="pl" data-title="Stan splątany" data-language-autonym="Polski" data-language-local-name="Polish" class="interlanguage-link-target"><span>Polski</span></a></li><li class="interlanguage-link interwiki-pt mw-list-item"><a href="https://pt.wikipedia.org/wiki/Entrela%C3%A7amento_qu%C3%A2ntico" title="Entrelaçamento quântico – Portuguese" lang="pt" hreflang="pt" data-title="Entrelaçamento quântico" data-language-autonym="Português" data-language-local-name="Portuguese" class="interlanguage-link-target"><span>Português</span></a></li><li class="interlanguage-link interwiki-ro mw-list-item"><a href="https://ro.wikipedia.org/wiki/Inseparabilitate_cuantic%C4%83" title="Inseparabilitate cuantică – Romanian" lang="ro" hreflang="ro" data-title="Inseparabilitate cuantică" data-language-autonym="Română" data-language-local-name="Romanian" class="interlanguage-link-target"><span>Română</span></a></li><li class="interlanguage-link interwiki-ru mw-list-item"><a href="https://ru.wikipedia.org/wiki/%D0%9A%D0%B2%D0%B0%D0%BD%D1%82%D0%BE%D0%B2%D0%B0%D1%8F_%D0%B7%D0%B0%D0%BF%D1%83%D1%82%D0%B0%D0%BD%D0%BD%D0%BE%D1%81%D1%82%D1%8C" title="Квантовая запутанность – Russian" lang="ru" hreflang="ru" data-title="Квантовая запутанность" data-language-autonym="Русский" data-language-local-name="Russian" class="interlanguage-link-target"><span>Русский</span></a></li><li class="interlanguage-link interwiki-scn mw-list-item"><a href="https://scn.wikipedia.org/wiki/Currilazzioni_qu%C3%A0ntica" title="Currilazzioni quàntica – Sicilian" lang="scn" hreflang="scn" data-title="Currilazzioni quàntica" data-language-autonym="Sicilianu" data-language-local-name="Sicilian" class="interlanguage-link-target"><span>Sicilianu</span></a></li><li class="interlanguage-link interwiki-simple mw-list-item"><a href="https://simple.wikipedia.org/wiki/Quantum_entanglement" title="Quantum entanglement – Simple English" lang="en-simple" hreflang="en-simple" data-title="Quantum entanglement" data-language-autonym="Simple English" data-language-local-name="Simple English" class="interlanguage-link-target"><span>Simple English</span></a></li><li class="interlanguage-link interwiki-sk mw-list-item"><a href="https://sk.wikipedia.org/wiki/Kvantov%C3%A9_previazanie" title="Kvantové previazanie – Slovak" lang="sk" hreflang="sk" data-title="Kvantové previazanie" data-language-autonym="Slovenčina" data-language-local-name="Slovak" class="interlanguage-link-target"><span>Slovenčina</span></a></li><li class="interlanguage-link interwiki-sl mw-list-item"><a href="https://sl.wikipedia.org/wiki/Kvantna_prepletenost" title="Kvantna prepletenost – Slovenian" lang="sl" hreflang="sl" data-title="Kvantna prepletenost" data-language-autonym="Slovenščina" data-language-local-name="Slovenian" class="interlanguage-link-target"><span>Slovenščina</span></a></li><li class="interlanguage-link interwiki-sr mw-list-item"><a href="https://sr.wikipedia.org/wiki/Kvantno_sprezanje" title="Kvantno sprezanje – Serbian" lang="sr" hreflang="sr" data-title="Kvantno sprezanje" data-language-autonym="Српски / srpski" data-language-local-name="Serbian" class="interlanguage-link-target"><span>Српски / srpski</span></a></li><li class="interlanguage-link interwiki-fi mw-list-item"><a href="https://fi.wikipedia.org/wiki/Lomittuminen" title="Lomittuminen – Finnish" lang="fi" hreflang="fi" data-title="Lomittuminen" data-language-autonym="Suomi" data-language-local-name="Finnish" class="interlanguage-link-target"><span>Suomi</span></a></li><li class="interlanguage-link interwiki-sv mw-list-item"><a href="https://sv.wikipedia.org/wiki/Kvantsammanfl%C3%A4tning" title="Kvantsammanflätning – Swedish" lang="sv" hreflang="sv" data-title="Kvantsammanflätning" data-language-autonym="Svenska" data-language-local-name="Swedish" class="interlanguage-link-target"><span>Svenska</span></a></li><li class="interlanguage-link interwiki-tl mw-list-item"><a href="https://tl.wikipedia.org/wiki/Pagkakabuhol_na_quantum" title="Pagkakabuhol na quantum – Tagalog" lang="tl" hreflang="tl" data-title="Pagkakabuhol na quantum" data-language-autonym="Tagalog" data-language-local-name="Tagalog" class="interlanguage-link-target"><span>Tagalog</span></a></li><li class="interlanguage-link interwiki-ta mw-list-item"><a href="https://ta.wikipedia.org/wiki/%E0%AE%95%E0%AF%81%E0%AE%B5%E0%AE%BE%E0%AE%A3%E0%AF%8D%E0%AE%9F%E0%AE%AE%E0%AF%8D_%E0%AE%AA%E0%AE%BF%E0%AE%A9%E0%AF%8D%E0%AE%A9%E0%AE%B2%E0%AF%8D" title="குவாண்டம் பின்னல் – Tamil" lang="ta" hreflang="ta" data-title="குவாண்டம் பின்னல்" data-language-autonym="தமிழ்" data-language-local-name="Tamil" class="interlanguage-link-target"><span>தமிழ்</span></a></li><li class="interlanguage-link interwiki-tt mw-list-item"><a href="https://tt.wikipedia.org/wiki/%D0%9A%D0%B2%D0%B0%D0%BD%D1%82_%D0%B1%D1%83%D1%82%D0%B0%D0%BB%D1%87%D1%8B%D0%BB%D1%8B%D0%B3%D1%8B" title="Квант буталчылыгы – Tatar" lang="tt" hreflang="tt" data-title="Квант буталчылыгы" data-language-autonym="Татарча / tatarça" data-language-local-name="Tatar" class="interlanguage-link-target"><span>Татарча / tatarça</span></a></li><li class="interlanguage-link interwiki-th mw-list-item"><a href="https://th.wikipedia.org/wiki/%E0%B8%81%E0%B8%B2%E0%B8%A3%E0%B8%9E%E0%B8%B1%E0%B8%A7%E0%B8%9E%E0%B8%B1%E0%B8%99%E0%B9%80%E0%B8%8A%E0%B8%B4%E0%B8%87%E0%B8%84%E0%B8%A7%E0%B8%AD%E0%B8%99%E0%B8%95%E0%B8%B1%E0%B8%A1" title="การพัวพันเชิงควอนตัม – Thai" lang="th" hreflang="th" data-title="การพัวพันเชิงควอนตัม" data-language-autonym="ไทย" data-language-local-name="Thai" class="interlanguage-link-target"><span>ไทย</span></a></li><li class="interlanguage-link interwiki-tg mw-list-item"><a href="https://tg.wikipedia.org/wiki/%D0%94%D0%B0%D1%80%D2%B3%D0%B0%D0%BC%D1%82%D0%B0%D0%BD%D0%B8%D0%B4%D0%B0%D0%B3%D0%B8%D0%B8_%D0%BA%D0%B2%D0%BE%D0%BD%D1%82%D1%83%D0%BC%D3%A3" title="Дарҳамтанидагии квонтумӣ – Tajik" lang="tg" hreflang="tg" data-title="Дарҳамтанидагии квонтумӣ" data-language-autonym="Тоҷикӣ" data-language-local-name="Tajik" class="interlanguage-link-target"><span>Тоҷикӣ</span></a></li><li class="interlanguage-link interwiki-tr mw-list-item"><a href="https://tr.wikipedia.org/wiki/Kuantum_dolan%C4%B1kl%C4%B1k" title="Kuantum dolanıklık – Turkish" lang="tr" hreflang="tr" data-title="Kuantum dolanıklık" data-language-autonym="Türkçe" data-language-local-name="Turkish" class="interlanguage-link-target"><span>Türkçe</span></a></li><li class="interlanguage-link interwiki-uk mw-list-item"><a href="https://uk.wikipedia.org/wiki/%D0%A1%D0%BF%D0%BB%D1%83%D1%82%D0%B0%D0%BD%D1%96_%D0%BA%D0%B2%D0%B0%D0%BD%D1%82%D0%BE%D0%B2%D1%96_%D1%81%D1%82%D0%B0%D0%BD%D0%B8" title="Сплутані квантові стани – Ukrainian" lang="uk" hreflang="uk" data-title="Сплутані квантові стани" data-language-autonym="Українська" data-language-local-name="Ukrainian" class="interlanguage-link-target"><span>Українська</span></a></li><li class="interlanguage-link interwiki-vi mw-list-item"><a href="https://vi.wikipedia.org/wiki/V%C6%B0%E1%BB%9Bng_m%E1%BA%AFc_l%C6%B0%E1%BB%A3ng_t%E1%BB%AD" title="Vướng mắc lượng tử – Vietnamese" lang="vi" hreflang="vi" data-title="Vướng mắc lượng tử" data-language-autonym="Tiếng Việt" data-language-local-name="Vietnamese" class="interlanguage-link-target"><span>Tiếng Việt</span></a></li><li class="interlanguage-link interwiki-wuu mw-list-item"><a href="https://wuu.wikipedia.org/wiki/%E9%87%8F%E5%AD%90%E7%BA%A0%E7%BC%A0" title="量子纠缠 – Wu" lang="wuu" hreflang="wuu" data-title="量子纠缠" data-language-autonym="吴语" data-language-local-name="Wu" class="interlanguage-link-target"><span>吴语</span></a></li><li class="interlanguage-link interwiki-zh-yue mw-list-item"><a href="https://zh-yue.wikipedia.org/wiki/%E9%87%8F%E5%AD%90%E7%B3%BE%E7%BA%8F" title="量子糾纏 – Cantonese" lang="yue" hreflang="yue" data-title="量子糾纏" data-language-autonym="粵語" data-language-local-name="Cantonese" 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Please see <a href="/wiki/Category:CS1_errors:_dates" title="Category:CS1 errors: dates">Category:CS1 errors: dates</a>.)</span></div></div><div id="mw-revision-nav">(<a href="/w/index.php?title=Quantum_entanglement&diff=prev&oldid=1259392593" title="Quantum entanglement">diff</a>) <a href="/w/index.php?title=Quantum_entanglement&direction=prev&oldid=1259392593" title="Quantum entanglement">← Previous revision</a> | Latest revision (diff) | Newer revision → (diff)</div></div></div></div></div> <div id="mw-content-text" class="mw-body-content"><div class="mw-content-ltr mw-parser-output" lang="en" dir="ltr"><div class="shortdescription nomobile noexcerpt noprint searchaux" style="display:none">Correlation between quantum systems</div> <p class="mw-empty-elt"> </p> <figure typeof="mw:File/Thumb"><a href="/wiki/File:SPDC_figure.png" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/2/2d/SPDC_figure.png/222px-SPDC_figure.png" decoding="async" width="222" 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style="width:19.0em;"><tbody><tr><td class="sidebar-pretitle">Part of a series of articles about</td></tr><tr><th class="sidebar-title-with-pretitle"><a href="/wiki/Quantum_mechanics" title="Quantum mechanics">Quantum mechanics</a></th></tr><tr><td class="sidebar-image"><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle i\hbar {\frac {d}{dt}}|\Psi \rangle ={\hat {H}}|\Psi \rangle }"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mi>i</mi> <mi class="MJX-variant">ℏ<!-- ℏ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mfrac> <mi>d</mi> <mrow> <mi>d</mi> <mi>t</mi> </mrow> </mfrac> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mi mathvariant="normal">Ψ<!-- Ψ --></mi> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mo>=</mo> <mrow class="MJX-TeXAtom-ORD"> <mrow class="MJX-TeXAtom-ORD"> <mover> <mi>H</mi> <mo stretchy="false">^<!-- ^ --></mo> </mover> </mrow> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mi mathvariant="normal">Ψ<!-- Ψ --></mi> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle i\hbar {\frac {d}{dt}}|\Psi \rangle ={\hat {H}}|\Psi \rangle }</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/1799e4a910c7d26396922a20ef5ceec25ca1871c" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -2.005ex; width:16.882ex; height:5.509ex;" alt="{\displaystyle i\hbar {\frac {d}{dt}}|\Psi \rangle ={\hat {H}}|\Psi \rangle }"></span><div class="sidebar-caption" style="font-size:90%;padding-top:0.4em;font-style:italic;"><a href="/wiki/Schr%C3%B6dinger_equation" title="Schrödinger equation">Schrödinger equation</a></div></td></tr><tr><td class="sidebar-above hlist nowrap" style="display:block;margin-bottom:0.4em;"> <ul><li><a href="/wiki/Introduction_to_quantum_mechanics" title="Introduction to quantum mechanics">Introduction</a></li> <li><a href="/wiki/Glossary_of_elementary_quantum_mechanics" title="Glossary of elementary quantum mechanics">Glossary</a></li> <li><a href="/wiki/History_of_quantum_mechanics" title="History of quantum mechanics">History</a></li></ul></td></tr><tr><td class="sidebar-content"> <div class="sidebar-list mw-collapsible mw-collapsed"><div class="sidebar-list-title" style="text-align:center;;color: var(--color-base)">Background</div><div class="sidebar-list-content mw-collapsible-content" style="border-top:1px solid #aaa;border-bottom:1px solid #aaa;"> <ul><li><a href="/wiki/Classical_mechanics" title="Classical mechanics">Classical mechanics</a></li> <li><a href="/wiki/Old_quantum_theory" title="Old quantum theory">Old quantum theory</a></li> <li><a href="/wiki/Bra%E2%80%93ket_notation" title="Bra–ket notation">Bra–ket notation</a></li></ul> <div class="hlist"> <ul><li><a href="/wiki/Hamiltonian_(quantum_mechanics)" title="Hamiltonian (quantum mechanics)">Hamiltonian</a></li> <li><a href="/wiki/Wave_interference" title="Wave interference">Interference</a></li></ul> </div></div></div></td> </tr><tr><td class="sidebar-content"> <div class="sidebar-list mw-collapsible"><div class="sidebar-list-title" style="text-align:center;;color: var(--color-base)">Fundamentals</div><div class="sidebar-list-content mw-collapsible-content" style="border-top:1px solid #aaa;border-bottom:1px solid #aaa;"><div class="hlist"> <ul><li><a href="/wiki/Complementarity_(physics)" title="Complementarity (physics)">Complementarity</a></li> <li><a href="/wiki/Quantum_decoherence" title="Quantum decoherence">Decoherence</a></li> <li><a class="mw-selflink selflink">Entanglement</a></li> <li><a href="/wiki/Energy_level" title="Energy level">Energy level</a></li> <li><a href="/wiki/Measurement_in_quantum_mechanics" title="Measurement in quantum mechanics">Measurement</a></li> <li><a href="/wiki/Quantum_nonlocality" title="Quantum nonlocality">Nonlocality</a></li> <li><a href="/wiki/Quantum_number" title="Quantum number">Quantum number</a></li> <li><a href="/wiki/Quantum_state" title="Quantum state">State</a></li> <li><a href="/wiki/Quantum_superposition" title="Quantum superposition">Superposition</a></li> <li><a href="/wiki/Symmetry_in_quantum_mechanics" title="Symmetry in quantum mechanics">Symmetry</a></li> <li><a href="/wiki/Quantum_tunnelling" title="Quantum tunnelling">Tunnelling</a></li> <li><a href="/wiki/Uncertainty_principle" title="Uncertainty principle">Uncertainty</a></li> <li><a href="/wiki/Wave_function" title="Wave function">Wave function</a> <ul><li><a href="/wiki/Wave_function_collapse" title="Wave function collapse">Collapse</a></li></ul></li></ul> </div></div></div></td> </tr><tr><td class="sidebar-content"> <div class="sidebar-list mw-collapsible mw-collapsed"><div class="sidebar-list-title" style="text-align:center;;color: var(--color-base)">Experiments</div><div class="sidebar-list-content mw-collapsible-content" style="border-top:1px solid #aaa;border-bottom:1px solid #aaa;"><div class="hlist"> <ul><li><a href="/wiki/Bell_test" title="Bell test">Bell's inequality</a></li> <li><a href="/wiki/CHSH_inequality" title="CHSH inequality">CHSH inequality</a></li> <li><a href="/wiki/Davisson%E2%80%93Germer_experiment" title="Davisson–Germer experiment">Davisson–Germer</a></li> <li><a href="/wiki/Double-slit_experiment" title="Double-slit experiment">Double-slit</a></li> <li><a href="/wiki/Elitzur%E2%80%93Vaidman_bomb_tester" title="Elitzur–Vaidman bomb tester">Elitzur–Vaidman</a></li> <li><a href="/wiki/Franck%E2%80%93Hertz_experiment" title="Franck–Hertz experiment">Franck–Hertz</a></li> <li><a href="/wiki/Leggett_inequality" title="Leggett inequality">Leggett inequality</a></li> <li><a href="/wiki/Leggett%E2%80%93Garg_inequality" title="Leggett–Garg inequality">Leggett–Garg inequality</a></li> <li><a href="/wiki/Mach%E2%80%93Zehnder_interferometer" title="Mach–Zehnder interferometer">Mach–Zehnder</a></li> <li><a href="/wiki/Popper%27s_experiment" title="Popper's experiment">Popper</a></li></ul> </div> <ul><li><a href="/wiki/Quantum_eraser_experiment" title="Quantum eraser experiment">Quantum eraser</a> <ul><li><a href="/wiki/Delayed-choice_quantum_eraser" title="Delayed-choice quantum eraser">Delayed-choice</a></li></ul></li></ul> <div class="hlist"> <ul><li><a href="/wiki/Schr%C3%B6dinger%27s_cat" title="Schrödinger's cat">Schrödinger's cat</a></li> <li><a href="/wiki/Stern%E2%80%93Gerlach_experiment" title="Stern–Gerlach experiment">Stern–Gerlach</a></li> <li><a href="/wiki/Wheeler%27s_delayed-choice_experiment" title="Wheeler's delayed-choice experiment">Wheeler's delayed-choice</a></li></ul> </div></div></div></td> </tr><tr><td class="sidebar-content"> <div class="sidebar-list mw-collapsible mw-collapsed"><div class="sidebar-list-title" style="text-align:center;;color: var(--color-base)">Formulations</div><div class="sidebar-list-content mw-collapsible-content" style="border-top:1px solid #aaa;border-bottom:1px solid #aaa;"> <ul><li><a href="/wiki/Mathematical_formulation_of_quantum_mechanics" title="Mathematical formulation of quantum mechanics">Overview</a></li></ul> <div class="hlist"> <ul><li><a href="/wiki/Heisenberg_picture" title="Heisenberg picture">Heisenberg</a></li> <li><a href="/wiki/Interaction_picture" title="Interaction picture">Interaction</a></li> <li><a href="/wiki/Matrix_mechanics" title="Matrix mechanics">Matrix</a></li> <li><a href="/wiki/Phase-space_formulation" title="Phase-space formulation">Phase-space</a></li> <li><a href="/wiki/Schr%C3%B6dinger_picture" title="Schrödinger picture">Schrödinger</a></li> <li><a href="/wiki/Path_integral_formulation" title="Path integral formulation">Sum-over-histories (path integral)</a></li></ul> </div></div></div></td> </tr><tr><td class="sidebar-content"> <div class="sidebar-list mw-collapsible mw-collapsed"><div class="sidebar-list-title" style="text-align:center;;color: var(--color-base)">Equations</div><div class="sidebar-list-content mw-collapsible-content" style="border-top:1px solid #aaa;border-bottom:1px solid #aaa;"><div class="hlist"> <ul><li><a href="/wiki/Dirac_equation" title="Dirac equation">Dirac</a></li> <li><a href="/wiki/Klein%E2%80%93Gordon_equation" title="Klein–Gordon equation">Klein–Gordon</a></li> <li><a href="/wiki/Pauli_equation" title="Pauli equation">Pauli</a></li> <li><a href="/wiki/Rydberg_formula" title="Rydberg formula">Rydberg</a></li> <li><a href="/wiki/Schr%C3%B6dinger_equation" title="Schrödinger equation">Schrödinger</a></li></ul> </div></div></div></td> </tr><tr><td class="sidebar-content"> <div class="sidebar-list mw-collapsible mw-collapsed"><div class="sidebar-list-title" style="text-align:center;;color: var(--color-base)"><a href="/wiki/Interpretations_of_quantum_mechanics" title="Interpretations of quantum mechanics">Interpretations</a></div><div class="sidebar-list-content mw-collapsible-content" style="border-top:1px solid #aaa;border-bottom:1px solid #aaa;"><div class="hlist"> <ul><li><a href="/wiki/Quantum_Bayesianism" title="Quantum Bayesianism">Bayesian</a></li> <li><a href="/wiki/Consistent_histories" title="Consistent histories">Consistent histories</a></li> <li><a href="/wiki/Copenhagen_interpretation" title="Copenhagen interpretation">Copenhagen</a></li> <li><a href="/wiki/De_Broglie%E2%80%93Bohm_theory" title="De Broglie–Bohm theory">de Broglie–Bohm</a></li> <li><a href="/wiki/Ensemble_interpretation" title="Ensemble interpretation">Ensemble</a></li> <li><a href="/wiki/Hidden-variable_theory" title="Hidden-variable theory">Hidden-variable</a> <ul><li><a href="/wiki/Local_hidden-variable_theory" title="Local hidden-variable theory">Local</a> <ul><li><a href="/wiki/Superdeterminism" title="Superdeterminism">Superdeterminism</a></li></ul></li></ul></li> <li><a href="/wiki/Many-worlds_interpretation" title="Many-worlds interpretation">Many-worlds</a></li> <li><a href="/wiki/Objective-collapse_theory" title="Objective-collapse theory">Objective-collapse</a></li> <li><a href="/wiki/Quantum_logic" title="Quantum logic">Quantum logic</a></li> <li><a href="/wiki/Relational_quantum_mechanics" title="Relational quantum mechanics">Relational</a></li> <li><a href="/wiki/Transactional_interpretation" title="Transactional interpretation">Transactional</a></li> <li><a href="/wiki/Von_Neumann%E2%80%93Wigner_interpretation" title="Von Neumann–Wigner interpretation">Von Neumann–Wigner</a></li></ul> </div></div></div></td> </tr><tr><td class="sidebar-content"> <div class="sidebar-list mw-collapsible mw-collapsed"><div class="sidebar-list-title" style="text-align:center;;color: var(--color-base)">Advanced topics</div><div class="sidebar-list-content mw-collapsible-content" style="border-top:1px solid #aaa;border-bottom:1px solid #aaa;"> <ul><li><a href="/wiki/Relativistic_quantum_mechanics" title="Relativistic quantum mechanics">Relativistic quantum mechanics</a></li> <li><a href="/wiki/Quantum_field_theory" title="Quantum field theory">Quantum field theory</a></li> <li><a href="/wiki/Quantum_information_science" title="Quantum information science">Quantum information science</a></li> <li><a href="/wiki/Quantum_computing" title="Quantum computing">Quantum computing</a></li> <li><a href="/wiki/Quantum_chaos" title="Quantum chaos">Quantum chaos</a></li> <li><a href="/wiki/Einstein%E2%80%93Podolsky%E2%80%93Rosen_paradox" title="Einstein–Podolsky–Rosen paradox">EPR paradox</a></li> <li><a href="/wiki/Density_matrix" title="Density matrix">Density matrix</a></li> <li><a href="/wiki/Scattering_theory" class="mw-redirect" title="Scattering theory">Scattering theory</a></li> <li><a href="/wiki/Quantum_statistical_mechanics" title="Quantum statistical mechanics">Quantum statistical mechanics</a></li> <li><a href="/wiki/Quantum_machine_learning" title="Quantum machine learning">Quantum machine learning</a></li></ul></div></div></td> </tr><tr><td class="sidebar-content"> <div class="sidebar-list mw-collapsible mw-collapsed"><div class="sidebar-list-title" style="text-align:center;;color: var(--color-base)">Scientists</div><div class="sidebar-list-content mw-collapsible-content" style="border-top:1px solid #aaa;border-bottom:1px solid #aaa;"><div class="hlist"> <ul><li><a href="/wiki/Yakir_Aharonov" title="Yakir Aharonov">Aharonov</a></li> <li><a href="/wiki/John_Stewart_Bell" title="John Stewart Bell">Bell</a></li> <li><a href="/wiki/Hans_Bethe" title="Hans Bethe">Bethe</a></li> <li><a href="/wiki/Patrick_Blackett" title="Patrick Blackett">Blackett</a></li> <li><a href="/wiki/Felix_Bloch" title="Felix Bloch">Bloch</a></li> <li><a href="/wiki/David_Bohm" title="David Bohm">Bohm</a></li> <li><a href="/wiki/Niels_Bohr" title="Niels Bohr">Bohr</a></li> <li><a href="/wiki/Max_Born" title="Max Born">Born</a></li> <li><a href="/wiki/Satyendra_Nath_Bose" title="Satyendra Nath Bose">Bose</a></li> <li><a href="/wiki/Louis_de_Broglie" title="Louis de Broglie">de Broglie</a></li> <li><a href="/wiki/Arthur_Compton" title="Arthur Compton">Compton</a></li> <li><a href="/wiki/Paul_Dirac" title="Paul Dirac">Dirac</a></li> <li><a href="/wiki/Clinton_Davisson" title="Clinton Davisson">Davisson</a></li> <li><a href="/wiki/Peter_Debye" title="Peter Debye">Debye</a></li> <li><a href="/wiki/Paul_Ehrenfest" title="Paul Ehrenfest">Ehrenfest</a></li> <li><a href="/wiki/Albert_Einstein" title="Albert Einstein">Einstein</a></li> <li><a href="/wiki/Hugh_Everett_III" title="Hugh Everett III">Everett</a></li> <li><a href="/wiki/Vladimir_Fock" title="Vladimir Fock">Fock</a></li> <li><a href="/wiki/Enrico_Fermi" title="Enrico Fermi">Fermi</a></li> <li><a href="/wiki/Richard_Feynman" title="Richard Feynman">Feynman</a></li> <li><a href="/wiki/Roy_J._Glauber" title="Roy J. Glauber">Glauber</a></li> <li><a href="/wiki/Martin_Gutzwiller" title="Martin Gutzwiller">Gutzwiller</a></li> <li><a href="/wiki/Werner_Heisenberg" title="Werner Heisenberg">Heisenberg</a></li> <li><a href="/wiki/David_Hilbert" title="David Hilbert">Hilbert</a></li> <li><a href="/wiki/Pascual_Jordan" title="Pascual Jordan">Jordan</a></li> <li><a href="/wiki/Hans_Kramers" title="Hans Kramers">Kramers</a></li> <li><a href="/wiki/Willis_Lamb" title="Willis Lamb">Lamb</a></li> <li><a href="/wiki/Lev_Landau" title="Lev Landau">Landau</a></li> <li><a href="/wiki/Max_von_Laue" title="Max von Laue">Laue</a></li> <li><a href="/wiki/Henry_Moseley" title="Henry Moseley">Moseley</a></li> <li><a href="/wiki/Robert_Andrews_Millikan" title="Robert Andrews Millikan">Millikan</a></li> <li><a href="/wiki/Heike_Kamerlingh_Onnes" title="Heike Kamerlingh Onnes">Onnes</a></li> <li><a href="/wiki/Wolfgang_Pauli" title="Wolfgang Pauli">Pauli</a></li> <li><a href="/wiki/Max_Planck" title="Max Planck">Planck</a></li> <li><a href="/wiki/Isidor_Isaac_Rabi" title="Isidor Isaac Rabi">Rabi</a></li> <li><a href="/wiki/C._V._Raman" title="C. V. Raman">Raman</a></li> <li><a href="/wiki/Johannes_Rydberg" title="Johannes Rydberg">Rydberg</a></li> <li><a href="/wiki/Erwin_Schr%C3%B6dinger" title="Erwin Schrödinger">Schrödinger</a></li> <li><a href="/wiki/Michelle_Simmons" title="Michelle Simmons">Simmons</a></li> <li><a href="/wiki/Arnold_Sommerfeld" title="Arnold Sommerfeld">Sommerfeld</a></li> <li><a href="/wiki/John_von_Neumann" title="John von Neumann">von Neumann</a></li> <li><a href="/wiki/Hermann_Weyl" title="Hermann Weyl">Weyl</a></li> <li><a href="/wiki/Wilhelm_Wien" title="Wilhelm Wien">Wien</a></li> <li><a href="/wiki/Eugene_Wigner" title="Eugene Wigner">Wigner</a></li> <li><a href="/wiki/Pieter_Zeeman" title="Pieter Zeeman">Zeeman</a></li> <li><a href="/wiki/Anton_Zeilinger" title="Anton Zeilinger">Zeilinger</a></li></ul> </div></div></div></td> </tr><tr><td class="sidebar-navbar" style="border-top:1px solid #aaa;padding-top:0.1em;"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1129693374"><style data-mw-deduplicate="TemplateStyles:r1239400231">.mw-parser-output .navbar{display:inline;font-size:88%;font-weight:normal}.mw-parser-output .navbar-collapse{float:left;text-align:left}.mw-parser-output .navbar-boxtext{word-spacing:0}.mw-parser-output .navbar ul{display:inline-block;white-space:nowrap;line-height:inherit}.mw-parser-output .navbar-brackets::before{margin-right:-0.125em;content:"[ "}.mw-parser-output .navbar-brackets::after{margin-left:-0.125em;content:" ]"}.mw-parser-output .navbar li{word-spacing:-0.125em}.mw-parser-output .navbar a>span,.mw-parser-output .navbar a>abbr{text-decoration:inherit}.mw-parser-output .navbar-mini abbr{font-variant:small-caps;border-bottom:none;text-decoration:none;cursor:inherit}.mw-parser-output .navbar-ct-full{font-size:114%;margin:0 7em}.mw-parser-output .navbar-ct-mini{font-size:114%;margin:0 4em}html.skin-theme-clientpref-night .mw-parser-output .navbar li a abbr{color:var(--color-base)!important}@media(prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .navbar li a abbr{color:var(--color-base)!important}}@media print{.mw-parser-output .navbar{display:none!important}}</style><div class="navbar plainlinks hlist navbar-mini"><ul><li class="nv-view"><a href="/wiki/Template:Quantum_mechanics" title="Template:Quantum mechanics"><abbr title="View this template">v</abbr></a></li><li class="nv-talk"><a href="/wiki/Template_talk:Quantum_mechanics" title="Template talk:Quantum mechanics"><abbr title="Discuss this template">t</abbr></a></li><li class="nv-edit"><a href="/wiki/Special:EditPage/Template:Quantum_mechanics" title="Special:EditPage/Template:Quantum mechanics"><abbr title="Edit this template">e</abbr></a></li></ul></div></td></tr></tbody></table> <p><b>Quantum entanglement</b> is the phenomenon of a group of <a href="/wiki/Particle" title="Particle">particles</a> being generated, interacting, or sharing spatial proximity in such a way that the <a href="/wiki/Quantum_state" title="Quantum state">quantum state</a> of each particle of the group cannot be described independently of the state of the others, including when the particles are separated by a large distance. The topic of quantum entanglement is at the heart of the disparity between <a href="/wiki/Classical_physics" title="Classical physics">classical</a> and <a href="/wiki/Quantum_physics" class="mw-redirect" title="Quantum physics">quantum physics</a>: entanglement is a primary feature of quantum mechanics not present in classical mechanics.<sup id="cite_ref-horodecki2007_1-0" class="reference"><a href="#cite_note-horodecki2007-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup><sup class="reference nowrap">: <span title="Page / location: 867 Quotation: "In this way entanglement is that feature of quantum formalism which makes it impossible to simulate quantum correlations within any classical formalism."" class="tooltip tooltip-dashed" style="border-bottom: 1px dashed;">867</span> </sup> </p><p><a href="/wiki/Measurement#Quantum_mechanics" title="Measurement">Measurements</a> of <a href="/wiki/Physical_properties" class="mw-redirect" title="Physical properties">physical properties</a> such as <a href="/wiki/Position_(vector)" class="mw-redirect" title="Position (vector)">position</a>, <a href="/wiki/Momentum" title="Momentum">momentum</a>, <a href="/wiki/Spin_(physics)" title="Spin (physics)">spin</a>, and <a href="/wiki/Polarization_(waves)" title="Polarization (waves)">polarization</a> performed on entangled particles can, in some cases, be found to be perfectly <a href="/wiki/Correlated" class="mw-redirect" title="Correlated">correlated</a>. For example, if a pair of entangled particles is generated such that their total spin is known to be zero, and one particle is found to have clockwise spin on a first axis, then the spin of the other particle, measured on the same axis, is found to be anticlockwise. However, this behavior gives rise to seemingly <a href="/wiki/Paradox" title="Paradox">paradoxical</a> effects: any measurement of a particle's properties results in an apparent and irreversible <a href="/wiki/Wave_function_collapse" title="Wave function collapse">wave function collapse</a> of that particle and changes the original quantum state. With entangled particles, such measurements affect the entangled system as a whole. </p><p>Such phenomena were the subject of a 1935 paper by <a href="/wiki/Albert_Einstein" title="Albert Einstein">Albert Einstein</a>, <a href="/wiki/Boris_Podolsky" title="Boris Podolsky">Boris Podolsky</a>, and <a href="/wiki/Nathan_Rosen" title="Nathan Rosen">Nathan Rosen</a>,<sup id="cite_ref-Einstein1935_2-0" class="reference"><a href="#cite_note-Einstein1935-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> and several papers by <a href="/wiki/Erwin_Schr%C3%B6dinger" title="Erwin Schrödinger">Erwin Schrödinger</a> shortly thereafter,<sup id="cite_ref-Schrödinger1935_3-0" class="reference"><a href="#cite_note-Schrödinger1935-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Schrödinger1936_4-0" class="reference"><a href="#cite_note-Schrödinger1936-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> describing what came to be known as the <a href="/wiki/EPR_paradox" class="mw-redirect" title="EPR paradox">EPR paradox</a>. Einstein and others considered such behavior impossible, as it violated the <a href="/wiki/Local_realism" class="mw-redirect" title="Local realism">local realism</a> view of <a href="/wiki/Causality" title="Causality">causality</a> (Einstein referring to it as "spooky <a href="/wiki/Action_at_a_distance" title="Action at a distance">action at a distance</a>")<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> and argued that the accepted formulation of <a href="/wiki/Quantum_mechanics" title="Quantum mechanics">quantum mechanics</a> must therefore be incomplete. </p><p>Later, however, the counterintuitive predictions of quantum mechanics were verified<sup id="cite_ref-:0_6-0" class="reference"><a href="#cite_note-:0-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-:1_7-0" class="reference"><a href="#cite_note-:1-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-:2_8-0" class="reference"><a href="#cite_note-:2-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> in tests where polarization or spin of entangled particles were measured at separate locations, statistically violating <a href="/wiki/Bell%27s_theorem" title="Bell's theorem">Bell's inequality</a>. In earlier tests, it could not be ruled out that the result at one point could have been <a href="/wiki/Loopholes_in_Bell_test_experiments" class="mw-redirect" title="Loopholes in Bell test experiments">subtly transmitted</a> to the remote point, affecting the outcome at the second location.<sup id="cite_ref-:2_8-1" class="reference"><a href="#cite_note-:2-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> However, so-called "loophole-free" Bell tests have since been performed where the locations were sufficiently separated that communications at the speed of light would have taken longer—in one case, 10,000 times longer—than the interval between the measurements.<sup id="cite_ref-:1_7-1" class="reference"><a href="#cite_note-:1-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-:0_6-1" class="reference"><a href="#cite_note-:0-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> </p><p>Entanglement produces <a href="/wiki/Correlation" title="Correlation">correlation</a> between the measurements, the <a href="/wiki/Mutual_information" title="Mutual information">mutual information</a> between the entangled particles can be exploited, but any transmission of information at <a href="/wiki/Faster-than-light" title="Faster-than-light">faster-than-light</a> speeds is impossible.<sup id="cite_ref-9" class="reference"><a href="#cite_note-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Griffiths2004_10-0" class="reference"><a href="#cite_note-Griffiths2004-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup> Quantum entanglement cannot be used for <a href="/wiki/Faster-than-light_communication" class="mw-redirect" title="Faster-than-light communication">faster-than-light communication</a>.<sup id="cite_ref-11" class="reference"><a href="#cite_note-11"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup> </p><p>Quantum entanglement has been demonstrated experimentally with <a href="/wiki/Photon" title="Photon">photons</a>,<sup id="cite_ref-Kocher1_12-0" class="reference"><a href="#cite_note-Kocher1-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Kocherphd_13-0" class="reference"><a href="#cite_note-Kocherphd-13"><span class="cite-bracket">[</span>13<span class="cite-bracket">]</span></a></sup> <a href="/wiki/Electron" title="Electron">electrons</a>,<sup id="cite_ref-NTR-20151021_14-0" class="reference"><a href="#cite_note-NTR-20151021-14"><span class="cite-bracket">[</span>14<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-NYT-20151021_15-0" class="reference"><a href="#cite_note-NYT-20151021-15"><span class="cite-bracket">[</span>15<span class="cite-bracket">]</span></a></sup> top quarks,<sup id="cite_ref-16" class="reference"><a href="#cite_note-16"><span class="cite-bracket">[</span>16<span class="cite-bracket">]</span></a></sup> molecules<sup id="cite_ref-17" class="reference"><a href="#cite_note-17"><span class="cite-bracket">[</span>17<span class="cite-bracket">]</span></a></sup> and even small diamonds.<sup id="cite_ref-18" class="reference"><a href="#cite_note-18"><span class="cite-bracket">[</span>18<span class="cite-bracket">]</span></a></sup> The use of entanglement in <a href="/wiki/Quantum_communication" class="mw-redirect" title="Quantum communication">communication</a>, <a href="/wiki/Quantum_computing" title="Quantum computing">computation</a> and <a href="/wiki/Quantum_radar" title="Quantum radar">quantum radar</a> is an active area of research and development. </p> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="History">History</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=1" title="Edit section: History"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1236090951">.mw-parser-output .hatnote{font-style:italic}.mw-parser-output div.hatnote{padding-left:1.6em;margin-bottom:0.5em}.mw-parser-output .hatnote i{font-style:normal}.mw-parser-output .hatnote+link+.hatnote{margin-top:-0.5em}@media print{body.ns-0 .mw-parser-output .hatnote{display:none!important}}</style><div role="note" class="hatnote navigation-not-searchable">Background: <a href="/wiki/History_of_quantum_mechanics" title="History of quantum mechanics">History of quantum mechanics</a></div> <figure class="mw-halign-right" typeof="mw:File/Thumb"><a href="/wiki/File:NYT_May_4,_1935.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/a/a0/NYT_May_4%2C_1935.jpg/224px-NYT_May_4%2C_1935.jpg" decoding="async" width="224" height="268" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/a/a0/NYT_May_4%2C_1935.jpg/336px-NYT_May_4%2C_1935.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/a/a0/NYT_May_4%2C_1935.jpg/448px-NYT_May_4%2C_1935.jpg 2x" data-file-width="569" data-file-height="680" /></a><figcaption>Article headline regarding the <a href="/wiki/EPR_paradox" class="mw-redirect" title="EPR paradox">Einstein–Podolsky–Rosen (EPR) paradox</a> paper, in the May 4, 1935 issue of <i><a href="/wiki/The_New_York_Times" title="The New York Times">The New York Times</a></i></figcaption></figure> <p>Albert Einstein and Niels Bohr engaged in a long-running collegial dispute about the meaning of quantum mechanics, now known as the <a href="/wiki/Bohr%E2%80%93Einstein_debates" title="Bohr–Einstein debates">Bohr–Einstein debates</a>. During these debates, Einstein introduced a <a href="/wiki/Thought_experiment" title="Thought experiment">thought experiment</a> about a box that emits a photon. He noted that the experimenter's choice of what measurement to make upon the box will change what can be predicted about the photon, even if the photon is very far away. This argument, which Einstein had formulated by 1931, was an early recognition of the phenomenon that would later be called entanglement.<sup id="cite_ref-19" class="reference"><a href="#cite_note-19"><span class="cite-bracket">[</span>19<span class="cite-bracket">]</span></a></sup> That same year, <a href="/wiki/Hermann_Weyl" title="Hermann Weyl">Hermann Weyl</a> observed in his textbook on <a href="/wiki/Group_theory" title="Group theory">group theory</a> and quantum mechanics that quantum systems made of multiple interacting pieces exhibit a kind of <i><a href="/wiki/Gestalt_psychology" title="Gestalt psychology">Gestalt</a></i>.<sup id="cite_ref-20" class="reference"><a href="#cite_note-20"><span class="cite-bracket">[</span>20<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-21" class="reference"><a href="#cite_note-21"><span class="cite-bracket">[</span>21<span class="cite-bracket">]</span></a></sup> In 1932, <a href="/wiki/Erwin_Schr%C3%B6dinger" title="Erwin Schrödinger">Erwin Schrödinger</a> wrote down the defining equations of quantum entnglement but set them aside, unpublished.<sup id="cite_ref-22" class="reference"><a href="#cite_note-22"><span class="cite-bracket">[</span>22<span class="cite-bracket">]</span></a></sup> In 1935, Einstein, <a href="/wiki/Boris_Podolsky" title="Boris Podolsky">Boris Podolsky</a> and <a href="/wiki/Nathan_Rosen" title="Nathan Rosen">Nathan Rosen</a> published a paper on what is now known as the <a href="/wiki/Einstein%E2%80%93Podolsky%E2%80%93Rosen_paradox" title="Einstein–Podolsky–Rosen paradox">Einstein–Podolsky–Rosen (EPR) paradox</a>, a thought experiment that attempted to show that "the <a href="/wiki/Quantum-mechanical" class="mw-redirect" title="Quantum-mechanical">quantum-mechanical</a> description of physical reality given by wave functions is not complete".<sup id="cite_ref-Einstein1935_2-1" class="reference"><a href="#cite_note-Einstein1935-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> Their concept had two systems interact, then separate, and they showed that afterwards quantum mechanics cannot describe the two systems individually. </p><p>Shortly after this paper appeared, <a href="/wiki/Erwin_Schr%C3%B6dinger" title="Erwin Schrödinger">Erwin Schrödinger</a> wrote a letter to Einstein in <a href="/wiki/German_language" title="German language">German</a> in which he used the word <i>Verschränkung</i> (translated by himself as <i>entanglement</i>) "to describe the correlations between two particles that interact and then separate, as in the EPR experiment".<sup id="cite_ref-MK_23-0" class="reference"><a href="#cite_note-MK-23"><span class="cite-bracket">[</span>23<span class="cite-bracket">]</span></a></sup> Schrödinger followed up with a full paper defining and discussing the notion of <i>entanglement</i>,<sup id="cite_ref-Schroeder-2017_24-0" class="reference"><a href="#cite_note-Schroeder-2017-24"><span class="cite-bracket">[</span>24<span class="cite-bracket">]</span></a></sup> saying "I would not call [entanglement] <i>one</i> but rather <i>the</i> characteristic trait of quantum mechanics, the one that enforces its entire departure from <a href="/wiki/Classical_mechanics" title="Classical mechanics">classical</a> lines of thought."<sup id="cite_ref-Schrödinger1935_3-1" class="reference"><a href="#cite_note-Schrödinger1935-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> Like Einstein, Schrödinger was dissatisfied with the concept of entanglement, because it seemed to violate the speed limit on the transmission of information implicit in the <a href="/wiki/Theory_of_relativity" title="Theory of relativity">theory of relativity</a>.<sup id="cite_ref-25" class="reference"><a href="#cite_note-25"><span class="cite-bracket">[</span>25<span class="cite-bracket">]</span></a></sup> Einstein later referred to this as "<i>spukhafte Fernwirkung</i>"<sup id="cite_ref-spukhafte_26-0" class="reference"><a href="#cite_note-spukhafte-26"><span class="cite-bracket">[</span>26<span class="cite-bracket">]</span></a></sup> or "<a href="/wiki/Spooky_action_at_a_distance" class="mw-redirect" title="Spooky action at a distance">spooky action at a distance</a>". </p><p>In 1946, <a href="/wiki/John_Archibald_Wheeler" title="John Archibald Wheeler">John Archibald Wheeler</a> suggested studying the <a href="/wiki/Polarization_(physics)" class="mw-redirect" title="Polarization (physics)">polarization</a> of pairs of <a href="/wiki/Gamma-ray" class="mw-redirect" title="Gamma-ray">gamma-ray</a> photons produced by electron–<a href="/wiki/Positron" title="Positron">positron</a> annihilation.<sup id="cite_ref-27" class="reference"><a href="#cite_note-27"><span class="cite-bracket">[</span>27<span class="cite-bracket">]</span></a></sup> <a href="/wiki/Chien-Shiung_Wu" title="Chien-Shiung Wu">Chien-Shiung Wu</a> and I. Shaknov carried out this experiment in 1949,<sup id="cite_ref-:3_28-0" class="reference"><a href="#cite_note-:3-28"><span class="cite-bracket">[</span>28<span class="cite-bracket">]</span></a></sup> thereby demonstrating that the entangled particle pairs considered by EPR could be created in the laboratory.<sup id="cite_ref-29" class="reference"><a href="#cite_note-29"><span class="cite-bracket">[</span>29<span class="cite-bracket">]</span></a></sup> </p><p>Despite Schrödinger's claim of its importance, little work on entanglement was published for decades after his paper was published.<sup id="cite_ref-Schroeder-2017_24-1" class="reference"><a href="#cite_note-Schroeder-2017-24"><span class="cite-bracket">[</span>24<span class="cite-bracket">]</span></a></sup> In 1964 <a href="/wiki/John_S._Bell" class="mw-redirect" title="John S. Bell">John S. Bell</a> demonstrated an upper limit, seen in <a href="/wiki/Bell%27s_inequality" class="mw-redirect" title="Bell's inequality">Bell's inequality</a>, regarding the strength of correlations that can be produced in any theory obeying <a href="/wiki/Local_realism" class="mw-redirect" title="Local realism">local realism</a>, and showed that quantum theory predicts violations of this limit for certain entangled systems.<sup id="cite_ref-30" class="reference"><a href="#cite_note-30"><span class="cite-bracket">[</span>30<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-31" class="reference"><a href="#cite_note-31"><span class="cite-bracket">[</span>31<span class="cite-bracket">]</span></a></sup><sup class="reference nowrap"><span title="Page / location: 405">: 405 </span></sup> His inequality is experimentally testable, and there have been numerous <a href="/wiki/Bell_test_experiments" class="mw-redirect" title="Bell test experiments">relevant experiments</a>, starting with the pioneering work of <a href="/wiki/Stuart_Freedman" title="Stuart Freedman">Stuart Freedman</a> and <a href="/wiki/John_Clauser" title="John Clauser">John Clauser</a> in 1972<sup id="cite_ref-Clauser_32-0" class="reference"><a href="#cite_note-Clauser-32"><span class="cite-bracket">[</span>32<span class="cite-bracket">]</span></a></sup> and <a href="/wiki/Alain_Aspect" title="Alain Aspect">Alain Aspect</a>'s experiments in 1982.<sup id="cite_ref-Aspect1982_33-0" class="reference"><a href="#cite_note-Aspect1982-33"><span class="cite-bracket">[</span>33<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-hanson_34-0" class="reference"><a href="#cite_note-hanson-34"><span class="cite-bracket">[</span>34<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-35" class="reference"><a href="#cite_note-35"><span class="cite-bracket">[</span>35<span class="cite-bracket">]</span></a></sup> </p><p>While Bell actively discouraged students from pursuing work like his as too esoteric, after a talk at Oxford a student named <a href="/wiki/Artur_Ekert" title="Artur Ekert">Artur Ekert</a> suggested that these super-strong correlations could be used as a resource for communication.<sup id="cite_ref-Gilder2009_36-0" class="reference"><a href="#cite_note-Gilder2009-36"><span class="cite-bracket">[</span>36<span class="cite-bracket">]</span></a></sup><sup class="reference nowrap"><span title="Page / location: 315">: 315 </span></sup> Ekert followed up by publishing a <a href="/wiki/Quantum_key_distribution" title="Quantum key distribution">quantum key distribution</a> protocol called <a href="/wiki/E91_protocol" class="mw-redirect" title="E91 protocol">E91</a> that uses the violation of a Bell inequality as a proof of security.<sup id="cite_ref-37" class="reference"><a href="#cite_note-37"><span class="cite-bracket">[</span>37<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-horodecki2007_1-1" class="reference"><a href="#cite_note-horodecki2007-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup><sup class="reference nowrap">: <span title="Page / location: 874 Quotation: "The first discovery within quantum information theory, which involves entanglement, is due to Ekert 1991."" class="tooltip tooltip-dashed" style="border-bottom: 1px dashed;">874</span> </sup> </p><p>In 1992, the entanglement concept was leveraged to propose <a href="/wiki/Quantum_teleportation" title="Quantum teleportation">quantum teleportation</a>,<sup id="cite_ref-BBCJPW93_38-0" class="reference"><a href="#cite_note-BBCJPW93-38"><span class="cite-bracket">[</span>38<span class="cite-bracket">]</span></a></sup> an effect that was realized experimentally in 1997.<sup id="cite_ref-39" class="reference"><a href="#cite_note-39"><span class="cite-bracket">[</span>39<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Bouwmeester-1997_40-0" class="reference"><a href="#cite_note-Bouwmeester-1997-40"><span class="cite-bracket">[</span>40<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Rome1998_41-0" class="reference"><a href="#cite_note-Rome1998-41"><span class="cite-bracket">[</span>41<span class="cite-bracket">]</span></a></sup> </p><p>Beginning in the mid-1990s, <a href="/wiki/Anton_Zeilinger" title="Anton Zeilinger">Anton Zeilinger</a> used the generation of entanglement via <a href="/wiki/Spontaneous_parametric_down-conversion" title="Spontaneous parametric down-conversion"> parametric down-conversion</a> to develop <a href="/wiki/Entanglement_swapping" class="mw-redirect" title="Entanglement swapping">entanglement swapping</a><sup id="cite_ref-Gilder2009_36-1" class="reference"><a href="#cite_note-Gilder2009-36"><span class="cite-bracket">[</span>36<span class="cite-bracket">]</span></a></sup><sup class="reference nowrap"><span title="Page / location: 317">: 317 </span></sup> and demonstrate <a href="/wiki/Quantum_cryptography" title="Quantum cryptography">quantum cryptography</a> with entangled photons.<sup id="cite_ref-42" class="reference"><a href="#cite_note-42"><span class="cite-bracket">[</span>42<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-43" class="reference"><a href="#cite_note-43"><span class="cite-bracket">[</span>43<span class="cite-bracket">]</span></a></sup> </p><p>In 2022, the <a href="/wiki/Nobel_Prize_in_Physics" title="Nobel Prize in Physics">Nobel Prize in Physics</a> was awarded to Aspect, Clauser, and Zeilinger "for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science".<sup id="cite_ref-NobelPrize_44-0" class="reference"><a href="#cite_note-NobelPrize-44"><span class="cite-bracket">[</span>44<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Concept">Concept</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=2" title="Edit section: Concept"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <div class="mw-heading mw-heading3"><h3 id="Meaning_of_entanglement">Meaning of entanglement</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=3" title="Edit section: Meaning of entanglement"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>An entangled system can be defined to be one whose quantum state cannot be factored as a product of states of its local constituents; that is to say, they are not individual particles but are an inseparable whole. In entanglement, one constituent cannot be fully described without considering the other(s). The state of a composite system is always expressible as a sum, or <a href="/wiki/Quantum_superposition" title="Quantum superposition">superposition</a>, of products of states of local constituents; it is entangled if this sum cannot be written as a single product term.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="definitions should be sourced (November 2024)">citation needed</span></a></i>]</sup> </p><p>Quantum <a href="/wiki/Physical_system" title="Physical system">systems</a> can become entangled through various types of interactions. For some ways in which entanglement may be achieved for experimental purposes, see the section below on <a href="#Methods_of_creating_entanglement">methods</a>. Entanglement is broken when the entangled particles <a href="/wiki/Quantum_decoherence" title="Quantum decoherence">decohere</a> through interaction with the environment; for example, when a measurement is made.<sup id="cite_ref-Peres1993_45-0" class="reference"><a href="#cite_note-Peres1993-45"><span class="cite-bracket">[</span>45<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-46" class="reference"><a href="#cite_note-46"><span class="cite-bracket">[</span>46<span class="cite-bracket">]</span></a></sup> </p><p>As an example of entanglement: a <a href="/wiki/Subatomic_particle" title="Subatomic particle">subatomic particle</a> <a href="/wiki/Particle_decay" title="Particle decay">decays</a> into an entangled pair of other particles. The decay events obey the various <a href="/wiki/Conservation_laws" class="mw-redirect" title="Conservation laws">conservation laws</a>, and as a result, the measurement outcomes of one daughter particle must be highly correlated with the measurement outcomes of the other daughter particle (so that the total momenta, angular momenta, energy, and so forth remains roughly the same before and after this process). For instance, a <a href="/wiki/Spin_(physics)" title="Spin (physics)">spin</a>-zero particle could decay into a pair of spin-1/2 particles. Since the total spin before and after this decay must be zero (conservation of angular momentum), whenever the first particle is measured to be <a href="/wiki/Spin_(physics)#Direction" title="Spin (physics)">spin up</a> on some axis, the other, when measured on the same axis, is always found to be <a href="/wiki/Spin_(physics)#Direction" title="Spin (physics)">spin down</a>. (This is called the spin anti-correlated case; and if the prior probabilities for measuring each spin are equal, the pair is said to be in the <a href="/wiki/Singlet_state" title="Singlet state">singlet state</a>.)<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (November 2024)">citation needed</span></a></i>]</sup> </p><p>The above result may or may not be perceived as surprising. A classical system would display the same property, and a <a href="/wiki/Hidden_variable_theory" class="mw-redirect" title="Hidden variable theory">hidden variable theory</a> would certainly be required to do so, based on conservation of angular momentum in classical and quantum mechanics alike. The difference is that a classical system has definite values for all the observables all along, while the quantum system does not. In a sense to be discussed below, the quantum system considered here seems to acquire a probability distribution for the outcome of a measurement of the spin along any axis of the other particle upon measurement of the first particle. This probability distribution is in general different from what it would be without measurement of the first particle. This may certainly be perceived as surprising in the case of spatially separated entangled particles.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (November 2024)">citation needed</span></a></i>]</sup> </p> <div class="mw-heading mw-heading3"><h3 id="Paradox">Paradox</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=4" title="Edit section: Paradox"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The paradox is that a measurement made on either of the particles apparently collapses the state of the entire entangled system—and does so instantaneously, before any information about the measurement result could have been communicated to the other particle (assuming that information cannot travel <a href="/wiki/Faster_than_light" class="mw-redirect" title="Faster than light">faster than light</a>) and hence assured the "proper" outcome of the measurement of the other part of the entangled pair. In the <a href="/wiki/Copenhagen_interpretation" title="Copenhagen interpretation">Copenhagen interpretation</a>, the result of a spin measurement on one of the particles is a collapse (of wave function) into a state in which each particle has a definite spin (either up or down) along the axis of measurement. The outcome is taken to be random, with each possibility having a probability of 50%. However, if both spins are measured along the same axis, they are found to be anti-correlated. This means that the random outcome of the measurement made on one particle seems to have been transmitted to the other, so that it can make the "right choice" when it too is measured.<sup id="cite_ref-47" class="reference"><a href="#cite_note-47"><span class="cite-bracket">[</span>47<span class="cite-bracket">]</span></a></sup><sup class="noprint Inline-Template" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Reliable_sources" title="Wikipedia:Reliable sources"><span title="The material near this tag may rely on an unreliable source. Interstellar travel? No physics cites in Google Scholar. Author not a scientist. (November 2024)">unreliable source?</span></a></i>]</sup> </p><p>The distance and timing of the measurements can be chosen so as to make the interval between the two measurements <a href="/wiki/Spacelike" class="mw-redirect" title="Spacelike">spacelike</a>, hence, any causal effect connecting the events would have to travel faster than light. According to the principles of <a href="/wiki/Special_relativity" title="Special relativity">special relativity</a>, it is not possible for any information to travel between two such measuring events. It is not even possible to say which of the measurements came first. For two spacelike separated events <span class="texhtml"><i>x</i><sub>1</sub></span> and <span class="texhtml"><i>x</i><sub>2</sub></span> there are <a href="/wiki/Inertial_frame" class="mw-redirect" title="Inertial frame">inertial frames</a> in which <span class="texhtml"><i>x</i><sub>1</sub></span> is first and others in which <span class="texhtml"><i>x</i><sub>2</sub></span> is first. Therefore, the correlation between the two measurements cannot be explained as one measurement determining the other: different observers would disagree about the role of cause and effect. </p> <div class="mw-heading mw-heading3"><h3 id="Hidden-variables_theory">Hidden-variables theory</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=5" title="Edit section: Hidden-variables theory"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Hidden-variables_theory" class="mw-redirect" title="Hidden-variables theory">Hidden-variables theory</a></div> <p>A possible resolution to the paradox is to assume that quantum theory is incomplete, and the result of measurements depends on predetermined "hidden variables".<sup id="cite_ref-Gibney2017_48-0" class="reference"><a href="#cite_note-Gibney2017-48"><span class="cite-bracket">[</span>48<span class="cite-bracket">]</span></a></sup> The state of the particles being measured contains some hidden variables, whose values effectively determine, right from the moment of separation, what the outcomes of the spin measurements are going to be. This would mean that each particle carries all the required information with it, and nothing needs to be transmitted from one particle to the other at the time of measurement. Einstein and others (see the previous section) originally believed this was the only way out of the paradox, and the accepted quantum mechanical description (with a random measurement outcome) must be incomplete. </p> <div class="mw-heading mw-heading3"><h3 id="Violations_of_Bell's_inequality"><span id="Violations_of_Bell.27s_inequality"></span>Violations of Bell's inequality</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=6" title="Edit section: Violations of Bell's inequality"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p><a href="/wiki/Local_hidden-variable_theory" title="Local hidden-variable theory">Local hidden variable theories</a> fail, however, when measurements of the spin of entangled particles along different axes are considered. If a large number of pairs of such measurements are made (on a large number of pairs of entangled particles), then statistically, if the local realist or hidden variables view were correct, the results would always satisfy <a href="/wiki/Bell%27s_inequality" class="mw-redirect" title="Bell's inequality">Bell's inequality</a>. A <a href="/wiki/Bell_test" title="Bell test">number of experiments</a> have shown in practice that Bell's inequality is not satisfied.<sup id="cite_ref-49" class="reference"><a href="#cite_note-49"><span class="cite-bracket">[</span>49<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-50" class="reference"><a href="#cite_note-50"><span class="cite-bracket">[</span>50<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-51" class="reference"><a href="#cite_note-51"><span class="cite-bracket">[</span>51<span class="cite-bracket">]</span></a></sup> Moreover, when measurements of the entangled particles are made in moving <a href="/wiki/Special_relativity" title="Special relativity">relativistic</a> reference frames, in which each measurement (in its own relativistic time frame) occurs before the other, the measurement results remain correlated.<sup id="cite_ref-52" class="reference"><a href="#cite_note-52"><span class="cite-bracket">[</span>52<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Gilder2009_36-2" class="reference"><a href="#cite_note-Gilder2009-36"><span class="cite-bracket">[</span>36<span class="cite-bracket">]</span></a></sup><sup class="reference nowrap"><span title="Page / location: 321–324">: 321–324 </span></sup> </p><p>The fundamental issue about measuring spin along different axes is that these measurements cannot have definite values at the same time―they are <a href="/wiki/Incompatible_observables" class="mw-redirect" title="Incompatible observables">incompatible</a> in the sense that these measurements' maximum simultaneous precision is constrained by the <a href="/wiki/Uncertainty_principle" title="Uncertainty principle">uncertainty principle</a>. This is contrary to what is found in classical physics, where any number of properties can be measured simultaneously with arbitrary accuracy. It has been proven mathematically that compatible measurements cannot show Bell-inequality-violating correlations,<sup id="cite_ref-53" class="reference"><a href="#cite_note-53"><span class="cite-bracket">[</span>53<span class="cite-bracket">]</span></a></sup> and thus entanglement is a fundamentally non-classical phenomenon. </p> <div class="mw-heading mw-heading3"><h3 id="Quantum_teleportation_and_entanglement_swapping">Quantum teleportation and entanglement swapping</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=7" title="Edit section: Quantum teleportation and entanglement swapping"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>If Alice and Bob share an entangled state,<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="'Mike and Ike' ref says this is an EPR state, not just any entangled state (November 2024)">citation needed</span></a></i>]</sup> Alice can tell Bob over a telephone call how to reproduce a quantum state <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle |\Psi \rangle }"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mi mathvariant="normal">Ψ<!-- Ψ --></mi> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle |\Psi \rangle }</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/6e77f6b1e903837c5765c9683da41dd93199621c" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:3.36ex; height:2.843ex;" alt="{\displaystyle |\Psi \rangle }"></span> she has in her lab. Alice performs a joint measurement on <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle |\Psi \rangle }"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mi mathvariant="normal">Ψ<!-- Ψ --></mi> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle |\Psi \rangle }</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/6e77f6b1e903837c5765c9683da41dd93199621c" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:3.36ex; height:2.843ex;" alt="{\displaystyle |\Psi \rangle }"></span> together with her half of the entangled state and tells Bob the results. Using Alice's results Bob operates on his half of the entangled state to make it equal to <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle |\Psi \rangle }"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mi mathvariant="normal">Ψ<!-- Ψ --></mi> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle |\Psi \rangle }</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/6e77f6b1e903837c5765c9683da41dd93199621c" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:3.36ex; height:2.843ex;" alt="{\displaystyle |\Psi \rangle }"></span>. Since Alice's original state is necessarily destroyed during the process,<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="'Mike and Ike' ref does not say this (November 2024)">citation needed</span></a></i>]</sup> it is said to be "<a href="/wiki/Quantum_teleportation" title="Quantum teleportation">quantum teleported</a>" to Bob's laboratory through this protocol.<sup id="cite_ref-Nielsen-2010_54-0" class="reference"><a href="#cite_note-Nielsen-2010-54"><span class="cite-bracket">[</span>54<span class="cite-bracket">]</span></a></sup><sup class="reference nowrap"><span title="Page / location: 27">: 27 </span></sup><sup id="cite_ref-horodecki2007_1-2" class="reference"><a href="#cite_note-horodecki2007-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup><sup class="reference nowrap"><span title="Page / location: 875">: 875 </span></sup> </p> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Entanglement_swapping.svg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/7/74/Entanglement_swapping.svg/220px-Entanglement_swapping.svg.png" decoding="async" width="220" height="193" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/7/74/Entanglement_swapping.svg/330px-Entanglement_swapping.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/7/74/Entanglement_swapping.svg/440px-Entanglement_swapping.svg.png 2x" data-file-width="512" data-file-height="450" /></a><figcaption>Entanglement of states from independent sources can be swapped through Bell state measurement.<sup id="cite_ref-GuoReview2023_55-0" class="reference"><a href="#cite_note-GuoReview2023-55"><span class="cite-bracket">[</span>55<span class="cite-bracket">]</span></a></sup><sup class="reference nowrap"><span title="Page / location: 341">: 341 </span></sup></figcaption></figure> <p><a href="/wiki/Entanglement_swapping" class="mw-redirect" title="Entanglement swapping">Entanglement swapping</a> is an application of teleportation to make two parties that never interacted share an entangled state. We start with three parties, Alice, Bob, and Carol. Alice and Bob share an entangled state, and so do Bob and Carol, but Alice and Carol do not. Using the teleportation protocol, Bob teleports to Carol the half of the entangled state that he shares with Alice. Since teleportation preserves entanglement, this results in Alice and Carol sharing an entangled state.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="this is not what horodecki says (November 2024)">citation needed</span></a></i>]</sup> </p> <div class="mw-heading mw-heading3"><h3 id="Emergent_time">Emergent time</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=8" title="Edit section: Emergent time"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Problem_of_time" title="Problem of time">Problem of time</a></div> <p>There is a fundamental conflict, referred to as the <b>problem of time</b>, between the way the concept of <i>time</i> is used in <a href="/wiki/Quantum_mechanics" title="Quantum mechanics">quantum mechanics</a>, and the role it plays in <a href="/wiki/General_relativity" title="General relativity">general relativity</a>. In standard quantum theories time acts as an independent background through which states evolve, while general relativity treats time as a dynamical variable which relates directly with matter. Part of the effort to reconcile these approaches to time results in the <a href="/wiki/Wheeler%E2%80%93DeWitt_equation" title="Wheeler–DeWitt equation">Wheeler–DeWitt equation</a>, which predicts the state of the universe is timeless or static, contrary to ordinary experience.<sup id="cite_ref-Moreva2014_56-0" class="reference"><a href="#cite_note-Moreva2014-56"><span class="cite-bracket">[</span>56<span class="cite-bracket">]</span></a></sup> Work started by <a href="/wiki/Don_Page_(physicist)" title="Don Page (physicist)">Don Page</a> and <a href="/wiki/William_Wootters" title="William Wootters">William Wootters</a><sup id="cite_ref-57" class="reference"><a href="#cite_note-57"><span class="cite-bracket">[</span>57<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-58" class="reference"><a href="#cite_note-58"><span class="cite-bracket">[</span>58<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-59" class="reference"><a href="#cite_note-59"><span class="cite-bracket">[</span>59<span class="cite-bracket">]</span></a></sup> suggests that the universe appears to evolve for observers on the inside because of energy entanglement between an evolving system and a clock system, both within the universe.<sup id="cite_ref-Moreva2014_56-1" class="reference"><a href="#cite_note-Moreva2014-56"><span class="cite-bracket">[</span>56<span class="cite-bracket">]</span></a></sup> In this way the overall system can remain timeless while parts experience time via entanglement. The issue remains an open question closely related to attempts at theories of <a href="/wiki/Quantum_gravity" title="Quantum gravity">quantum gravity</a>.<sup id="cite_ref-60" class="reference"><a href="#cite_note-60"><span class="cite-bracket">[</span>60<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-61" class="reference"><a href="#cite_note-61"><span class="cite-bracket">[</span>61<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Emergent_gravity">Emergent gravity</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=9" title="Edit section: Emergent gravity"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In general relativity gravity arises from the curvature of spacetime and that curvature derives from the distribution of matter. However, matter is governed by quantum mechanics. Integration of these two theories faces many problems. In an (unrealistic) model space called the <a href="/wiki/Anti-de_Sitter_space" title="Anti-de Sitter space">anti-de Sitter space</a>, the <a href="/wiki/AdS/CFT_correspondence" title="AdS/CFT correspondence">AdS/CFT correspondence</a> allows a quantum gravitational system to be related to a quantum field theory without gravity.<sup id="cite_ref-Swingle2018_62-0" class="reference"><a href="#cite_note-Swingle2018-62"><span class="cite-bracket">[</span>62<span class="cite-bracket">]</span></a></sup> Using this correspondence, <a href="/wiki/Mark_Van_Raamsdonk" title="Mark Van Raamsdonk">Mark Van Raamsdonk</a> suggested that <a href="/wiki/Spacetime" title="Spacetime">spacetime</a> arises as an emergent phenomenon of the quantum degrees of freedom that are entangled and live in the boundary of the spacetime.<sup id="cite_ref-63" class="reference"><a href="#cite_note-63"><span class="cite-bracket">[</span>63<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Non-locality_and_entanglement">Non-locality and entanglement</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=10" title="Edit section: Non-locality and entanglement"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In the media and popular science, <a href="/wiki/Quantum_nonlocality" title="Quantum nonlocality">quantum non-locality</a> is often portrayed as being equivalent to entanglement. While this is true for pure bipartite quantum states, in general entanglement is only necessary for non-local correlations, but there exist mixed entangled states that do not produce such correlations.<sup id="cite_ref-Brunner-RMP2014_64-0" class="reference"><a href="#cite_note-Brunner-RMP2014-64"><span class="cite-bracket">[</span>64<span class="cite-bracket">]</span></a></sup> One example is the <a href="/wiki/Werner_state" title="Werner state">Werner states</a> that are entangled for certain values of <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle p_{\text{sym}}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msub> <mi>p</mi> <mrow class="MJX-TeXAtom-ORD"> <mtext>sym</mtext> </mrow> </msub> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle p_{\text{sym}}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/e4eeb2d2922ed0e273cbf1f7440d85f3cca4d736" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -1.005ex; margin-left: -0.089ex; width:4.376ex; height:2.343ex;" alt="{\displaystyle p_{\text{sym}}}"></span>, but can always be described using local hidden variables.<sup id="cite_ref-werner1989_65-0" class="reference"><a href="#cite_note-werner1989-65"><span class="cite-bracket">[</span>65<span class="cite-bracket">]</span></a></sup> Moreover, it was shown that, for arbitrary numbers of particles, there exist states that are genuinely entangled but admit a local model.<sup id="cite_ref-Augusiak2015_66-0" class="reference"><a href="#cite_note-Augusiak2015-66"><span class="cite-bracket">[</span>66<span class="cite-bracket">]</span></a></sup> </p><p>The mentioned proofs about the existence of local models assume that there is only one copy of the quantum state available at a time. If the particles are allowed to perform local measurements on many copies of such states, then many apparently local states (e.g., the qubit Werner states) can no longer be described by a local model. This is, in particular, true for all <a href="/wiki/Entanglement_distillation" title="Entanglement distillation">distillable</a> states. However, it remains an open question whether all entangled states become non-local given sufficiently many copies.<sup id="cite_ref-Vertesi2014_67-0" class="reference"><a href="#cite_note-Vertesi2014-67"><span class="cite-bracket">[</span>67<span class="cite-bracket">]</span></a></sup> </p><p>Entanglement of a state shared by two particles is necessary, but not sufficient for that state to be non-local. Entanglement is more commonly viewed as an algebraic concept, noted for being a prerequisite to non-locality as well as to <a href="/wiki/Quantum_teleportation" title="Quantum teleportation">quantum teleportation</a> and to <a href="/wiki/Superdense_coding" title="Superdense coding">superdense coding</a>, whereas non-locality is defined according to experimental statistics and is much more involved with the <a href="/wiki/Quantum_foundations" title="Quantum foundations">foundations</a> and <a href="/wiki/Interpretations_of_quantum_mechanics" title="Interpretations of quantum mechanics">interpretations of quantum mechanics</a>.<sup id="cite_ref-68" class="reference"><a href="#cite_note-68"><span class="cite-bracket">[</span>68<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Quantum-mechanical_framework">Quantum-mechanical framework</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=11" title="Edit section: Quantum-mechanical framework"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The following subsections use the formalism and theoretical framework developed in the articles <a href="/wiki/Bra%E2%80%93ket_notation" title="Bra–ket notation">bra–ket notation</a> and <a href="/wiki/Mathematical_formulation_of_quantum_mechanics" title="Mathematical formulation of quantum mechanics">mathematical formulation of quantum mechanics</a>. </p> <div class="mw-heading mw-heading3"><h3 id="Pure_states">Pure states</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=12" title="Edit section: Pure states"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Consider two arbitrary quantum systems <span class="texhtml mvar" style="font-style:italic;">A</span> and <span class="texhtml mvar" style="font-style:italic;">B</span>, with respective <a href="/wiki/Hilbert_space" title="Hilbert space">Hilbert spaces</a> <span class="texhtml mvar" style="font-style:italic;">H<sub>A</sub></span> and <span class="texhtml mvar" style="font-style:italic;">H<sub>B</sub></span>. The Hilbert space of the composite system is the <a href="/wiki/Tensor_product" title="Tensor product">tensor product</a> </p> <dl><dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle H_{A}\otimes H_{B}.}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msub> <mi>H</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mo>⊗<!-- ⊗ --></mo> <msub> <mi>H</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msub> <mo>.</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle H_{A}\otimes H_{B}.}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/19fca01f34c847473055f02b00ae2c3e51409901" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:10.294ex; height:2.509ex;" alt="{\displaystyle H_{A}\otimes H_{B}.}"></span></dd></dl> <p>If the first system is in state <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle |\psi \rangle _{A}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mi>ψ<!-- ψ --></mi> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle |\psi \rangle _{A}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/435c58e1961f1e68f74f8734418e7c386efd5ef4" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:4.529ex; height:2.843ex;" alt="{\displaystyle |\psi \rangle _{A}}"></span> and the second in state <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle |\phi \rangle _{B}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mi>ϕ<!-- ϕ --></mi> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msub> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle |\phi \rangle _{B}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/2be1ace3f2710536cfa89e755db7a9f4f9f5136e" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:4.417ex; height:2.843ex;" alt="{\displaystyle |\phi \rangle _{B}}"></span>, the state of the composite system is </p> <dl><dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle |\psi \rangle _{A}\otimes |\phi \rangle _{B}.}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mi>ψ<!-- ψ --></mi> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mo>⊗<!-- ⊗ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mi>ϕ<!-- ϕ --></mi> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msub> <mo>.</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle |\psi \rangle _{A}\otimes |\phi \rangle _{B}.}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/c828da659b247951f5792107fc723ebe7c5577e9" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:12.433ex; height:2.843ex;" alt="{\displaystyle |\psi \rangle _{A}\otimes |\phi \rangle _{B}.}"></span></dd></dl> <p>States of the composite system that can be represented in this form are called separable states, or <a href="/wiki/Product_state" class="mw-redirect" title="Product state">product states</a>. </p><p>Not all states are separable states (and thus product states). Fix a <a href="/wiki/Basis_(linear_algebra)" title="Basis (linear algebra)">basis</a> <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \{|i\rangle _{A}\}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mo fence="false" stretchy="false">{</mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mi>i</mi> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mo fence="false" stretchy="false">}</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \{|i\rangle _{A}\}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/3744b6c619227428bd614c13bb9aac750b0e82fe" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:6.144ex; height:2.843ex;" alt="{\displaystyle \{|i\rangle _{A}\}}"></span> for <span class="texhtml mvar" style="font-style:italic;">H<sub>A</sub></span> and a basis <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \{|j\rangle _{B}\}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mo fence="false" stretchy="false">{</mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mi>j</mi> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msub> <mo fence="false" stretchy="false">}</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \{|j\rangle _{B}\}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/c4e8a89a80bddaf7a469e3ecc3ec555eede209a7" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:6.314ex; height:2.843ex;" alt="{\displaystyle \{|j\rangle _{B}\}}"></span> for <span class="texhtml mvar" style="font-style:italic;">H<sub>B</sub></span>. The most general state in <span class="texhtml"><i>H<sub>A</sub></i> ⊗ <i>H<sub>B</sub></i></span> is of the form </p> <dl><dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle |\psi \rangle _{AB}=\sum _{i,j}c_{ij}|i\rangle _{A}\otimes |j\rangle _{B}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mi>ψ<!-- ψ --></mi> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> <mi>B</mi> </mrow> </msub> <mo>=</mo> <munder> <mo>∑<!-- ∑ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> <mo>,</mo> <mi>j</mi> </mrow> </munder> <msub> <mi>c</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> <mi>j</mi> </mrow> </msub> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mi>i</mi> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mo>⊗<!-- ⊗ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mi>j</mi> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msub> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle |\psi \rangle _{AB}=\sum _{i,j}c_{ij}|i\rangle _{A}\otimes |j\rangle _{B}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/d211d5745f4b52151d8489776dfbdf1fee13dc1f" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -3.338ex; width:25.75ex; height:5.843ex;" alt="{\displaystyle |\psi \rangle _{AB}=\sum _{i,j}c_{ij}|i\rangle _{A}\otimes |j\rangle _{B}}"></span>.</dd></dl> <p>This state is separable if there exist vectors <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle [c_{i}^{A}],[c_{j}^{B}]}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mo stretchy="false">[</mo> <msubsup> <mi>c</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msubsup> <mo stretchy="false">]</mo> <mo>,</mo> <mo stretchy="false">[</mo> <msubsup> <mi>c</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>j</mi> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msubsup> <mo stretchy="false">]</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle [c_{i}^{A}],[c_{j}^{B}]}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/a2b47f980dac5bc92f73609aef7a6280f2f2be7f" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -1.338ex; width:8.579ex; height:3.509ex;" alt="{\displaystyle [c_{i}^{A}],[c_{j}^{B}]}"></span> so that <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle c_{ij}=c_{i}^{A}c_{j}^{B},}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msub> <mi>c</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> <mi>j</mi> </mrow> </msub> <mo>=</mo> <msubsup> <mi>c</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msubsup> <msubsup> <mi>c</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>j</mi> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msubsup> <mo>,</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle c_{ij}=c_{i}^{A}c_{j}^{B},}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/febfdcca5d8a1c72ab47af48880d828bb7d60aab" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -1.338ex; width:11.187ex; height:3.509ex;" alt="{\displaystyle c_{ij}=c_{i}^{A}c_{j}^{B},}"></span> yielding <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\textstyle |\psi \rangle _{A}=\sum _{i}c_{i}^{A}|i\rangle _{A}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="false" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mi>ψ<!-- ψ --></mi> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mo>=</mo> <munder> <mo>∑<!-- ∑ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> </munder> <msubsup> <mi>c</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msubsup> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mi>i</mi> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> </mstyle> </mrow> <annotation encoding="application/x-tex">{\textstyle |\psi \rangle _{A}=\sum _{i}c_{i}^{A}|i\rangle _{A}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/7f0bbef833d99d88a2228377927c3b0c01229d78" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -1.005ex; width:17.559ex; height:3.176ex;" alt="{\textstyle |\psi \rangle _{A}=\sum _{i}c_{i}^{A}|i\rangle _{A}}"></span> and <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\textstyle |\phi \rangle _{B}=\sum _{j}c_{j}^{B}|j\rangle _{B}.}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="false" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mi>ϕ<!-- ϕ --></mi> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msub> <mo>=</mo> <munder> <mo>∑<!-- ∑ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>j</mi> </mrow> </munder> <msubsup> <mi>c</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>j</mi> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msubsup> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mi>j</mi> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msub> <mo>.</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\textstyle |\phi \rangle _{B}=\sum _{j}c_{j}^{B}|j\rangle _{B}.}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/d82f19111902fc3a2c36f633c6925a9314f48d72" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -1.338ex; width:18.388ex; height:3.509ex;" alt="{\textstyle |\phi \rangle _{B}=\sum _{j}c_{j}^{B}|j\rangle _{B}.}"></span> It is inseparable if for any vectors <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle [c_{i}^{A}],[c_{j}^{B}]}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mo stretchy="false">[</mo> <msubsup> <mi>c</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msubsup> <mo stretchy="false">]</mo> <mo>,</mo> <mo stretchy="false">[</mo> <msubsup> <mi>c</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>j</mi> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msubsup> <mo stretchy="false">]</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle [c_{i}^{A}],[c_{j}^{B}]}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/a2b47f980dac5bc92f73609aef7a6280f2f2be7f" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -1.338ex; width:8.579ex; height:3.509ex;" alt="{\displaystyle [c_{i}^{A}],[c_{j}^{B}]}"></span> at least for one pair of coordinates <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle c_{i}^{A},c_{j}^{B}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msubsup> <mi>c</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msubsup> <mo>,</mo> <msubsup> <mi>c</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>j</mi> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msubsup> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle c_{i}^{A},c_{j}^{B}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/79bd35827efb95e0fcfebd87e1b5071ecc50709f" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -1.338ex; width:5.992ex; height:3.509ex;" alt="{\displaystyle c_{i}^{A},c_{j}^{B}}"></span> we have <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle c_{ij}\neq c_{i}^{A}c_{j}^{B}.}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msub> <mi>c</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> <mi>j</mi> </mrow> </msub> <mo>≠<!-- ≠ --></mo> <msubsup> <mi>c</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msubsup> <msubsup> <mi>c</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>j</mi> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msubsup> <mo>.</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle c_{ij}\neq c_{i}^{A}c_{j}^{B}.}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/edc74f42a3e75058769623c147eb649b8e284d1c" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -1.338ex; width:11.187ex; height:3.509ex;" alt="{\displaystyle c_{ij}\neq c_{i}^{A}c_{j}^{B}.}"></span> If a state is inseparable, it is called an 'entangled state'. </p><p>For example, given two basis vectors <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \{|0\rangle _{A},|1\rangle _{A}\}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mo fence="false" stretchy="false">{</mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>0</mn> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mo>,</mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>1</mn> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mo fence="false" stretchy="false">}</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \{|0\rangle _{A},|1\rangle _{A}\}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/29ccb462de8b00e179e027c5d0834ac3691bc45b" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:11.716ex; height:2.843ex;" alt="{\displaystyle \{|0\rangle _{A},|1\rangle _{A}\}}"></span> of <span class="texhtml mvar" style="font-style:italic;">H<sub>A</sub></span> and two basis vectors <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \{|0\rangle _{B},|1\rangle _{B}\}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mo fence="false" stretchy="false">{</mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>0</mn> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msub> <mo>,</mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>1</mn> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msub> <mo fence="false" stretchy="false">}</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \{|0\rangle _{B},|1\rangle _{B}\}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/357e2a7e30db6187849c070e3398ff8d031d1178" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:11.746ex; height:2.843ex;" alt="{\displaystyle \{|0\rangle _{B},|1\rangle _{B}\}}"></span> of <span class="texhtml mvar" style="font-style:italic;">H<sub>B</sub></span>, the following is an entangled state: </p> <dl><dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle {\tfrac {1}{\sqrt {2}}}\left(|0\rangle _{A}\otimes |1\rangle _{B}-|1\rangle _{A}\otimes |0\rangle _{B}\right).}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="false" scriptlevel="0"> <mfrac> <mn>1</mn> <msqrt> <mn>2</mn> </msqrt> </mfrac> </mstyle> </mrow> <mrow> <mo>(</mo> <mrow> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>0</mn> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mo>⊗<!-- ⊗ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>1</mn> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msub> <mo>−<!-- − --></mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>1</mn> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mo>⊗<!-- ⊗ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>0</mn> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msub> </mrow> <mo>)</mo> </mrow> <mo>.</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle {\tfrac {1}{\sqrt {2}}}\left(|0\rangle _{A}\otimes |1\rangle _{B}-|1\rangle _{A}\otimes |0\rangle _{B}\right).}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/4053aa0fcea6250c60262618348a170fc18ac03b" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -1.838ex; width:31.523ex; height:4.176ex;" alt="{\displaystyle {\tfrac {1}{\sqrt {2}}}\left(|0\rangle _{A}\otimes |1\rangle _{B}-|1\rangle _{A}\otimes |0\rangle _{B}\right).}"></span></dd></dl> <p>If the composite system is in this state, it is impossible to attribute to either system <span class="texhtml mvar" style="font-style:italic;">A</span> or system <span class="texhtml mvar" style="font-style:italic;">B</span> a definite <a href="/wiki/Pure_state" class="mw-redirect" title="Pure state">pure state</a>. Another way to say this is that while the <a href="/wiki/Von_Neumann_entropy" title="Von Neumann entropy">von Neumann entropy</a> of the whole state is zero (as it is for any pure state), the entropy of the subsystems is greater than zero. In this sense, the systems are "entangled". This has specific empirical ramifications for interferometry.<sup id="cite_ref-69" class="reference"><a href="#cite_note-69"><span class="cite-bracket">[</span>69<span class="cite-bracket">]</span></a></sup> The above example is one of four <a href="/wiki/Bell_states" class="mw-redirect" title="Bell states">Bell states</a>, which are (maximally) entangled pure states (pure states of the <span class="texhtml"><i>H<sub>A</sub></i> ⊗ <i>H<sub>B</sub></i></span> space, but which cannot be separated into pure states of each <span class="texhtml mvar" style="font-style:italic;">H<sub>A</sub></span> and <span class="texhtml mvar" style="font-style:italic;">H<sub>B</sub></span>). </p><p>Now suppose Alice is an observer for system <span class="texhtml mvar" style="font-style:italic;">A</span>, and Bob is an observer for system <span class="texhtml mvar" style="font-style:italic;">B</span>. If in the entangled state given above Alice makes a measurement in the <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \{|0\rangle ,|1\rangle \}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mo fence="false" stretchy="false">{</mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>0</mn> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mo>,</mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>1</mn> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mo fence="false" stretchy="false">}</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \{|0\rangle ,|1\rangle \}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/7a72099dd8347d05e92c98f19db855b1651f8402" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:8.787ex; height:2.843ex;" alt="{\displaystyle \{|0\rangle ,|1\rangle \}}"></span> eigenbasis of <span class="texhtml mvar" style="font-style:italic;">A</span>, there are two possible outcomes, occurring with equal probability:<sup id="cite_ref-70" class="reference"><a href="#cite_note-70"><span class="cite-bracket">[</span>70<span class="cite-bracket">]</span></a></sup><sup class="noprint Inline-Template" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Verifiability" title="Wikipedia:Verifiability"><span title="The material near this tag failed verification of its source citation(s). (November 2024)">failed verification</span></a> – <a href="/wiki/Talk:Quantum_entanglement#Unverified_and_incorrect_content_in_section_Pure_states." title="Talk:Quantum entanglement">see discussion</a></i>]</sup> </p> <ol><li>Alice measures 0, and the state of the system collapses to <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle |0\rangle _{A}|1\rangle _{B}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>0</mn> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>1</mn> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msub> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle |0\rangle _{A}|1\rangle _{B}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/d322139160494277bc5be26f86b7606e841f5c8c" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:8.372ex; height:2.843ex;" alt="{\displaystyle |0\rangle _{A}|1\rangle _{B}}"></span>.</li> <li>Alice measures 1, and the state of the system collapses to <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle |1\rangle _{A}|0\rangle _{B}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>1</mn> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>0</mn> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msub> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle |1\rangle _{A}|0\rangle _{B}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/25496f3649c84d1757b6dce70e36e099b0f18945" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:8.372ex; height:2.843ex;" alt="{\displaystyle |1\rangle _{A}|0\rangle _{B}}"></span>.</li></ol> <p>If the former occurs, then any subsequent measurement performed by Bob, in the same basis, will always return 1. If the latter occurs, (Alice measures 1) then Bob's measurement will return 0 with certainty. Thus, the quantum state that describes system <span class="texhtml mvar" style="font-style:italic;">B</span> has been altered by Alice performing a local measurement on system <span class="texhtml mvar" style="font-style:italic;">A</span>. This remains true even if the systems <span class="texhtml mvar" style="font-style:italic;">A</span> and <span class="texhtml mvar" style="font-style:italic;">B</span> are spatially separated. This is the foundation of the EPR paradox. </p><p>The outcome of Alice's measurement is random. Alice cannot decide which state to collapse the composite system into, and therefore cannot transmit information to Bob by acting on her system. Causality is thus preserved, in this particular scheme. For the general argument, see <a href="/wiki/No-communication_theorem" title="No-communication theorem">no-communication theorem</a>. </p> <div class="mw-heading mw-heading3"><h3 id="Ensembles">Ensembles</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=13" title="Edit section: Ensembles"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>As mentioned above, a state of a quantum system is given by a unit vector in a Hilbert space. More generally, if one has less information about the system, then one calls it an 'ensemble' and describes it by a <a href="/wiki/Density_matrix" title="Density matrix">density matrix</a>, which is a <a href="/wiki/Positive-semidefinite_matrix" class="mw-redirect" title="Positive-semidefinite matrix">positive-semidefinite matrix</a>, or a <a href="/wiki/Trace_class" title="Trace class">trace class</a> when the state space is infinite-dimensional, and has trace 1. Again, by the <a href="/wiki/Spectral_theorem" title="Spectral theorem">spectral theorem</a>, such a matrix takes the general form: </p> <dl><dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \rho =\sum _{i}w_{i}|\alpha _{i}\rangle \langle \alpha _{i}|,}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mi>ρ<!-- ρ --></mi> <mo>=</mo> <munder> <mo>∑<!-- ∑ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> </munder> <msub> <mi>w</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> </msub> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <msub> <mi>α<!-- α --></mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> </msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mo fence="false" stretchy="false">⟨<!-- ⟨ --></mo> <msub> <mi>α<!-- α --></mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> </msub> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mo>,</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \rho =\sum _{i}w_{i}|\alpha _{i}\rangle \langle \alpha _{i}|,}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/6b4708b78f8713fd851ab3f5d8e97d8f85b873c0" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -3.005ex; width:18.831ex; height:5.509ex;" alt="{\displaystyle \rho =\sum _{i}w_{i}|\alpha _{i}\rangle \langle \alpha _{i}|,}"></span></dd></dl> <p>where the <i>w</i><sub><i>i</i></sub> are positive-valued probabilities (they sum up to 1), the vectors <span class="texhtml"><i>α</i><sub><i>i</i></sub></span> are unit vectors, and in the infinite-dimensional case, we would take the closure of such states in the trace norm. We can interpret <span class="texhtml mvar" style="font-style:italic;">ρ</span> as representing an ensemble where <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle w_{i}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msub> <mi>w</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> </msub> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle w_{i}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/fe22f0329d3ecb2e1880d44d191aba0e5475db68" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:2.464ex; height:2.009ex;" alt="{\displaystyle w_{i}}"></span> is the proportion of the ensemble whose states are <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle |\alpha _{i}\rangle }"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <msub> <mi>α<!-- α --></mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> </msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle |\alpha _{i}\rangle }</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/0894b0cce03f69124a81e6f240e13af70918a098" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:3.839ex; height:2.843ex;" alt="{\displaystyle |\alpha _{i}\rangle }"></span>. When a mixed state has rank 1, it therefore describes a 'pure ensemble'. When there is less than total information about the state of a quantum system we need <a href="#Reduced_density_matrices">density matrices</a> to represent the state. </p><p>Experimentally, a mixed ensemble might be realized as follows. Consider a "black box" apparatus that spits <a href="/wiki/Electron" title="Electron">electrons</a> towards an observer. The electrons' Hilbert spaces are <a href="/wiki/Identical_particles" class="mw-redirect" title="Identical particles">identical</a>. The apparatus might produce electrons that are all in the same state; in this case, the electrons received by the observer are then a pure ensemble. However, the apparatus could produce electrons in different states. For example, it could produce two populations of electrons: one with state <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle |\mathbf {z} +\rangle }"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi mathvariant="bold">z</mi> </mrow> <mo>+</mo> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle |\mathbf {z} +\rangle }</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/567f15dd6efff02e103e9637ee7cf4954ff92662" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:4.548ex; height:2.843ex;" alt="{\displaystyle |\mathbf {z} +\rangle }"></span> with spins aligned in the positive <span class="texhtml"><b>z</b></span> direction, and the other with state <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle |\mathbf {y} -\rangle }"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi mathvariant="bold">y</mi> </mrow> <mo>−<!-- − --></mo> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle |\mathbf {y} -\rangle }</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/ee2032e156f9a7256cfb8095258bce23a33e0345" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:4.771ex; height:2.843ex;" alt="{\displaystyle |\mathbf {y} -\rangle }"></span> with spins aligned in the negative <span class="texhtml"><b>y</b></span> direction. Generally, this is a mixed ensemble, as there can be any number of populations, each corresponding to a different state. </p><p>Following the definition above, for a bipartite composite system, mixed states are just density matrices on <span class="texhtml"><i>H<sub>A</sub></i> ⊗ <i>H<sub>B</sub></i></span>. That is, it has the general form </p> <dl><dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \rho =\sum _{i}w_{i}\left[\sum _{j}{\bar {c}}_{ij}(|\alpha _{ij}\rangle \otimes |\beta _{ij}\rangle )\right]\left[\sum _{k}c_{ik}(\langle \alpha _{ik}|\otimes \langle \beta _{ik}|)\right]}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mi>ρ<!-- ρ --></mi> <mo>=</mo> <munder> <mo>∑<!-- ∑ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> </munder> <msub> <mi>w</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> </msub> <mrow> <mo>[</mo> <mrow> <munder> <mo>∑<!-- ∑ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>j</mi> </mrow> </munder> <msub> <mrow class="MJX-TeXAtom-ORD"> <mrow class="MJX-TeXAtom-ORD"> <mover> <mi>c</mi> <mo stretchy="false">¯<!-- ¯ --></mo> </mover> </mrow> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> <mi>j</mi> </mrow> </msub> <mo stretchy="false">(</mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <msub> <mi>α<!-- α --></mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> <mi>j</mi> </mrow> </msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mo>⊗<!-- ⊗ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <msub> <mi>β<!-- β --></mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> <mi>j</mi> </mrow> </msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mo stretchy="false">)</mo> </mrow> <mo>]</mo> </mrow> <mrow> <mo>[</mo> <mrow> <munder> <mo>∑<!-- ∑ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>k</mi> </mrow> </munder> <msub> <mi>c</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> <mi>k</mi> </mrow> </msub> <mo stretchy="false">(</mo> <mo fence="false" stretchy="false">⟨<!-- ⟨ --></mo> <msub> <mi>α<!-- α --></mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> <mi>k</mi> </mrow> </msub> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mo>⊗<!-- ⊗ --></mo> <mo fence="false" stretchy="false">⟨<!-- ⟨ --></mo> <msub> <mi>β<!-- β --></mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> <mi>k</mi> </mrow> </msub> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mo stretchy="false">)</mo> </mrow> <mo>]</mo> </mrow> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \rho =\sum _{i}w_{i}\left[\sum _{j}{\bar {c}}_{ij}(|\alpha _{ij}\rangle \otimes |\beta _{ij}\rangle )\right]\left[\sum _{k}c_{ik}(\langle \alpha _{ik}|\otimes \langle \beta _{ik}|)\right]}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/cbf2f7c56adc4e273d3e3c73ee6a66e190c3f35c" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -3.338ex; width:56.996ex; height:7.676ex;" alt="{\displaystyle \rho =\sum _{i}w_{i}\left[\sum _{j}{\bar {c}}_{ij}(|\alpha _{ij}\rangle \otimes |\beta _{ij}\rangle )\right]\left[\sum _{k}c_{ik}(\langle \alpha _{ik}|\otimes \langle \beta _{ik}|)\right]}"></span></dd></dl> <p>where the <i>w</i><sub><i>i</i></sub> are positively valued probabilities, <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\textstyle \sum _{j}|c_{ij}|^{2}=1}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="false" scriptlevel="0"> <munder> <mo>∑<!-- ∑ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>j</mi> </mrow> </munder> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <msub> <mi>c</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> <mi>j</mi> </mrow> </msub> <msup> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> </msup> <mo>=</mo> <mn>1</mn> </mstyle> </mrow> <annotation encoding="application/x-tex">{\textstyle \sum _{j}|c_{ij}|^{2}=1}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/407c8b79636e8aafbad96aa6aaaeeefd52d73008" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -1.338ex; width:12.843ex; height:3.843ex;" alt="{\textstyle \sum _{j}|c_{ij}|^{2}=1}"></span>, and the vectors are unit vectors. This is self-adjoint and positive and has trace 1. </p><p>Extending the definition of separability from the pure case, we say that a mixed state is separable if it can be written as<sup id="cite_ref-Laloe_71-0" class="reference"><a href="#cite_note-Laloe-71"><span class="cite-bracket">[</span>71<span class="cite-bracket">]</span></a></sup><sup class="reference nowrap"><span title="Page / location: 131–132">: 131–132 </span></sup> </p> <dl><dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \rho =\sum _{i}w_{i}\rho _{i}^{A}\otimes \rho _{i}^{B},}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mi>ρ<!-- ρ --></mi> <mo>=</mo> <munder> <mo>∑<!-- ∑ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> </munder> <msub> <mi>w</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> </msub> <msubsup> <mi>ρ<!-- ρ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msubsup> <mo>⊗<!-- ⊗ --></mo> <msubsup> <mi>ρ<!-- ρ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msubsup> <mo>,</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \rho =\sum _{i}w_{i}\rho _{i}^{A}\otimes \rho _{i}^{B},}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/6597264612d7732f42fd586b145e28e9b04aa329" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -3.005ex; width:19.342ex; height:5.509ex;" alt="{\displaystyle \rho =\sum _{i}w_{i}\rho _{i}^{A}\otimes \rho _{i}^{B},}"></span></dd></dl> <p>where the <span class="texhtml"><i>w</i><sub><i>i</i></sub></span> are positively valued probabilities and the <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \rho _{i}^{A}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msubsup> <mi>ρ<!-- ρ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msubsup> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \rho _{i}^{A}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/60b5bc5b7612e68b45a4ce589c0e4c289df55b34" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -1.005ex; width:2.667ex; height:3.176ex;" alt="{\displaystyle \rho _{i}^{A}}"></span>s and <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \rho _{i}^{B}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msubsup> <mi>ρ<!-- ρ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msubsup> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \rho _{i}^{B}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/475221b5b58c88f20750357c28e1c45b9087f9bf" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -1.005ex; width:2.682ex; height:3.176ex;" alt="{\displaystyle \rho _{i}^{B}}"></span>s are themselves mixed states (density operators) on the subsystems <span class="texhtml mvar" style="font-style:italic;">A</span> and <span class="texhtml mvar" style="font-style:italic;">B</span> respectively. In other words, a state is separable if it is a probability distribution over uncorrelated states, or product states. By writing the density matrices as sums of pure ensembles and expanding, we may assume without loss of generality that <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \rho _{i}^{A}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msubsup> <mi>ρ<!-- ρ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msubsup> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \rho _{i}^{A}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/60b5bc5b7612e68b45a4ce589c0e4c289df55b34" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -1.005ex; width:2.667ex; height:3.176ex;" alt="{\displaystyle \rho _{i}^{A}}"></span> and <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \rho _{i}^{B}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msubsup> <mi>ρ<!-- ρ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msubsup> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \rho _{i}^{B}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/475221b5b58c88f20750357c28e1c45b9087f9bf" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -1.005ex; width:2.682ex; height:3.176ex;" alt="{\displaystyle \rho _{i}^{B}}"></span> are themselves pure ensembles. A state is then said to be entangled if it is not separable. </p><p>In general, finding out whether or not a mixed state is entangled is considered difficult. The general bipartite case has been shown to be <a href="/wiki/NP-hard" class="mw-redirect" title="NP-hard">NP-hard</a>.<sup id="cite_ref-72" class="reference"><a href="#cite_note-72"><span class="cite-bracket">[</span>72<span class="cite-bracket">]</span></a></sup> For the <span class="texhtml">2 × 2</span> and <span class="texhtml">2 × 3</span> cases, a necessary and sufficient criterion for separability is given by the famous <a href="/wiki/Peres-Horodecki_criterion" class="mw-redirect" title="Peres-Horodecki criterion">Positive Partial Transpose (PPT)</a> condition.<sup id="cite_ref-73" class="reference"><a href="#cite_note-73"><span class="cite-bracket">[</span>73<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Reduced_density_matrices">Reduced density matrices</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=14" title="Edit section: Reduced density matrices"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The idea of a reduced density matrix was introduced by <a href="/wiki/Paul_Dirac" title="Paul Dirac">Paul Dirac</a> in 1930.<sup id="cite_ref-Dirac1930_74-0" class="reference"><a href="#cite_note-Dirac1930-74"><span class="cite-bracket">[</span>74<span class="cite-bracket">]</span></a></sup> Consider as above systems <span class="texhtml mvar" style="font-style:italic;">A</span> and <span class="texhtml mvar" style="font-style:italic;">B</span> each with a Hilbert space <span class="texhtml mvar" style="font-style:italic;">H<sub>A</sub>, H<sub>B</sub></span>. Let the state of the composite system be </p> <dl><dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle |\Psi \rangle \in H_{A}\otimes H_{B}.}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mi mathvariant="normal">Ψ<!-- Ψ --></mi> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mo>∈<!-- ∈ --></mo> <msub> <mi>H</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mo>⊗<!-- ⊗ --></mo> <msub> <mi>H</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msub> <mo>.</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle |\Psi \rangle \in H_{A}\otimes H_{B}.}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/b222fc006a0cca79472f2fbb1313f03c2b512490" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:16.494ex; height:2.843ex;" alt="{\displaystyle |\Psi \rangle \in H_{A}\otimes H_{B}.}"></span></dd></dl> <p>As indicated above, in general there is no way to associate a pure state to the component system <span class="texhtml mvar" style="font-style:italic;">A</span>. However, it still is possible to associate a density matrix. Let </p> <dl><dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \rho _{T}=|\Psi \rangle \;\langle \Psi |}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msub> <mi>ρ<!-- ρ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mi>T</mi> </mrow> </msub> <mo>=</mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mi mathvariant="normal">Ψ<!-- Ψ --></mi> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mspace width="thickmathspace" /> <mo fence="false" stretchy="false">⟨<!-- ⟨ --></mo> <mi mathvariant="normal">Ψ<!-- Ψ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \rho _{T}=|\Psi \rangle \;\langle \Psi |}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/354e47cf72ebfb19333d551ac631ca85d9fc2c0b" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:13.054ex; height:2.843ex;" alt="{\displaystyle \rho _{T}=|\Psi \rangle \;\langle \Psi |}"></span>.</dd></dl> <p>which is the <a href="/wiki/Projection_operator" class="mw-redirect" title="Projection operator">projection operator</a> onto this state. The state of <span class="texhtml mvar" style="font-style:italic;">A</span> is the <a href="/wiki/Partial_trace" title="Partial trace">partial trace</a> of <span class="texhtml mvar" style="font-style:italic;">ρ<sub>T</sub></span> over the basis of system <span class="texhtml mvar" style="font-style:italic;">B</span>: </p> <dl><dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \rho _{A}\ {\stackrel {\mathrm {def} }{=}}\ \sum _{j}^{N_{B}}\left(I_{A}\otimes \langle j|_{B}\right)\left(|\Psi \rangle \langle \Psi |\right)\left(I_{A}\otimes |j\rangle _{B}\right)={\hbox{Tr}}_{B}\;\rho _{T}.}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msub> <mi>ρ<!-- ρ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mtext> </mtext> <mrow class="MJX-TeXAtom-ORD"> <mrow class="MJX-TeXAtom-REL"> <mover> <mrow class="MJX-TeXAtom-OP MJX-fixedlimits"> <mo>=</mo> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mrow class="MJX-TeXAtom-ORD"> <mi mathvariant="normal">d</mi> <mi mathvariant="normal">e</mi> <mi mathvariant="normal">f</mi> </mrow> </mrow> </mover> </mrow> </mrow> <mtext> </mtext> <munderover> <mo>∑<!-- ∑ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>j</mi> </mrow> <mrow class="MJX-TeXAtom-ORD"> <msub> <mi>N</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msub> </mrow> </munderover> <mrow> <mo>(</mo> <mrow> <msub> <mi>I</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mo>⊗<!-- ⊗ --></mo> <mo fence="false" stretchy="false">⟨<!-- ⟨ --></mo> <mi>j</mi> <msub> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msub> </mrow> <mo>)</mo> </mrow> <mrow> <mo>(</mo> <mrow> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mi mathvariant="normal">Ψ<!-- Ψ --></mi> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mo fence="false" stretchy="false">⟨<!-- ⟨ --></mo> <mi mathvariant="normal">Ψ<!-- Ψ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> </mrow> <mo>)</mo> </mrow> <mrow> <mo>(</mo> <mrow> <msub> <mi>I</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mo>⊗<!-- ⊗ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mi>j</mi> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msub> </mrow> <mo>)</mo> </mrow> <mo>=</mo> <msub> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="false" scriptlevel="0"> <mtext>Tr</mtext> </mstyle> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msub> <mspace width="thickmathspace" /> <msub> <mi>ρ<!-- ρ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mi>T</mi> </mrow> </msub> <mo>.</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \rho _{A}\ {\stackrel {\mathrm {def} }{=}}\ \sum _{j}^{N_{B}}\left(I_{A}\otimes \langle j|_{B}\right)\left(|\Psi \rangle \langle \Psi |\right)\left(I_{A}\otimes |j\rangle _{B}\right)={\hbox{Tr}}_{B}\;\rho _{T}.}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/e603e36b10b46642f2ab810508663f112c64b376" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -3.338ex; width:52.82ex; height:7.843ex;" alt="{\displaystyle \rho _{A}\ {\stackrel {\mathrm {def} }{=}}\ \sum _{j}^{N_{B}}\left(I_{A}\otimes \langle j|_{B}\right)\left(|\Psi \rangle \langle \Psi |\right)\left(I_{A}\otimes |j\rangle _{B}\right)={\hbox{Tr}}_{B}\;\rho _{T}.}"></span></dd></dl> <p>The sum occurs over <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle N_{B}:=\dim(H_{B})}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msub> <mi>N</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msub> <mo>:=</mo> <mi>dim</mi> <mo>⁡<!-- --></mo> <mo stretchy="false">(</mo> <msub> <mi>H</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msub> <mo stretchy="false">)</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle N_{B}:=\dim(H_{B})}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/cc45ee38d11c298d2c78941259ec8477b0aee5cb" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:16.186ex; height:2.843ex;" alt="{\displaystyle N_{B}:=\dim(H_{B})}"></span> and <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle I_{A}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msub> <mi>I</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle I_{A}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/fad11afeef762471a89c8a258282a40af27d6516" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:2.488ex; height:2.509ex;" alt="{\displaystyle I_{A}}"></span> the identity operator in <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle H_{A}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msub> <mi>H</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle H_{A}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/b3f561a051435337d6fd20d1b3a2cb945fe97f3b" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:3.396ex; height:2.509ex;" alt="{\displaystyle H_{A}}"></span>. <span class="texhtml mvar" style="font-style:italic;">ρ<sub>A</sub></span> is sometimes called the reduced density matrix of <span class="texhtml mvar" style="font-style:italic;">ρ</span> on subsystem <span class="texhtml mvar" style="font-style:italic;">A</span>. Colloquially, we "trace out" system <span class="texhtml mvar" style="font-style:italic;">B</span> to obtain the reduced density matrix on <span class="texhtml mvar" style="font-style:italic;">A</span>. </p><p>For example, the reduced density matrix of <span class="texhtml mvar" style="font-style:italic;">A</span> for the entangled state </p> <dl><dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle {\tfrac {1}{\sqrt {2}}}\left(|0\rangle _{A}\otimes |1\rangle _{B}-|1\rangle _{A}\otimes |0\rangle _{B}\right),}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="false" scriptlevel="0"> <mfrac> <mn>1</mn> <msqrt> <mn>2</mn> </msqrt> </mfrac> </mstyle> </mrow> <mrow> <mo>(</mo> <mrow> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>0</mn> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mo>⊗<!-- ⊗ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>1</mn> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msub> <mo>−<!-- − --></mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>1</mn> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mo>⊗<!-- ⊗ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>0</mn> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msub> </mrow> <mo>)</mo> </mrow> <mo>,</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle {\tfrac {1}{\sqrt {2}}}\left(|0\rangle _{A}\otimes |1\rangle _{B}-|1\rangle _{A}\otimes |0\rangle _{B}\right),}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/0a819774559f3e64569ce18b2c7de993d2066d24" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -1.838ex; width:31.523ex; height:4.176ex;" alt="{\displaystyle {\tfrac {1}{\sqrt {2}}}\left(|0\rangle _{A}\otimes |1\rangle _{B}-|1\rangle _{A}\otimes |0\rangle _{B}\right),}"></span></dd></dl> <p>discussed above is </p> <dl><dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \rho _{A}={\tfrac {1}{2}}\left(|0\rangle _{A}\langle 0|_{A}+|1\rangle _{A}\langle 1|_{A}\right).}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msub> <mi>ρ<!-- ρ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mo>=</mo> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="false" scriptlevel="0"> <mfrac> <mn>1</mn> <mn>2</mn> </mfrac> </mstyle> </mrow> <mrow> <mo>(</mo> <mrow> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>0</mn> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mo fence="false" stretchy="false">⟨<!-- ⟨ --></mo> <mn>0</mn> <msub> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mo>+</mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>1</mn> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mo fence="false" stretchy="false">⟨<!-- ⟨ --></mo> <mn>1</mn> <msub> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> </mrow> <mo>)</mo> </mrow> <mo>.</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \rho _{A}={\tfrac {1}{2}}\left(|0\rangle _{A}\langle 0|_{A}+|1\rangle _{A}\langle 1|_{A}\right).}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/96d96b1e42a9c740ebe571acad39d0844a8473c8" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -1.171ex; width:30.209ex; height:3.509ex;" alt="{\displaystyle \rho _{A}={\tfrac {1}{2}}\left(|0\rangle _{A}\langle 0|_{A}+|1\rangle _{A}\langle 1|_{A}\right).}"></span></dd></dl> <p>This demonstrates that, as expected, the reduced density matrix for an entangled pure ensemble is a mixed ensemble. Also not surprisingly, the density matrix of <span class="texhtml mvar" style="font-style:italic;">A</span> for the pure product state <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle |\psi \rangle _{A}\otimes |\phi \rangle _{B}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mi>ψ<!-- ψ --></mi> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mo>⊗<!-- ⊗ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mi>ϕ<!-- ϕ --></mi> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msub> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle |\psi \rangle _{A}\otimes |\phi \rangle _{B}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/3ff74a24657c12842e203b89a4427f0cfabd067d" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:11.786ex; height:2.843ex;" alt="{\displaystyle |\psi \rangle _{A}\otimes |\phi \rangle _{B}}"></span> discussed above is </p> <dl><dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \rho _{A}=|\psi \rangle _{A}\langle \psi |_{A}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msub> <mi>ρ<!-- ρ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mo>=</mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mi>ψ<!-- ψ --></mi> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mo fence="false" stretchy="false">⟨<!-- ⟨ --></mo> <mi>ψ<!-- ψ --></mi> <msub> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \rho _{A}=|\psi \rangle _{A}\langle \psi |_{A}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/2b672eec721c2dde05973d40e3a730ae07568e54" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:14.824ex; height:2.843ex;" alt="{\displaystyle \rho _{A}=|\psi \rangle _{A}\langle \psi |_{A}}"></span>.</dd></dl> <p>In general, a bipartite pure state <i>ρ</i> is entangled if and only if its reduced states are mixed rather than pure. </p> <div class="mw-heading mw-heading3"><h3 id="Two_applications_that_use_them">Two applications that use them</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=15" title="Edit section: Two applications that use them"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Reduced density matrices were explicitly calculated in different spin chains with unique ground state. An example is the one-dimensional <a href="/wiki/AKLT_Model" class="mw-redirect" title="AKLT Model">AKLT spin chain</a>:<sup id="cite_ref-Fan2004_75-0" class="reference"><a href="#cite_note-Fan2004-75"><span class="cite-bracket">[</span>75<span class="cite-bracket">]</span></a></sup> the ground state can be divided into a block and an environment. The reduced density matrix of the block is <a href="/wiki/Proportionality_(mathematics)" title="Proportionality (mathematics)">proportional</a> to a projector to a degenerate ground state of another Hamiltonian. </p><p>The reduced density matrix also was evaluated for <a href="/wiki/Heisenberg_model_(quantum)" class="mw-redirect" title="Heisenberg model (quantum)">XY spin chains</a>, where it has full rank. It was proved that in the thermodynamic limit, the spectrum of the reduced density matrix of a large block of spins is an exact geometric sequence<sup id="cite_ref-76" class="reference"><a href="#cite_note-76"><span class="cite-bracket">[</span>76<span class="cite-bracket">]</span></a></sup> in this case. </p> <div class="mw-heading mw-heading3"><h3 id="Entanglement_as_a_resource">Entanglement as a resource</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=16" title="Edit section: Entanglement as a resource"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In quantum information theory, entangled states are considered a 'resource', i.e., something costly to produce and that allows implementing valuable transformations.<sup id="cite_ref-Chitambar2019_77-0" class="reference"><a href="#cite_note-Chitambar2019-77"><span class="cite-bracket">[</span>77<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-GG-2022_78-0" class="reference"><a href="#cite_note-GG-2022-78"><span class="cite-bracket">[</span>78<span class="cite-bracket">]</span></a></sup> The setting in which this perspective is most evident is that of "distant labs", i.e., two quantum systems labeled "A" and "B" on each of which arbitrary <a href="/wiki/Quantum_operation" title="Quantum operation">quantum operations</a> can be performed, but which do not interact with each other quantum mechanically. The only interaction allowed is the exchange of classical information, which combined with the most general local quantum operations gives rise to the class of operations called <a href="/wiki/LOCC" title="LOCC">LOCC</a> (local operations and classical communication). These operations do not allow the production of entangled states between systems A and B. But if A and B are provided with a supply of entangled states, then these, together with LOCC operations can enable a larger class of transformations. For example, an interaction between a qubit of A and a qubit of B can be realized by first teleporting A's qubit to B, then letting it interact with B's qubit (which is now a LOCC operation, since both qubits are in B's lab) and then teleporting the qubit back to A. Two maximally entangled states of two qubits are used up in this process. Thus entangled states are a resource that enables the realization of quantum interactions (or of quantum channels) in a setting where only LOCC are available, but they are consumed in the process. There are other applications where entanglement can be seen as a resource, e.g., private communication or distinguishing quantum states.<sup id="cite_ref-horodecki2007_1-3" class="reference"><a href="#cite_note-horodecki2007-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Classification_of_entanglement">Classification of entanglement</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=17" title="Edit section: Classification of entanglement"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Not all quantum states are equally valuable as a resource. To quantify this value, different <a href="#Entanglement_measures">entanglement measures</a> (see below) can be used, that assign a numerical value to each quantum state. However, it is often interesting to settle for a coarser way to compare quantum states. This gives rise to different classification schemes. Most entanglement classes are defined based on whether states can be converted to other states using LOCC or a subclass of these operations. The smaller the set of allowed operations, the finer the classification. Important examples are: </p> <ul><li>If two states can be transformed into each other by a local unitary operation, they are said to be in the same <i>LU class</i>. This is the finest of the usually considered classes. Two states in the same LU class have the same value for entanglement measures and the same value as a resource in the distant-labs setting. There is an infinite number of different LU classes (even in the simplest case of two qubits in a pure state).<sup id="cite_ref-GRB1998_79-0" class="reference"><a href="#cite_note-GRB1998-79"><span class="cite-bracket">[</span>79<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Kraus2010_80-0" class="reference"><a href="#cite_note-Kraus2010-80"><span class="cite-bracket">[</span>80<span class="cite-bracket">]</span></a></sup></li> <li>If two states can be transformed into each other by local operations including measurements with probability larger than 0, they are said to be in the same 'SLOCC class' ("stochastic LOCC"). Qualitatively, two states <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \rho _{1}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msub> <mi>ρ<!-- ρ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mn>1</mn> </mrow> </msub> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \rho _{1}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/1e0f2d347f2a0ed7f7c9808c427a89813b957017" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:2.256ex; height:2.176ex;" alt="{\displaystyle \rho _{1}}"></span> and <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \rho _{2}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msub> <mi>ρ<!-- ρ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> </msub> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \rho _{2}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/793b211571b3ffe34c4639654d567296d29d7f72" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:2.256ex; height:2.176ex;" alt="{\displaystyle \rho _{2}}"></span> in the same SLOCC class are equally powerful (since I can transform one into the other and then do whatever it allows me to do), but since the transformations <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \rho _{1}\to \rho _{2}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msub> <mi>ρ<!-- ρ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mn>1</mn> </mrow> </msub> <mo stretchy="false">→<!-- → --></mo> <msub> <mi>ρ<!-- ρ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> </msub> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \rho _{1}\to \rho _{2}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/942457dace61f7de964e603f92a3f2f16138d687" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:8.126ex; height:2.343ex;" alt="{\displaystyle \rho _{1}\to \rho _{2}}"></span> and <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \rho _{2}\to \rho _{1}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msub> <mi>ρ<!-- ρ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> </msub> <mo stretchy="false">→<!-- → --></mo> <msub> <mi>ρ<!-- ρ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mn>1</mn> </mrow> </msub> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \rho _{2}\to \rho _{1}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/7190e9057b105b0847bbb547e18f29bb647b6996" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:8.126ex; height:2.343ex;" alt="{\displaystyle \rho _{2}\to \rho _{1}}"></span> may succeed with different probability, they are no longer equally valuable. E.g., for two pure qubits there are only two SLOCC classes: the entangled states (which contains both the (maximally entangled) Bell states and weakly entangled states like <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle |00\rangle +0.01|11\rangle }"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>00</mn> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mo>+</mo> <mn>0.01</mn> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>11</mn> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle |00\rangle +0.01|11\rangle }</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/587cf78fb2f0efce5b5bc27af0fa8fe01d95bf25" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:14.727ex; height:2.843ex;" alt="{\displaystyle |00\rangle +0.01|11\rangle }"></span>) and the separable ones (i.e., product states like <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle |00\rangle }"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>00</mn> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle |00\rangle }</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/79016644d7bb5d4282f69bf8af1befa3131445fb" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:3.876ex; height:2.843ex;" alt="{\displaystyle |00\rangle }"></span>).<sup id="cite_ref-81" class="reference"><a href="#cite_note-81"><span class="cite-bracket">[</span>81<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-GoWa2010_82-0" class="reference"><a href="#cite_note-GoWa2010-82"><span class="cite-bracket">[</span>82<span class="cite-bracket">]</span></a></sup></li> <li>Instead of considering transformations of single copies of a state (like <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \rho _{1}\to \rho _{2}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msub> <mi>ρ<!-- ρ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mn>1</mn> </mrow> </msub> <mo stretchy="false">→<!-- → --></mo> <msub> <mi>ρ<!-- ρ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> </msub> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \rho _{1}\to \rho _{2}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/942457dace61f7de964e603f92a3f2f16138d687" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:8.126ex; height:2.343ex;" alt="{\displaystyle \rho _{1}\to \rho _{2}}"></span>) one can define classes based on the possibility of multi-copy transformations. E.g., there are examples when <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \rho _{1}\to \rho _{2}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msub> <mi>ρ<!-- ρ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mn>1</mn> </mrow> </msub> <mo stretchy="false">→<!-- → --></mo> <msub> <mi>ρ<!-- ρ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> </msub> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \rho _{1}\to \rho _{2}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/942457dace61f7de964e603f92a3f2f16138d687" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:8.126ex; height:2.343ex;" alt="{\displaystyle \rho _{1}\to \rho _{2}}"></span> is impossible by LOCC, but <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \rho _{1}\otimes \rho _{1}\to \rho _{2}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msub> <mi>ρ<!-- ρ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mn>1</mn> </mrow> </msub> <mo>⊗<!-- ⊗ --></mo> <msub> <mi>ρ<!-- ρ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mn>1</mn> </mrow> </msub> <mo stretchy="false">→<!-- → --></mo> <msub> <mi>ρ<!-- ρ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> </msub> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \rho _{1}\otimes \rho _{1}\to \rho _{2}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/18f007d3601506321810c4cea20dd31586650876" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:13.223ex; height:2.509ex;" alt="{\displaystyle \rho _{1}\otimes \rho _{1}\to \rho _{2}}"></span> is possible. A very important (and very coarse) classification is based on the property whether it is possible to transform an arbitrarily large number of copies of a state <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \rho }"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mi>ρ<!-- ρ --></mi> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \rho }</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/1f7d439671d1289b6a816e6af7a304be40608d64" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:1.202ex; height:2.176ex;" alt="{\displaystyle \rho }"></span> into at least one pure entangled state. States that have this property are called distillable. These states are the most useful quantum states since, given enough of them, they can be transformed (with local operations) into any entangled state and hence allow for all possible uses. It came initially as a surprise that not all entangled states are distillable, those that are not are called '<a href="/wiki/Bound_entanglement" title="Bound entanglement">bound entangled</a>'.<sup id="cite_ref-HHH97_83-0" class="reference"><a href="#cite_note-HHH97-83"><span class="cite-bracket">[</span>83<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-horodecki2007_1-4" class="reference"><a href="#cite_note-horodecki2007-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup></li></ul> <p>A different entanglement classification is based on what the quantum correlations present in a state allow A and B to do: one distinguishes three subsets of entangled states: (1) the <i><a href="/wiki/Quantum_nonlocality" title="Quantum nonlocality">non-local</a> states</i>, which produce correlations that cannot be explained by a local hidden variable model and thus violate a Bell inequality, (2) the <i><a href="/wiki/Quantum_steering" title="Quantum steering">steerable</a> states</i> that contain sufficient correlations for A to modify ("steer") by local measurements the conditional reduced state of B in such a way, that A can prove to B that the state they possess is indeed entangled, and finally (3) those entangled states that are neither non-local nor steerable. All three sets are non-empty.<sup id="cite_ref-WJD2007_84-0" class="reference"><a href="#cite_note-WJD2007-84"><span class="cite-bracket">[</span>84<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Entropy">Entropy</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=18" title="Edit section: Entropy"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In this section, the entropy of a mixed state is discussed as well as how it can be viewed as a measure of quantum entanglement. </p> <div class="mw-heading mw-heading4"><h4 id="Definition">Definition</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=19" title="Edit section: Definition"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-halign-right" typeof="mw:File/Thumb"><a href="/wiki/File:Von_Neumann_entropy_for_bipartite_system_plot.svg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/3/3e/Von_Neumann_entropy_for_bipartite_system_plot.svg/200px-Von_Neumann_entropy_for_bipartite_system_plot.svg.png" decoding="async" width="200" height="188" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/3/3e/Von_Neumann_entropy_for_bipartite_system_plot.svg/300px-Von_Neumann_entropy_for_bipartite_system_plot.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/3/3e/Von_Neumann_entropy_for_bipartite_system_plot.svg/400px-Von_Neumann_entropy_for_bipartite_system_plot.svg.png 2x" data-file-width="170" data-file-height="160" /></a><figcaption>The plot of von Neumann entropy Vs Eigenvalue for a bipartite 2-level pure state. When the eigenvalue has value 0.5, von Neumann entropy is at a maximum, corresponding to maximum entanglement.</figcaption></figure> <p>In classical <a href="/wiki/Information_theory" title="Information theory">information theory</a> <span class="texhtml mvar" style="font-style:italic;">H</span>, the <a href="/wiki/Shannon_entropy" class="mw-redirect" title="Shannon entropy">Shannon entropy</a>, is associated to a probability distribution, <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle p_{1},\cdots ,p_{n}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msub> <mi>p</mi> <mrow class="MJX-TeXAtom-ORD"> <mn>1</mn> </mrow> </msub> <mo>,</mo> <mo>⋯<!-- ⋯ --></mo> <mo>,</mo> <msub> <mi>p</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>n</mi> </mrow> </msub> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle p_{1},\cdots ,p_{n}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/3711f3a8dc0f30eca8b99ff7fb660d911007ab7a" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; margin-left: -0.089ex; width:9.879ex; height:2.009ex;" alt="{\displaystyle p_{1},\cdots ,p_{n}}"></span>, in the following way:<sup id="cite_ref-SE_85-0" class="reference"><a href="#cite_note-SE-85"><span class="cite-bracket">[</span>85<span class="cite-bracket">]</span></a></sup> </p> <dl><dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle H(p_{1},\cdots ,p_{n})=-\sum _{i}p_{i}\log _{2}p_{i}.}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mi>H</mi> <mo stretchy="false">(</mo> <msub> <mi>p</mi> <mrow class="MJX-TeXAtom-ORD"> <mn>1</mn> </mrow> </msub> <mo>,</mo> <mo>⋯<!-- ⋯ --></mo> <mo>,</mo> <msub> <mi>p</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>n</mi> </mrow> </msub> <mo stretchy="false">)</mo> <mo>=</mo> <mo>−<!-- − --></mo> <munder> <mo>∑<!-- ∑ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> </munder> <msub> <mi>p</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> </msub> <msub> <mi>log</mi> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> </msub> <mo>⁡<!-- --></mo> <msub> <mi>p</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> </msub> <mo>.</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle H(p_{1},\cdots ,p_{n})=-\sum _{i}p_{i}\log _{2}p_{i}.}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/875a53f0d12455b69f2b0ee8230fc98e151bc87a" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -3.005ex; width:32.084ex; height:5.509ex;" alt="{\displaystyle H(p_{1},\cdots ,p_{n})=-\sum _{i}p_{i}\log _{2}p_{i}.}"></span></dd></dl> <p>Since a mixed state <span class="texhtml mvar" style="font-style:italic;">ρ</span> is a probability distribution over an ensemble, this leads naturally to the definition of the <a href="/wiki/Von_Neumann_entropy" title="Von Neumann entropy">von Neumann entropy</a>: </p> <dl><dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle S(\rho )=-{\hbox{Tr}}\left(\rho \log _{2}{\rho }\right).}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mi>S</mi> <mo stretchy="false">(</mo> <mi>ρ<!-- ρ --></mi> <mo stretchy="false">)</mo> <mo>=</mo> <mo>−<!-- − --></mo> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="false" scriptlevel="0"> <mtext>Tr</mtext> </mstyle> </mrow> <mrow> <mo>(</mo> <mrow> <mi>ρ<!-- ρ --></mi> <msub> <mi>log</mi> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> </msub> <mo>⁡<!-- --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>ρ<!-- ρ --></mi> </mrow> </mrow> <mo>)</mo> </mrow> <mo>.</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle S(\rho )=-{\hbox{Tr}}\left(\rho \log _{2}{\rho }\right).}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/58c1e4f3606b03e38fd2c986ec9f25f30f917a83" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:22.441ex; height:2.843ex;" alt="{\displaystyle S(\rho )=-{\hbox{Tr}}\left(\rho \log _{2}{\rho }\right).}"></span></dd></dl> <p>In general, one uses the <a href="/wiki/Borel_functional_calculus" title="Borel functional calculus">Borel functional calculus</a> to calculate a non-polynomial function such as <span class="texhtml">log<sub>2</sub>(<i>ρ</i>)</span>. If the nonnegative operator <span class="texhtml mvar" style="font-style:italic;">ρ</span> acts on a finite-dimensional Hilbert space and has eigenvalues <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \lambda _{1},\cdots ,\lambda _{n}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msub> <mi>λ<!-- λ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mn>1</mn> </mrow> </msub> <mo>,</mo> <mo>⋯<!-- ⋯ --></mo> <mo>,</mo> <msub> <mi>λ<!-- λ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mi>n</mi> </mrow> </msub> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \lambda _{1},\cdots ,\lambda _{n}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/5417ed32af96eccaa187bb395916020ef108ba2f" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:10.161ex; height:2.509ex;" alt="{\displaystyle \lambda _{1},\cdots ,\lambda _{n}}"></span>, <span class="texhtml">log<sub>2</sub>(<i>ρ</i>)</span> turns out to be nothing more than the operator with the same eigenvectors, but the eigenvalues <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \log _{2}(\lambda _{1}),\cdots ,\log _{2}(\lambda _{n})}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msub> <mi>log</mi> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> </msub> <mo>⁡<!-- --></mo> <mo stretchy="false">(</mo> <msub> <mi>λ<!-- λ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mn>1</mn> </mrow> </msub> <mo stretchy="false">)</mo> <mo>,</mo> <mo>⋯<!-- ⋯ --></mo> <mo>,</mo> <msub> <mi>log</mi> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> </msub> <mo>⁡<!-- --></mo> <mo stretchy="false">(</mo> <msub> <mi>λ<!-- λ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mi>n</mi> </mrow> </msub> <mo stretchy="false">)</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \log _{2}(\lambda _{1}),\cdots ,\log _{2}(\lambda _{n})}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/1576682bf3e6ee7a8fe1c9b2e99703c28f7e7649" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:21.832ex; height:2.843ex;" alt="{\displaystyle \log _{2}(\lambda _{1}),\cdots ,\log _{2}(\lambda _{n})}"></span>. The Shannon entropy is then: </p> <dl><dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle S(\rho )=-{\hbox{Tr}}\left(\rho \log _{2}{\rho }\right)=-\sum _{i}\lambda _{i}\log _{2}\lambda _{i}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mi>S</mi> <mo stretchy="false">(</mo> <mi>ρ<!-- ρ --></mi> <mo stretchy="false">)</mo> <mo>=</mo> <mo>−<!-- − --></mo> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="false" scriptlevel="0"> <mtext>Tr</mtext> </mstyle> </mrow> <mrow> <mo>(</mo> <mrow> <mi>ρ<!-- ρ --></mi> <msub> <mi>log</mi> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> </msub> <mo>⁡<!-- --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>ρ<!-- ρ --></mi> </mrow> </mrow> <mo>)</mo> </mrow> <mo>=</mo> <mo>−<!-- − --></mo> <munder> <mo>∑<!-- ∑ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> </munder> <msub> <mi>λ<!-- λ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> </msub> <msub> <mi>log</mi> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> </msub> <mo>⁡<!-- --></mo> <msub> <mi>λ<!-- λ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mi>i</mi> </mrow> </msub> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle S(\rho )=-{\hbox{Tr}}\left(\rho \log _{2}{\rho }\right)=-\sum _{i}\lambda _{i}\log _{2}\lambda _{i}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/928c2d6ade5615fad5fe82e67e17ca9b07640bb4" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -3.005ex; width:39.553ex; height:5.509ex;" alt="{\displaystyle S(\rho )=-{\hbox{Tr}}\left(\rho \log _{2}{\rho }\right)=-\sum _{i}\lambda _{i}\log _{2}\lambda _{i}}"></span>.</dd></dl> <p>Since an event of probability 0 should not contribute to the entropy, and given that </p> <dl><dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \lim _{p\to 0}p\log p=0,}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <munder> <mo movablelimits="true" form="prefix">lim</mo> <mrow class="MJX-TeXAtom-ORD"> <mi>p</mi> <mo stretchy="false">→<!-- → --></mo> <mn>0</mn> </mrow> </munder> <mi>p</mi> <mi>log</mi> <mo>⁡<!-- --></mo> <mi>p</mi> <mo>=</mo> <mn>0</mn> <mo>,</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \lim _{p\to 0}p\log p=0,}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/1e4050c3b4a3183e1f8af387f25474c7e83bd980" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -2.338ex; margin-left: -0.063ex; width:14.735ex; height:4.176ex;" alt="{\displaystyle \lim _{p\to 0}p\log p=0,}"></span></dd></dl> <p>the convention <span class="texhtml">0 log(0) = 0</span> is adopted. This extends to the infinite-dimensional case as well: if <span class="texhtml mvar" style="font-style:italic;">ρ</span> has <a href="/wiki/Projection-valued_measure" title="Projection-valued measure">spectral resolution</a> </p> <dl><dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \rho =\int \lambda dP_{\lambda },}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mi>ρ<!-- ρ --></mi> <mo>=</mo> <mo>∫<!-- ∫ --></mo> <mi>λ<!-- λ --></mi> <mi>d</mi> <msub> <mi>P</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>λ<!-- λ --></mi> </mrow> </msub> <mo>,</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \rho =\int \lambda dP_{\lambda },}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/fe3ed8c6820ba4611ceaa17f37092fa6b1225f76" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -2.338ex; width:12.782ex; height:5.676ex;" alt="{\displaystyle \rho =\int \lambda dP_{\lambda },}"></span></dd></dl> <p>assume the same convention when calculating </p> <dl><dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \rho \log _{2}\rho =\int \lambda \log _{2}\lambda \ dP_{\lambda }.}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mi>ρ<!-- ρ --></mi> <msub> <mi>log</mi> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> </msub> <mo>⁡<!-- --></mo> <mi>ρ<!-- ρ --></mi> <mo>=</mo> <mo>∫<!-- ∫ --></mo> <mi>λ<!-- λ --></mi> <msub> <mi>log</mi> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> </msub> <mo>⁡<!-- --></mo> <mi>λ<!-- λ --></mi> <mtext> </mtext> <mi>d</mi> <msub> <mi>P</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>λ<!-- λ --></mi> </mrow> </msub> <mo>.</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \rho \log _{2}\rho =\int \lambda \log _{2}\lambda \ dP_{\lambda }.}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/e23e352105423f1405a28029e447f58c3ad6a4e5" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -2.338ex; width:25.52ex; height:5.676ex;" alt="{\displaystyle \rho \log _{2}\rho =\int \lambda \log _{2}\lambda \ dP_{\lambda }.}"></span></dd></dl> <p>As in <a href="/wiki/Entropy" title="Entropy">statistical mechanics</a>, the more uncertainty (number of microstates) the system should possess, the larger the entropy. For example, the entropy of any pure state is zero, which is unsurprising since there is no uncertainty about a system in a pure state. The entropy of any of the two subsystems of the entangled state discussed above is <span class="texhtml">log(2)</span> (which can be shown to be the maximum entropy for <span class="texhtml">2 × 2</span> mixed states). </p> <div class="mw-heading mw-heading4"><h4 id="As_a_measure_of_entanglement">As a measure of entanglement</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=20" title="Edit section: As a measure of entanglement"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Entropy provides one tool that can be used to quantify entanglement, although other entanglement measures exist.<sup id="cite_ref-Plenio_86-0" class="reference"><a href="#cite_note-Plenio-86"><span class="cite-bracket">[</span>86<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Vedral2002_87-0" class="reference"><a href="#cite_note-Vedral2002-87"><span class="cite-bracket">[</span>87<span class="cite-bracket">]</span></a></sup> If the overall system is pure, the entropy of one subsystem can be used to measure its degree of entanglement with the other subsystems. For bipartite pure states, the von Neumann entropy of reduced states is the unique measure of entanglement in the sense that it is the only function on the family of states that satisfies certain axioms required of an entanglement measure.<sup id="cite_ref-88" class="reference"><a href="#cite_note-88"><span class="cite-bracket">[</span>88<span class="cite-bracket">]</span></a></sup> </p><p>It is a classical result that the Shannon entropy achieves its maximum at, and only at, the uniform probability distribution {1/<i>n</i>, ..., 1/<i>n</i>}. Therefore, a bipartite pure state <span class="texhtml"><i>ρ</i> ∈ <i>H</i><sub>A</sub> ⊗ <i>H</i><sub>B</sub></span> is said to be a <b>maximally entangled state</b> if the reduced state of each subsystem of <span class="texhtml mvar" style="font-style:italic;">ρ</span> is the diagonal matrix </p> <dl><dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle {\begin{bmatrix}{\frac {1}{n}}&&\\&\ddots &\\&&{\frac {1}{n}}\end{bmatrix}}.}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mrow> <mo>[</mo> <mtable rowspacing="4pt" columnspacing="1em"> <mtr> <mtd> <mrow class="MJX-TeXAtom-ORD"> <mfrac> <mn>1</mn> <mi>n</mi> </mfrac> </mrow> </mtd> <mtd /> <mtd /> </mtr> <mtr> <mtd /> <mtd> <mo>⋱<!-- ⋱ --></mo> </mtd> <mtd /> </mtr> <mtr> <mtd /> <mtd /> <mtd> <mrow class="MJX-TeXAtom-ORD"> <mfrac> <mn>1</mn> <mi>n</mi> </mfrac> </mrow> </mtd> </mtr> </mtable> <mo>]</mo> </mrow> </mrow> <mo>.</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle {\begin{bmatrix}{\frac {1}{n}}&&\\&\ddots &\\&&{\frac {1}{n}}\end{bmatrix}}.}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/0ac2c4a2dbfa47eec57c86a3f4939f6aaf0adc23" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -5.505ex; width:15.768ex; height:12.176ex;" alt="{\displaystyle {\begin{bmatrix}{\frac {1}{n}}&&\\&\ddots &\\&&{\frac {1}{n}}\end{bmatrix}}.}"></span></dd></dl> <p>For mixed states, the reduced von Neumann entropy is not the only reasonable entanglement measure. </p><p>As an aside, the information-theoretic definition is closely related to <a href="/wiki/Entropy_(statistical_views)" class="mw-redirect" title="Entropy (statistical views)">entropy</a> in the sense of statistical mechanics<sup id="cite_ref-89" class="reference"><a href="#cite_note-89"><span class="cite-bracket">[</span>89<span class="cite-bracket">]</span></a></sup> (comparing the two definitions in the present context, it is customary to set the <a href="/wiki/Boltzmann_constant" title="Boltzmann constant">Boltzmann constant</a> <span class="texhtml"><i>k</i> = 1</span>). For example, by properties of the <a href="/wiki/Borel_functional_calculus" title="Borel functional calculus">Borel functional calculus</a>, we see that for any <a href="/wiki/Unitary_operator" title="Unitary operator">unitary operator</a> <span class="texhtml mvar" style="font-style:italic;">U</span>, </p> <dl><dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle S(\rho )=S\left(U\rho U^{*}\right).}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mi>S</mi> <mo stretchy="false">(</mo> <mi>ρ<!-- ρ --></mi> <mo stretchy="false">)</mo> <mo>=</mo> <mi>S</mi> <mrow> <mo>(</mo> <mrow> <mi>U</mi> <mi>ρ<!-- ρ --></mi> <msup> <mi>U</mi> <mrow class="MJX-TeXAtom-ORD"> <mo>∗<!-- ∗ --></mo> </mrow> </msup> </mrow> <mo>)</mo> </mrow> <mo>.</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle S(\rho )=S\left(U\rho U^{*}\right).}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/c5d02f56f32da80d33bbfa1145e42f65612c57e7" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:18.218ex; height:2.843ex;" alt="{\displaystyle S(\rho )=S\left(U\rho U^{*}\right).}"></span></dd></dl> <p>Indeed, without this property, the von Neumann entropy would not be well-defined. </p><p>In particular, <span class="texhtml mvar" style="font-style:italic;">U</span> could be the time evolution operator of the system, i.e., </p> <dl><dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle U(t)=\exp \left({\frac {-iHt}{\hbar }}\right),}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mi>U</mi> <mo stretchy="false">(</mo> <mi>t</mi> <mo stretchy="false">)</mo> <mo>=</mo> <mi>exp</mi> <mo>⁡<!-- --></mo> <mrow> <mo>(</mo> <mrow class="MJX-TeXAtom-ORD"> <mfrac> <mrow> <mo>−<!-- − --></mo> <mi>i</mi> <mi>H</mi> <mi>t</mi> </mrow> <mi class="MJX-variant">ℏ<!-- ℏ --></mi> </mfrac> </mrow> <mo>)</mo> </mrow> <mo>,</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle U(t)=\exp \left({\frac {-iHt}{\hbar }}\right),}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/17432addf55e212c0242d9c0ca3cba98e3c95894" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -2.505ex; width:21.5ex; height:6.176ex;" alt="{\displaystyle U(t)=\exp \left({\frac {-iHt}{\hbar }}\right),}"></span></dd></dl> <p>where <span class="texhtml mvar" style="font-style:italic;">H</span> is the <a href="/wiki/Hamiltonian_(quantum_mechanics)" title="Hamiltonian (quantum mechanics)">Hamiltonian</a> of the system. Here the entropy is unchanged. </p><p><a href="/wiki/R%C3%A9nyi_entropy" title="Rényi entropy">Rényi entropy</a> also can be used as a measure of entanglement.<sup id="cite_ref-90" class="reference"><a href="#cite_note-90"><span class="cite-bracket">[</span>90<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Entanglement_measures">Entanglement measures</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=21" title="Edit section: Entanglement measures"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Entanglement measures quantify the amount of entanglement in a (often viewed as a bipartite) quantum state. As aforementioned, <a href="/wiki/Entropy_of_entanglement" title="Entropy of entanglement">entanglement entropy</a> is the standard measure of entanglement for pure states (but no longer a measure of entanglement for mixed states). For mixed states, there are some entanglement measures in the literature<sup id="cite_ref-Plenio_86-1" class="reference"><a href="#cite_note-Plenio-86"><span class="cite-bracket">[</span>86<span class="cite-bracket">]</span></a></sup> and no single one is standard. </p> <ul><li>Entanglement cost</li> <li><a href="/wiki/Entanglement_distillation" title="Entanglement distillation">Distillable entanglement</a></li> <li><a href="/wiki/Entanglement_of_formation" title="Entanglement of formation">Entanglement of formation</a></li> <li><a href="/wiki/Concurrence_(quantum_computing)" title="Concurrence (quantum computing)">Concurrence</a></li> <li><a href="/wiki/Quantum_relative_entropy" title="Quantum relative entropy">Relative entropy of entanglement</a></li> <li><a href="/wiki/Squashed_entanglement" title="Squashed entanglement">Squashed entanglement</a></li> <li><a href="/wiki/Negativity_(quantum_mechanics)#Logarithmic_negativity" title="Negativity (quantum mechanics)">Logarithmic negativity</a></li></ul> <p>Most (but not all) of these entanglement measures reduce for pure states to entanglement entropy, and are difficult (<a href="/wiki/NP-hard" class="mw-redirect" title="NP-hard">NP-hard</a>) to compute for mixed states as the dimension of the entangled system grows.<sup id="cite_ref-91" class="reference"><a href="#cite_note-91"><span class="cite-bracket">[</span>91<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Quantum_field_theory">Quantum field theory</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=22" title="Edit section: Quantum field theory"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The <a href="/wiki/Reeh%E2%80%93Schlieder_theorem" title="Reeh–Schlieder theorem">Reeh–Schlieder theorem</a> of <a href="/wiki/Quantum_field_theory" title="Quantum field theory">quantum field theory</a> is sometimes seen as an analogue of quantum entanglement. </p> <div class="mw-heading mw-heading2"><h2 id="Applications">Applications</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=23" title="Edit section: Applications"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Entanglement has many applications in <a href="/wiki/Quantum_information_theory" class="mw-redirect" title="Quantum information theory">quantum information theory</a>. With the aid of entanglement, otherwise impossible tasks may be achieved. </p><p>Among the best-known applications of entanglement are <a href="/wiki/Superdense_coding" title="Superdense coding">superdense coding</a> and quantum teleportation.<sup id="cite_ref-92" class="reference"><a href="#cite_note-92"><span class="cite-bracket">[</span>92<span class="cite-bracket">]</span></a></sup> </p><p>Most researchers believe that entanglement is necessary to realize <a href="/wiki/Quantum_computer" class="mw-redirect" title="Quantum computer">quantum computing</a> (although this is disputed by some).<sup id="cite_ref-jozsa02_93-0" class="reference"><a href="#cite_note-jozsa02-93"><span class="cite-bracket">[</span>93<span class="cite-bracket">]</span></a></sup> </p><p>Entanglement is used in some protocols of <a href="/wiki/Quantum_cryptography" title="Quantum cryptography">quantum cryptography</a>,<sup id="cite_ref-ekert91_94-0" class="reference"><a href="#cite_note-ekert91-94"><span class="cite-bracket">[</span>94<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-horodecki10_95-0" class="reference"><a href="#cite_note-horodecki10-95"><span class="cite-bracket">[</span>95<span class="cite-bracket">]</span></a></sup> but to prove the security of <a href="/wiki/Quantum_key_distribution" title="Quantum key distribution">quantum key distribution</a> (QKD) under standard assumptions does not require entanglement.<sup id="cite_ref-96" class="reference"><a href="#cite_note-96"><span class="cite-bracket">[</span>96<span class="cite-bracket">]</span></a></sup> However, the <i><a href="/wiki/Device-independent_quantum_cryptography" title="Device-independent quantum cryptography">device independent</a></i> security of QKD is shown exploiting entanglement between the communication partners.<sup id="cite_ref-97" class="reference"><a href="#cite_note-97"><span class="cite-bracket">[</span>97<span class="cite-bracket">]</span></a></sup> </p><p>In August 2014, Brazilian researcher Gabriela Barreto Lemos, from the University of Vienna, and team were able to "take pictures" of objects using photons that had not interacted with the subjects, but were entangled with photons that did interact with such objects.<sup id="cite_ref-98" class="reference"><a href="#cite_note-98"><span class="cite-bracket">[</span>98<span class="cite-bracket">]</span></a></sup> The idea has been adapted to make infrared images using only standard cameras that are insensitive to infrared.<sup id="cite_ref-99" class="reference"><a href="#cite_note-99"><span class="cite-bracket">[</span>99<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Entangled_states">Entangled states</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=24" title="Edit section: Entangled states"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>There are several canonical entangled states that appear often in theory and experiments. </p><p>For two <a href="/wiki/Qubits" class="mw-redirect" title="Qubits">qubits</a>, the <a href="/wiki/Bell_state" title="Bell state">Bell states</a> are </p> <dl><dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle |\Phi ^{\pm }\rangle ={\frac {1}{\sqrt {2}}}(|0\rangle _{A}\otimes |0\rangle _{B}\pm |1\rangle _{A}\otimes |1\rangle _{B})}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <msup> <mi mathvariant="normal">Φ<!-- Φ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mo>±<!-- ± --></mo> </mrow> </msup> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mo>=</mo> <mrow class="MJX-TeXAtom-ORD"> <mfrac> <mn>1</mn> <msqrt> <mn>2</mn> </msqrt> </mfrac> </mrow> <mo stretchy="false">(</mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>0</mn> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mo>⊗<!-- ⊗ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>0</mn> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msub> <mo>±<!-- ± --></mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>1</mn> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mo>⊗<!-- ⊗ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>1</mn> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msub> <mo stretchy="false">)</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle |\Phi ^{\pm }\rangle ={\frac {1}{\sqrt {2}}}(|0\rangle _{A}\otimes |0\rangle _{B}\pm |1\rangle _{A}\otimes |1\rangle _{B})}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/0c5e3e97f9337b2928160879b1039b554fa4efd2" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -2.838ex; width:38.848ex; height:6.176ex;" alt="{\displaystyle |\Phi ^{\pm }\rangle ={\frac {1}{\sqrt {2}}}(|0\rangle _{A}\otimes |0\rangle _{B}\pm |1\rangle _{A}\otimes |1\rangle _{B})}"></span></dd> <dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle |\Psi ^{\pm }\rangle ={\frac {1}{\sqrt {2}}}(|0\rangle _{A}\otimes |1\rangle _{B}\pm |1\rangle _{A}\otimes |0\rangle _{B}).}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <msup> <mi mathvariant="normal">Ψ<!-- Ψ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mo>±<!-- ± --></mo> </mrow> </msup> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mo>=</mo> <mrow class="MJX-TeXAtom-ORD"> <mfrac> <mn>1</mn> <msqrt> <mn>2</mn> </msqrt> </mfrac> </mrow> <mo stretchy="false">(</mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>0</mn> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mo>⊗<!-- ⊗ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>1</mn> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msub> <mo>±<!-- ± --></mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>1</mn> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>A</mi> </mrow> </msub> <mo>⊗<!-- ⊗ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>0</mn> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>B</mi> </mrow> </msub> <mo stretchy="false">)</mo> <mo>.</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle |\Psi ^{\pm }\rangle ={\frac {1}{\sqrt {2}}}(|0\rangle _{A}\otimes |1\rangle _{B}\pm |1\rangle _{A}\otimes |0\rangle _{B}).}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/c3ad61f595b438e703d3fdf2ff88bbe5946b7e3b" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -2.838ex; width:39.625ex; height:6.176ex;" alt="{\displaystyle |\Psi ^{\pm }\rangle ={\frac {1}{\sqrt {2}}}(|0\rangle _{A}\otimes |1\rangle _{B}\pm |1\rangle _{A}\otimes |0\rangle _{B}).}"></span></dd></dl> <p>These four pure states are all maximally entangled (according to the <a href="/wiki/Entropy_of_entanglement" title="Entropy of entanglement">entropy of entanglement</a>) and form an <a href="/wiki/Orthonormal" class="mw-redirect" title="Orthonormal">orthonormal</a> <a href="/wiki/Basis_(linear_algebra)" title="Basis (linear algebra)">basis (linear algebra)</a> of the Hilbert space of the two qubits. They play a fundamental role in <a href="/wiki/Bell%27s_theorem" title="Bell's theorem">Bell's theorem</a>. </p><p>For <span class="nowrap"><i>M</i> > 2</span> qubits, the <a href="/wiki/Greenberger%E2%80%93Horne%E2%80%93Zeilinger_state" title="Greenberger–Horne–Zeilinger state">GHZ state</a> is </p> <dl><dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle |\mathrm {GHZ} \rangle ={\frac {|0\rangle ^{\otimes M}+|1\rangle ^{\otimes M}}{\sqrt {2}}},}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi mathvariant="normal">G</mi> <mi mathvariant="normal">H</mi> <mi mathvariant="normal">Z</mi> </mrow> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mo>=</mo> <mrow class="MJX-TeXAtom-ORD"> <mfrac> <mrow> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>0</mn> <msup> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mo>⊗<!-- ⊗ --></mo> <mi>M</mi> </mrow> </msup> <mo>+</mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>1</mn> <msup> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mo>⊗<!-- ⊗ --></mo> <mi>M</mi> </mrow> </msup> </mrow> <msqrt> <mn>2</mn> </msqrt> </mfrac> </mrow> <mo>,</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle |\mathrm {GHZ} \rangle ={\frac {|0\rangle ^{\otimes M}+|1\rangle ^{\otimes M}}{\sqrt {2}}},}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/e51e220f2b023d20c1e3d4c7461928e4bbafc1bf" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -2.838ex; width:25.864ex; height:6.843ex;" alt="{\displaystyle |\mathrm {GHZ} \rangle ={\frac {|0\rangle ^{\otimes M}+|1\rangle ^{\otimes M}}{\sqrt {2}}},}"></span></dd></dl> <p>which reduces to the Bell state <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle |\Phi ^{+}\rangle }"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <msup> <mi mathvariant="normal">Φ<!-- Φ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mo>+</mo> </mrow> </msup> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle |\Phi ^{+}\rangle }</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/4570afa1cbf91ef240b83f9ad1476558042f4361" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:4.74ex; height:3.009ex;" alt="{\displaystyle |\Phi ^{+}\rangle }"></span> for <span class="nowrap"><i>M</i> = 2</span>. The traditional GHZ state was defined for <span class="nowrap"><i>M</i> = 3</span>. GHZ states are occasionally extended to <a href="/wiki/Qudit" class="mw-redirect" title="Qudit">qudits</a>, i.e., systems of <i>d</i> rather than 2 dimensions. </p><p>Also for <span class="nowrap"><i>M</i> > 2</span> qubits, there are <a href="/wiki/Spin_squeezing" title="Spin squeezing">spin squeezed states</a>, a class of <a href="/wiki/Squeezed_coherent_states" class="mw-redirect" title="Squeezed coherent states">squeezed coherent states</a> satisfying certain restrictions on the uncertainty of spin measurements, which are necessarily entangled.<sup id="cite_ref-100" class="reference"><a href="#cite_note-100"><span class="cite-bracket">[</span>100<span class="cite-bracket">]</span></a></sup> Spin squeezed states are good candidates for enhancing precision measurements using quantum entanglement.<sup id="cite_ref-101" class="reference"><a href="#cite_note-101"><span class="cite-bracket">[</span>101<span class="cite-bracket">]</span></a></sup> </p><p>For two <a href="/wiki/Boson" title="Boson">bosonic</a> modes, a <a href="/wiki/NOON_state" title="NOON state">NOON state</a> is </p> <dl><dd><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle |\psi _{\text{NOON}}\rangle ={\frac {|N\rangle _{a}|0\rangle _{b}+|{0}\rangle _{a}|{N}\rangle _{b}}{\sqrt {2}}},}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <msub> <mi>ψ<!-- ψ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mtext>NOON</mtext> </mrow> </msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mo>=</mo> <mrow class="MJX-TeXAtom-ORD"> <mfrac> <mrow> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mi>N</mi> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>a</mi> </mrow> </msub> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mn>0</mn> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>b</mi> </mrow> </msub> <mo>+</mo> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mn>0</mn> </mrow> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>a</mi> </mrow> </msub> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mi>N</mi> </mrow> <msub> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>b</mi> </mrow> </msub> </mrow> <msqrt> <mn>2</mn> </msqrt> </mfrac> </mrow> <mo>,</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle |\psi _{\text{NOON}}\rangle ={\frac {|N\rangle _{a}|0\rangle _{b}+|{0}\rangle _{a}|{N}\rangle _{b}}{\sqrt {2}}},}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/350d83720810b698c62488cca1fd6852bde4076d" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -2.838ex; width:32.478ex; height:6.676ex;" alt="{\displaystyle |\psi _{\text{NOON}}\rangle ={\frac {|N\rangle _{a}|0\rangle _{b}+|{0}\rangle _{a}|{N}\rangle _{b}}{\sqrt {2}}},}"></span></dd></dl> <p>This is like the Bell state <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle |\Psi ^{+}\rangle }"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">|</mo> </mrow> <msup> <mi mathvariant="normal">Ψ<!-- Ψ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mo>+</mo> </mrow> </msup> <mo fence="false" stretchy="false">⟩<!-- ⟩ --></mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle |\Psi ^{+}\rangle }</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/3fb516a603ad0d72c91c8e3e736aaad102d073f2" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:4.87ex; height:3.009ex;" alt="{\displaystyle |\Psi ^{+}\rangle }"></span> except the basis kets 0 and 1 have been replaced with "the <i>N</i> photons are in one mode" and "the <i>N</i> photons are in the other mode". </p><p>Finally, there also exist <a href="/w/index.php?title=Twin_Fock_states&action=edit&redlink=1" class="new" title="Twin Fock states (page does not exist)">twin Fock states</a> for bosonic modes, which can be created by feeding a <a href="/wiki/Fock_state" title="Fock state">Fock state</a> into two arms leading to a beam splitter. They are the sum of multiple of NOON states, and can be used to achieve the Heisenberg limit.<sup id="cite_ref-102" class="reference"><a href="#cite_note-102"><span class="cite-bracket">[</span>102<span class="cite-bracket">]</span></a></sup> </p><p>For the appropriately chosen measures of entanglement, Bell, GHZ, and NOON states are maximally entangled while spin squeezed and twin Fock states are only partially entangled. The partially entangled states are generally easier to prepare experimentally. </p> <div class="mw-heading mw-heading3"><h3 id="Methods_of_creating_entanglement">Methods of creating entanglement</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=25" title="Edit section: Methods of creating entanglement"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Entanglement is usually created by direct interactions between subatomic particles. These interactions can take numerous forms. One of the most commonly used methods is <a href="/wiki/Spontaneous_parametric_down-conversion" title="Spontaneous parametric down-conversion">spontaneous parametric down-conversion</a> to generate a pair of photons entangled in polarization.<sup id="cite_ref-horodecki2007_1-5" class="reference"><a href="#cite_note-horodecki2007-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Shadbolt2012_103-0" class="reference"><a href="#cite_note-Shadbolt2012-103"><span class="cite-bracket">[</span>103<span class="cite-bracket">]</span></a></sup> Other methods include the use of a <a href="/wiki/Fiber_coupler" class="mw-redirect" title="Fiber coupler">fiber coupler</a> to confine and mix photons, photons emitted from decay cascade of the bi-exciton in a <a href="/wiki/Quantum_dot" title="Quantum dot">quantum dot</a>,<sup id="cite_ref-104" class="reference"><a href="#cite_note-104"><span class="cite-bracket">[</span>104<span class="cite-bracket">]</span></a></sup> or the use of the <a href="/wiki/Hong%E2%80%93Ou%E2%80%93Mandel_effect" title="Hong–Ou–Mandel effect">Hong–Ou–Mandel effect</a>. Quantum entanglement of a <a href="/wiki/Elementary_particle" title="Elementary particle">particle</a> and its <a href="/wiki/Antiparticle" title="Antiparticle">antiparticle</a>, such as an electron and a <a href="/wiki/Positron" title="Positron">positron</a>, can be created by partial overlap of the corresponding <a href="/wiki/Quantum_wave_function" class="mw-redirect" title="Quantum wave function">quantum wave functions</a> in <a href="/wiki/Hardy%27s_paradox" title="Hardy's paradox">Hardy's interferometer</a>.<sup id="cite_ref-Hardy1992_105-0" class="reference"><a href="#cite_note-Hardy1992-105"><span class="cite-bracket">[</span>105<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Georgiev2022_106-0" class="reference"><a href="#cite_note-Georgiev2022-106"><span class="cite-bracket">[</span>106<span class="cite-bracket">]</span></a></sup> In the earliest tests of Bell's theorem, the entangled particles were generated using <a href="/wiki/Atomic_cascade" class="mw-redirect" title="Atomic cascade">atomic cascades</a>.<sup id="cite_ref-Clauser_32-1" class="reference"><a href="#cite_note-Clauser-32"><span class="cite-bracket">[</span>32<span class="cite-bracket">]</span></a></sup> </p><p>It is also possible to create entanglement between quantum systems that never directly interacted, through the use of <a href="/wiki/Quantum_teleportation#Entanglement_swapping" title="Quantum teleportation">entanglement swapping</a>. Two independently prepared, identical particles may also be entangled if their wave functions merely spatially overlap, at least partially.<sup id="cite_ref-107" class="reference"><a href="#cite_note-107"><span class="cite-bracket">[</span>107<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Testing_a_system_for_entanglement">Testing a system for entanglement</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=26" title="Edit section: Testing a system for entanglement"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>A density matrix <i>ρ</i> is called separable if it can be written as a convex sum of product states, namely <span class="mwe-math-element"><span class="mwe-math-mathml-display mwe-math-mathml-a11y" style="display: none;"><math display="block" xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle {\rho =\sum _{j}p_{j}\rho _{j}^{(A)}\otimes \rho _{j}^{(B)}}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mi>ρ<!-- ρ --></mi> <mo>=</mo> <munder> <mo>∑<!-- ∑ --></mo> <mrow class="MJX-TeXAtom-ORD"> <mi>j</mi> </mrow> </munder> <msub> <mi>p</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>j</mi> </mrow> </msub> <msubsup> <mi>ρ<!-- ρ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mi>j</mi> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">(</mo> <mi>A</mi> <mo stretchy="false">)</mo> </mrow> </msubsup> <mo>⊗<!-- ⊗ --></mo> <msubsup> <mi>ρ<!-- ρ --></mi> <mrow class="MJX-TeXAtom-ORD"> <mi>j</mi> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mo stretchy="false">(</mo> <mi>B</mi> <mo stretchy="false">)</mo> </mrow> </msubsup> </mrow> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle {\rho =\sum _{j}p_{j}\rho _{j}^{(A)}\otimes \rho _{j}^{(B)}}}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/595cfaf3cbe5191d7b447eecd2003515a410ae2a" class="mwe-math-fallback-image-display mw-invert skin-invert" aria-hidden="true" style="vertical-align: -3.338ex; width:20.869ex; height:6.009ex;" alt="{\displaystyle {\rho =\sum _{j}p_{j}\rho _{j}^{(A)}\otimes \rho _{j}^{(B)}}}"></span> with <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle 0\leq p_{j}\leq 1}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mn>0</mn> <mo>≤<!-- ≤ --></mo> <msub> <mi>p</mi> <mrow class="MJX-TeXAtom-ORD"> <mi>j</mi> </mrow> </msub> <mo>≤<!-- ≤ --></mo> <mn>1</mn> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle 0\leq p_{j}\leq 1}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/be98598a3b3c77ddc979cf9a2617effaa0f62c47" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -1.005ex; width:10.601ex; height:2.843ex;" alt="{\displaystyle 0\leq p_{j}\leq 1}"></span> probabilities. By definition, a state is entangled if it is not separable. </p><p>For 2-qubit and qubit-qutrit systems (2 × 2 and 2 × 3 respectively) the simple <a href="/wiki/Peres%E2%80%93Horodecki_criterion" title="Peres–Horodecki criterion">Peres–Horodecki criterion</a> provides both a necessary and a sufficient criterion for separability, and thus—inadvertently—for detecting entanglement. However, for the general case, the criterion is merely a necessary one for separability, as the problem becomes NP-hard when generalized.<sup id="cite_ref-NP-hard1_108-0" class="reference"><a href="#cite_note-NP-hard1-108"><span class="cite-bracket">[</span>108<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-NP-hard2_109-0" class="reference"><a href="#cite_note-NP-hard2-109"><span class="cite-bracket">[</span>109<span class="cite-bracket">]</span></a></sup> Other separability criteria include (but not limited to) the <a href="/wiki/Range_criterion" title="Range criterion">range criterion</a>, <a href="/wiki/Reduction_criterion" title="Reduction criterion">reduction criterion</a>, and those based on uncertainty relations.<sup id="cite_ref-110" class="reference"><a href="#cite_note-110"><span class="cite-bracket">[</span>110<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-111" class="reference"><a href="#cite_note-111"><span class="cite-bracket">[</span>111<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-112" class="reference"><a href="#cite_note-112"><span class="cite-bracket">[</span>112<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-113" class="reference"><a href="#cite_note-113"><span class="cite-bracket">[</span>113<span class="cite-bracket">]</span></a></sup> See Ref.<sup id="cite_ref-114" class="reference"><a href="#cite_note-114"><span class="cite-bracket">[</span>114<span class="cite-bracket">]</span></a></sup> for a review of separability criteria in discrete-variable systems and Ref.<sup id="cite_ref-FriisEtAl2019entanglement_115-0" class="reference"><a href="#cite_note-FriisEtAl2019entanglement-115"><span class="cite-bracket">[</span>115<span class="cite-bracket">]</span></a></sup> for a review on techniques and challenges in experimental entanglement certification in discrete-variable systems. </p><p>A numerical approach to the problem is suggested by <a href="/wiki/Jon_Magne_Leinaas" title="Jon Magne Leinaas">Jon Magne Leinaas</a>, <a href="/wiki/Jan_Myrheim" title="Jan Myrheim">Jan Myrheim</a> and <a href="/w/index.php?title=Eirik_Ovrum&action=edit&redlink=1" class="new" title="Eirik Ovrum (page does not exist)">Eirik Ovrum</a> in their paper "Geometrical aspects of entanglement".<sup id="cite_ref-116" class="reference"><a href="#cite_note-116"><span class="cite-bracket">[</span>116<span class="cite-bracket">]</span></a></sup> Leinaas et al. offer a numerical approach, iteratively refining an estimated separable state towards the target state to be tested, and checking if the target state can indeed be reached. An implementation of the algorithm (including a built-in <a href="/wiki/Peres%E2%80%93Horodecki_criterion" title="Peres–Horodecki criterion">Peres–Horodecki criterion</a> testing) is "StateSeparator" web-app. </p><p>In continuous variable systems, the Peres–Horodecki criterion also applies. Specifically, Simon<sup id="cite_ref-117" class="reference"><a href="#cite_note-117"><span class="cite-bracket">[</span>117<span class="cite-bracket">]</span></a></sup> formulated a particular version of the Peres–Horodecki criterion in terms of the second-order moments of canonical operators and showed that it is necessary and sufficient for <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle 1\oplus 1}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mn>1</mn> <mo>⊕<!-- ⊕ --></mo> <mn>1</mn> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle 1\oplus 1}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/910ec7d56dcdfb30a81400532fa3399299261342" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.505ex; width:5.165ex; height:2.343ex;" alt="{\displaystyle 1\oplus 1}"></span>-mode Gaussian states (see Ref.<sup id="cite_ref-118" class="reference"><a href="#cite_note-118"><span class="cite-bracket">[</span>118<span class="cite-bracket">]</span></a></sup> for a seemingly different but essentially equivalent approach). It was later found<sup id="cite_ref-119" class="reference"><a href="#cite_note-119"><span class="cite-bracket">[</span>119<span class="cite-bracket">]</span></a></sup> that Simon's condition is also necessary and sufficient for <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle 1\oplus n}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mn>1</mn> <mo>⊕<!-- ⊕ --></mo> <mi>n</mi> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle 1\oplus n}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/49fdecc4a94f6e2bf4ea50e1cebd24d9e346173c" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.505ex; width:5.398ex; height:2.343ex;" alt="{\displaystyle 1\oplus n}"></span>-mode Gaussian states, but no longer sufficient for <span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle 2\oplus 2}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mn>2</mn> <mo>⊕<!-- ⊕ --></mo> <mn>2</mn> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle 2\oplus 2}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/60ea3c2d231414b3e14766f90606b747314d3fd8" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.505ex; width:5.165ex; height:2.343ex;" alt="{\displaystyle 2\oplus 2}"></span>-mode Gaussian states. Simon's condition can be generalized by taking into account the higher order moments of canonical operators<sup id="cite_ref-120" class="reference"><a href="#cite_note-120"><span class="cite-bracket">[</span>120<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-121" class="reference"><a href="#cite_note-121"><span class="cite-bracket">[</span>121<span class="cite-bracket">]</span></a></sup> or by using entropic measures.<sup id="cite_ref-122" class="reference"><a href="#cite_note-122"><span class="cite-bracket">[</span>122<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-123" class="reference"><a href="#cite_note-123"><span class="cite-bracket">[</span>123<span class="cite-bracket">]</span></a></sup> </p><p>On August 16, 2016, the world's first quantum communications satellite was launched from the <a href="/wiki/Jiuquan_Satellite_Launch_Center" title="Jiuquan Satellite Launch Center">Jiuquan Satellite Launch Center</a> in China, the <a href="/wiki/Quantum_Experiments_at_Space_Scale" title="Quantum Experiments at Space Scale">Quantum Experiments at Space Scale</a> (QUESS) mission, nicknamed "<a href="/wiki/Micius" class="mw-redirect" title="Micius">Micius</a>" after the ancient Chinese philosopher. The satellite was intended to demonstrate the feasibility of quantum communication between Earth and space, and test quantum entanglement over unprecedented distances.<sup id="cite_ref-124" class="reference"><a href="#cite_note-124"><span class="cite-bracket">[</span>124<span class="cite-bracket">]</span></a></sup> </p><p>In the 16 June 2017, issue of <i>Science</i>, Yin et al. report setting a new quantum entanglement distance record of 1,203 km, demonstrating the survival of a two-photon pair and a violation of a Bell inequality, reaching a CHSH valuation of <span class="nowrap"><span data-sort-value="7000237000000000000♠"></span>2.37<span style="margin-left:0.3em;margin-right:0.15em;">±</span>0.09</span>, under strict Einstein locality conditions, from the Micius satellite to bases in Lijian, Yunnan and Delingha, Quinhai, increasing the efficiency of transmission over prior fiberoptic experiments by an order of magnitude.<sup id="cite_ref-125" class="reference"><a href="#cite_note-125"><span class="cite-bracket">[</span>125<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-126" class="reference"><a href="#cite_note-126"><span class="cite-bracket">[</span>126<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading4"><h4 id="Entanglement_of_top_quarks">Entanglement of top quarks</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=27" title="Edit section: Entanglement of top quarks"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In 2023 the <a href="/wiki/Large_Hadron_Collider" title="Large Hadron Collider">LHC</a> using techniques from <a href="/wiki/Quantum_tomography" title="Quantum tomography">quantum tomography</a> measured entanglement at the highest energy so far,<sup id="cite_ref-127" class="reference"><a href="#cite_note-127"><span class="cite-bracket">[</span>127<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-128" class="reference"><a href="#cite_note-128"><span class="cite-bracket">[</span>128<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-129" class="reference"><a href="#cite_note-129"><span class="cite-bracket">[</span>129<span class="cite-bracket">]</span></a></sup> a rare intersection between quantum information and high energy physics based on theoretical work first proposed in 2021.<sup id="cite_ref-130" class="reference"><a href="#cite_note-130"><span class="cite-bracket">[</span>130<span class="cite-bracket">]</span></a></sup> The experiment was carried by the <a href="/wiki/ATLAS_experiment" title="ATLAS experiment">ATLAS</a> detector measuring the spin of top-quark pair production and the effect was observed witha more than 5<a href="/wiki/Standard_deviation" title="Standard deviation"><i>σ</i></a> level of significance, the top quark is the heaviest known particle and therefore has a very short lifetime (<span class="nowrap"><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \tau }"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mi>τ<!-- τ --></mi> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \tau }</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/38a7dcde9730ef0853809fefc18d88771f95206c" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:1.202ex; height:1.676ex;" alt="{\displaystyle \tau }"></span> ≈ <span class="nowrap"><span data-sort-value="6975100000000000000♠"></span>10<sup>−25</sup> s</span></span>) being the only quark that decays before undergoing <a href="/wiki/Hadronization" title="Hadronization">hadronization</a> (~ <span class="nowrap"><span data-sort-value="6977099999999999999♠"></span>10<sup>−23</sup> s</span>) and spin decorrelation (~ <span class="nowrap"><span data-sort-value="6979099999999999999♠"></span>10<sup>−21</sup> s</span>), so the spin information is transferred without much loss to the leptonic decays products that will be caught by the detector.<sup id="cite_ref-131" class="reference"><a href="#cite_note-131"><span class="cite-bracket">[</span>131<span class="cite-bracket">]</span></a></sup> The <a href="/wiki/Spin_polarization" title="Spin polarization">spin polarization</a> and correlation of the particles was measured and tested for entanglement with <a href="/wiki/Concurrence_(quantum_computing)" title="Concurrence (quantum computing)">concurrence</a> as well as the <a href="/wiki/Peres%E2%80%93Horodecki_criterion" title="Peres–Horodecki criterion">Peres–Horodecki criterion</a> and subsequently the effect has been confirmed too in the <a href="/wiki/CMS_experiment" class="mw-redirect" title="CMS experiment">CMS</a> detector.<sup id="cite_ref-132" class="reference"><a href="#cite_note-132"><span class="cite-bracket">[</span>132<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-133" class="reference"><a href="#cite_note-133"><span class="cite-bracket">[</span>133<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Entanglement_of_macroscopic_objects">Entanglement of macroscopic objects</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=28" title="Edit section: Entanglement of macroscopic objects"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In 2020, researchers reported the quantum entanglement between the <a href="/wiki/Vibrations_of_a_circular_membrane" title="Vibrations of a circular membrane">motion of a millimeter-sized mechanical oscillator</a> and a disparate distant spin system of a cloud of atoms.<sup id="cite_ref-134" class="reference"><a href="#cite_note-134"><span class="cite-bracket">[</span>134<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-135" class="reference"><a href="#cite_note-135"><span class="cite-bracket">[</span>135<span class="cite-bracket">]</span></a></sup> Later work complemented this work by quantum-entangling two mechanical oscillators.<sup id="cite_ref-136" class="reference"><a href="#cite_note-136"><span class="cite-bracket">[</span>136<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-137" class="reference"><a href="#cite_note-137"><span class="cite-bracket">[</span>137<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-138" class="reference"><a href="#cite_note-138"><span class="cite-bracket">[</span>138<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Entanglement_of_elements_of_living_systems">Entanglement of elements of living systems</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=29" title="Edit section: Entanglement of elements of living systems"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In October 2018, physicists reported producing quantum entanglement using <a href="/wiki/Living_organism" class="mw-redirect" title="Living organism">living organisms</a>, particularly between photosynthetic molecules within living <a href="/wiki/Bacteria" title="Bacteria">bacteria</a> and <a href="/wiki/Photon" title="Photon">quantized light</a>.<sup id="cite_ref-JPC-20181010_139-0" class="reference"><a href="#cite_note-JPC-20181010-139"><span class="cite-bracket">[</span>139<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-SA-20181029_140-0" class="reference"><a href="#cite_note-SA-20181029-140"><span class="cite-bracket">[</span>140<span class="cite-bracket">]</span></a></sup> </p><p>Living organisms (green sulphur bacteria) have been studied as mediators to create quantum entanglement between otherwise non-interacting light modes, showing high entanglement between light and bacterial modes, and to some extent, even entanglement within the bacteria.<sup id="cite_ref-141" class="reference"><a href="#cite_note-141"><span class="cite-bracket">[</span>141<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=30" title="Edit section: See also"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1184024115">.mw-parser-output .div-col{margin-top:0.3em;column-width:30em}.mw-parser-output .div-col-small{font-size:90%}.mw-parser-output .div-col-rules{column-rule:1px solid #aaa}.mw-parser-output .div-col dl,.mw-parser-output .div-col ol,.mw-parser-output .div-col ul{margin-top:0}.mw-parser-output .div-col li,.mw-parser-output .div-col dd{page-break-inside:avoid;break-inside:avoid-column}</style><div class="div-col" style="column-width: 16em;"> <ul><li><a href="/wiki/Bound_entanglement" title="Bound entanglement">Bound entanglement</a></li> <li><a href="/wiki/Concurrence_(quantum_computing)" title="Concurrence (quantum computing)">Concurrence</a></li> <li><a href="/wiki/Controlled_NOT_gate" title="Controlled NOT gate">CNOT gate</a></li> <li><a href="/wiki/Einstein%27s_thought_experiments" title="Einstein's thought experiments">Einstein's thought experiments</a></li> <li><a href="/wiki/Entanglement_distillation" title="Entanglement distillation">Entanglement distillation</a></li> <li><a href="/wiki/Entanglement_witness" title="Entanglement witness">Entanglement witness</a></li> <li><a href="/wiki/ER_%3D_EPR" title="ER = EPR">ER = EPR</a></li> <li><a href="/wiki/Faster-than-light_communication" class="mw-redirect" title="Faster-than-light communication">Faster-than-light communication</a></li> <li><a href="/wiki/Multipartite_entanglement" title="Multipartite entanglement">Multipartite entanglement</a></li> <li><a href="/wiki/Normally_distributed_and_uncorrelated_does_not_imply_independent" class="mw-redirect" title="Normally distributed and uncorrelated does not imply independent">Normally distributed and uncorrelated does not imply independent</a></li> <li><a href="/wiki/Pauli_exclusion_principle" title="Pauli exclusion principle">Pauli exclusion principle</a></li> <li><a href="/wiki/Quantum_coherence" class="mw-redirect" title="Quantum coherence">Quantum coherence</a></li> <li><a href="/wiki/Quantum_computing" title="Quantum computing">Quantum computing</a></li> <li><a href="/wiki/Quantum_discord" title="Quantum discord">Quantum discord</a></li> <li><a href="/wiki/Quantum_entanglement_swapping" title="Quantum entanglement swapping">Quantum entanglement swapping</a></li> <li><a href="/wiki/Quantum_network" title="Quantum network">Quantum network</a></li> <li><a href="/wiki/Quantum_phase_transition" title="Quantum phase transition">Quantum phase transition</a></li> <li><a href="/wiki/Quantum_pseudo-telepathy" title="Quantum pseudo-telepathy">Quantum pseudo-telepathy</a></li> <li><a href="/wiki/Quantum_teleportation" title="Quantum teleportation">Quantum teleportation</a></li> <li><a href="/wiki/Retrocausality" title="Retrocausality">Retrocausality</a></li> <li><a href="/wiki/Separable_state" title="Separable state">Separable state</a></li> <li><a href="/wiki/Spontaneous_parametric_down-conversion" title="Spontaneous parametric down-conversion">Spontaneous parametric down-conversion</a></li> <li><a href="/wiki/Squashed_entanglement" title="Squashed entanglement">Squashed entanglement</a></li> <li><a href="/wiki/Stern%E2%80%93Gerlach_experiment" title="Stern–Gerlach experiment">Stern–Gerlach experiment</a></li> <li><a href="/wiki/John_Clive_Ward" title="John Clive Ward">Ward's probability amplitude</a></li></ul> </div> <style data-mw-deduplicate="TemplateStyles:r1239009302">.mw-parser-output .portalbox{padding:0;margin:0.5em 0;display:table;box-sizing:border-box;max-width:175px;list-style:none}.mw-parser-output .portalborder{border:1px solid var(--border-color-base,#a2a9b1);padding:0.1em;background:var(--background-color-neutral-subtle,#f8f9fa)}.mw-parser-output .portalbox-entry{display:table-row;font-size:85%;line-height:110%;height:1.9em;font-style:italic;font-weight:bold}.mw-parser-output .portalbox-image{display:table-cell;padding:0.2em;vertical-align:middle;text-align:center}.mw-parser-output .portalbox-link{display:table-cell;padding:0.2em 0.2em 0.2em 0.3em;vertical-align:middle}@media(min-width:720px){.mw-parser-output .portalleft{clear:left;float:left;margin:0.5em 1em 0.5em 0}.mw-parser-output .portalright{clear:right;float:right;margin:0.5em 0 0.5em 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.references{font-size:100%;margin-bottom:0;list-style-type:inherit}.mw-parser-output .reflist-columns-2{column-width:30em}.mw-parser-output .reflist-columns-3{column-width:25em}.mw-parser-output .reflist-columns{margin-top:0.3em}.mw-parser-output .reflist-columns ol{margin-top:0}.mw-parser-output .reflist-columns li{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .reflist-upper-alpha{list-style-type:upper-alpha}.mw-parser-output .reflist-upper-roman{list-style-type:upper-roman}.mw-parser-output .reflist-lower-alpha{list-style-type:lower-alpha}.mw-parser-output .reflist-lower-greek{list-style-type:lower-greek}.mw-parser-output .reflist-lower-roman{list-style-type:lower-roman}</style><div class="reflist reflist-columns references-column-width" style="column-width: 30em;"> <ol class="references"> <li id="cite_note-horodecki2007-1"><span class="mw-cite-backlink">^ <a href="#cite_ref-horodecki2007_1-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-horodecki2007_1-1"><sup><i><b>b</b></i></sup></a> <a href="#cite_ref-horodecki2007_1-2"><sup><i><b>c</b></i></sup></a> <a href="#cite_ref-horodecki2007_1-3"><sup><i><b>d</b></i></sup></a> <a href="#cite_ref-horodecki2007_1-4"><sup><i><b>e</b></i></sup></a> <a href="#cite_ref-horodecki2007_1-5"><sup><i><b>f</b></i></sup></a></span> <span class="reference-text"> <style data-mw-deduplicate="TemplateStyles:r1238218222">.mw-parser-output cite.citation{font-style:inherit;word-wrap:break-word}.mw-parser-output .citation q{quotes:"\"""\"""'""'"}.mw-parser-output .citation:target{background-color:rgba(0,127,255,0.133)}.mw-parser-output .id-lock-free.id-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited.id-lock-limited a,.mw-parser-output .id-lock-registration.id-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription.id-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat}body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-free a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-limited a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-registration a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-subscription a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .cs1-ws-icon a{background-size:contain;padding:0 1em 0 0}.mw-parser-output .cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;color:var(--color-error,#d33)}.mw-parser-output .cs1-visible-error{color:var(--color-error,#d33)}.mw-parser-output .cs1-maint{display:none;color:#085;margin-left:0.3em}.mw-parser-output .cs1-kern-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}@media screen{.mw-parser-output .cs1-format{font-size:95%}html.skin-theme-clientpref-night .mw-parser-output .cs1-maint{color:#18911f}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .cs1-maint{color:#18911f}}</style><cite id="CITEREFHorodeckiHorodeckiHorodeckiHorodecki2009" class="citation journal cs1">Horodecki, Ryszard; Horodecki, Pawel; Horodecki, Michal; Horodecki, Karol (2009). "Quantum entanglement". <i>Reviews of Modern Physics</i>. <b>81</b> (2): 865–942. <a href="/wiki/ArXiv_(identifier)" class="mw-redirect" title="ArXiv (identifier)">arXiv</a>:<span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://arxiv.org/abs/quant-ph/0702225">quant-ph/0702225</a></span>. <a href="/wiki/Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/2009RvMP...81..865H">2009RvMP...81..865H</a>. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1103%2FRevModPhys.81.865">10.1103/RevModPhys.81.865</a>. <a href="/wiki/S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:59577352">59577352</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=Reviews+of+Modern+Physics&rft.atitle=Quantum+entanglement&rft.volume=81&rft.issue=2&rft.pages=865-942&rft.date=2009&rft_id=info%3Aarxiv%2Fquant-ph%2F0702225&rft_id=https%3A%2F%2Fapi.semanticscholar.org%2FCorpusID%3A59577352%23id-name%3DS2CID&rft_id=info%3Adoi%2F10.1103%2FRevModPhys.81.865&rft_id=info%3Abibcode%2F2009RvMP...81..865H&rft.aulast=Horodecki&rft.aufirst=Ryszard&rft.au=Horodecki%2C+Pawel&rft.au=Horodecki%2C+Michal&rft.au=Horodecki%2C+Karol&rfr_id=info%3Asid%2Fen.wikipedia.org%3AQuantum+entanglement" class="Z3988"></span></span> </li> <li id="cite_note-Einstein1935-2"><span class="mw-cite-backlink">^ <a href="#cite_ref-Einstein1935_2-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Einstein1935_2-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFEinsteinPodolskyRosen1935" class="citation journal cs1"><a href="/wiki/Albert_Einstein" title="Albert Einstein">Einstein, Albert</a>; <a href="/wiki/Boris_Podolsky" title="Boris Podolsky">Podolsky, Boris</a>; <a href="/wiki/Nathan_Rosen" title="Nathan Rosen">Rosen, Nathan</a> (1935). <a rel="nofollow" class="external text" href="https://doi.org/10.1103%2FPhysRev.47.777">"Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?"</a>. <i>Phys. Rev</i>. <b>47</b> (10): 777–780. <a href="/wiki/Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/1935PhRv...47..777E">1935PhRv...47..777E</a>. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://doi.org/10.1103%2FPhysRev.47.777">10.1103/PhysRev.47.777</a></span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=Phys.+Rev.&rft.atitle=Can+Quantum-Mechanical+Description+of+Physical+Reality+Be+Considered+Complete%3F&rft.volume=47&rft.issue=10&rft.pages=777-780&rft.date=1935&rft_id=info%3Adoi%2F10.1103%2FPhysRev.47.777&rft_id=info%3Abibcode%2F1935PhRv...47..777E&rft.aulast=Einstein&rft.aufirst=Albert&rft.au=Podolsky%2C+Boris&rft.au=Rosen%2C+Nathan&rft_id=https%3A%2F%2Fdoi.org%2F10.1103%252FPhysRev.47.777&rfr_id=info%3Asid%2Fen.wikipedia.org%3AQuantum+entanglement" class="Z3988"></span></span> </li> <li id="cite_note-Schrödinger1935-3"><span class="mw-cite-backlink">^ <a href="#cite_ref-Schrödinger1935_3-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Schrödinger1935_3-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFSchrödinger1935" class="citation journal cs1">Schrödinger, Erwin (1935). "Discussion of probability relations between separated systems". <i>Mathematical Proceedings of the Cambridge Philosophical Society</i>. <b>31</b> (4): 555–563. <a href="/wiki/Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/1935PCPS...31..555S">1935PCPS...31..555S</a>. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1017%2FS0305004100013554">10.1017/S0305004100013554</a>. <a href="/wiki/S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:121278681">121278681</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=Mathematical+Proceedings+of+the+Cambridge+Philosophical+Society&rft.atitle=Discussion+of+probability+relations+between+separated+systems&rft.volume=31&rft.issue=4&rft.pages=555-563&rft.date=1935&rft_id=https%3A%2F%2Fapi.semanticscholar.org%2FCorpusID%3A121278681%23id-name%3DS2CID&rft_id=info%3Adoi%2F10.1017%2FS0305004100013554&rft_id=info%3Abibcode%2F1935PCPS...31..555S&rft.aulast=Schr%C3%B6dinger&rft.aufirst=Erwin&rfr_id=info%3Asid%2Fen.wikipedia.org%3AQuantum+entanglement" class="Z3988"></span></span> </li> <li id="cite_note-Schrödinger1936-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-Schrödinger1936_4-0">^</a></b></span> <span class="reference-text"> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFSchrödinger1936" class="citation journal cs1">Schrödinger, Erwin (1936). "Probability relations between separated systems". <i>Mathematical Proceedings of the Cambridge Philosophical Society</i>. <b>32</b> (3): 446–452. <a href="/wiki/Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/1936PCPS...32..446S">1936PCPS...32..446S</a>. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1017%2FS0305004100019137">10.1017/S0305004100019137</a>. <a href="/wiki/S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:122822435">122822435</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=Mathematical+Proceedings+of+the+Cambridge+Philosophical+Society&rft.atitle=Probability+relations+between+separated+systems&rft.volume=32&rft.issue=3&rft.pages=446-452&rft.date=1936&rft_id=https%3A%2F%2Fapi.semanticscholar.org%2FCorpusID%3A122822435%23id-name%3DS2CID&rft_id=info%3Adoi%2F10.1017%2FS0305004100019137&rft_id=info%3Abibcode%2F1936PCPS...32..446S&rft.aulast=Schr%C3%B6dinger&rft.aufirst=Erwin&rfr_id=info%3Asid%2Fen.wikipedia.org%3AQuantum+entanglement" class="Z3988"></span></span> </li> <li id="cite_note-5"><span class="mw-cite-backlink"><b><a href="#cite_ref-5">^</a></b></span> <span class="reference-text">Physicist John Bell depicts the Einstein camp in this debate in his article entitled "Bertlmann's socks and the nature of reality", p. 143 of <i>Speakable and unspeakable in quantum mechanics</i>: "For EPR that would be an unthinkable 'spooky action at a distance'. To avoid such action at a distance they have to attribute, to the space-time regions in question, real properties in advance of observation, correlated properties, which predetermine the outcomes of these particular observations. Since these real properties, fixed in advance of observation, are not contained in quantum formalism, that formalism for EPR is incomplete. It may be correct, as far as it goes, but the usual quantum formalism cannot be the whole story." And again on p. 144 Bell says: "Einstein had no difficulty accepting that affairs in different places could be correlated. What he could not accept was that an intervention at one place could influence, immediately, affairs at the other." Downloaded 5 July 2011 from <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFBell1987" class="citation book cs1">Bell, J. S. (1987). <a rel="nofollow" class="external text" href="https://web.archive.org/web/20150412044550/http://philosophyfaculty.ucsd.edu/faculty/wuthrich/GSSPP09/Files/BellJohnS1981Speakable_BertlmannsSocks.pdf"><i>Speakable and Unspeakable in Quantum Mechanics</i></a> <span class="cs1-format">(PDF)</span>. <a href="/wiki/CERN" title="CERN">CERN</a>. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/0521334950" title="Special:BookSources/0521334950"><bdi>0521334950</bdi></a>. Archived from <a rel="nofollow" class="external text" href="http://philosophyfaculty.ucsd.edu/faculty/wuthrich/GSSPP09/Files/BellJohnS1981Speakable_BertlmannsSocks.pdf">the original</a> <span class="cs1-format">(PDF)</span> on 12 April 2015<span class="reference-accessdate">. Retrieved <span class="nowrap">14 June</span> 2014</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=Speakable+and+Unspeakable+in+Quantum+Mechanics&rft.pub=CERN&rft.date=1987&rft.isbn=0521334950&rft.aulast=Bell&rft.aufirst=J.+S.&rft_id=http%3A%2F%2Fphilosophyfaculty.ucsd.edu%2Ffaculty%2Fwuthrich%2FGSSPP09%2FFiles%2FBellJohnS1981Speakable_BertlmannsSocks.pdf&rfr_id=info%3Asid%2Fen.wikipedia.org%3AQuantum+entanglement" class="Z3988"></span></span> </li> <li id="cite_note-:0-6"><span class="mw-cite-backlink">^ <a href="#cite_ref-:0_6-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-:0_6-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFYinCaoYongRen2013" class="citation journal cs1">Yin, Juan; Cao, Yuan; Yong, Hai-Lin; Ren, Ji-Gang; et al. (2013). "Bounding the speed of 'spooky action at a distance". <i>Physical Review Letters</i>. <b>110</b> (26): 260407. <a href="/wiki/ArXiv_(identifier)" class="mw-redirect" title="ArXiv (identifier)">arXiv</a>:<span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://arxiv.org/abs/1303.0614">1303.0614</a></span>. <a href="/wiki/Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/2013PhRvL.110z0407Y">2013PhRvL.110z0407Y</a>. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1103%2FPhysRevLett.110.260407">10.1103/PhysRevLett.110.260407</a>. <a href="/wiki/PMID_(identifier)" class="mw-redirect" title="PMID (identifier)">PMID</a> <a rel="nofollow" class="external text" href="https://pubmed.ncbi.nlm.nih.gov/23848853">23848853</a>. <a href="/wiki/S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:119293698">119293698</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=Physical+Review+Letters&rft.atitle=Bounding+the+speed+of+%27spooky+action+at+a+distance&rft.volume=110&rft.issue=26&rft.pages=260407&rft.date=2013&rft_id=https%3A%2F%2Fapi.semanticscholar.org%2FCorpusID%3A119293698%23id-name%3DS2CID&rft_id=info%3Abibcode%2F2013PhRvL.110z0407Y&rft_id=info%3Aarxiv%2F1303.0614&rft_id=info%3Apmid%2F23848853&rft_id=info%3Adoi%2F10.1103%2FPhysRevLett.110.260407&rft.aulast=Yin&rft.aufirst=Juan&rft.au=Cao%2C+Yuan&rft.au=Yong%2C+Hai-Lin&rft.au=Ren%2C+Ji-Gang&rft.au=Liang%2C+Hao&rft.au=Liao%2C+Sheng-Kai&rft.au=Zhou%2C+Fei&rft.au=Liu%2C+Chang&rft.au=Wu%2C+Yu-Ping&rft.au=Pan%2C+Ge-Sheng&rft.au=Li%2C+Li&rft.au=Liu%2C+Nai-Le&rft.au=Zhang%2C+Qiang&rft.au=Peng%2C+Cheng-Zhi&rft.au=Pan%2C+Jian-Wei&rfr_id=info%3Asid%2Fen.wikipedia.org%3AQuantum+entanglement" class="Z3988"></span></span> </li> <li id="cite_note-:1-7"><span class="mw-cite-backlink">^ <a href="#cite_ref-:1_7-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-:1_7-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFMatson2012" class="citation journal cs1">Matson, John (13 August 2012). "Quantum teleportation achieved over record distances". <i>Nature News</i>. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1038%2Fnature.2012.11163">10.1038/nature.2012.11163</a>. <a href="/wiki/S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:124852641">124852641</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=Nature+News&rft.atitle=Quantum+teleportation+achieved+over+record+distances&rft.date=2012-08-13&rft_id=info%3Adoi%2F10.1038%2Fnature.2012.11163&rft_id=https%3A%2F%2Fapi.semanticscholar.org%2FCorpusID%3A124852641%23id-name%3DS2CID&rft.aulast=Matson&rft.aufirst=John&rfr_id=info%3Asid%2Fen.wikipedia.org%3AQuantum+entanglement" class="Z3988"></span></span> </li> <li id="cite_note-:2-8"><span class="mw-cite-backlink">^ <a href="#cite_ref-:2_8-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-:2_8-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFFrancis2012" class="citation web cs1">Francis, Matthew (30 October 2012). <a rel="nofollow" class="external text" href="https://arstechnica.com/science/2012/10/quantum-entanglement-shows-that-reality-cant-be-local/">"Quantum entanglement shows that reality can't be local"</a>. <i>Ars Technica</i><span class="reference-accessdate">. Retrieved <span class="nowrap">22 August</span> 2023</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=Ars+Technica&rft.atitle=Quantum+entanglement+shows+that+reality+can%27t+be+local&rft.date=2012-10-30&rft.aulast=Francis&rft.aufirst=Matthew&rft_id=https%3A%2F%2Farstechnica.com%2Fscience%2F2012%2F10%2Fquantum-entanglement-shows-that-reality-cant-be-local%2F&rfr_id=info%3Asid%2Fen.wikipedia.org%3AQuantum+entanglement" class="Z3988"></span></span> </li> <li id="cite_note-9"><span class="mw-cite-backlink"><b><a href="#cite_ref-9">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFPenrose2004" class="citation book cs1"><a href="/wiki/Roger_Penrose" title="Roger Penrose">Penrose, Roger</a> (2004). <i>The road to reality: a complete guide to the laws of the universe</i>. London: Jonathan Cape. p. 603. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-0-224-04447-9" title="Special:BookSources/978-0-224-04447-9"><bdi>978-0-224-04447-9</bdi></a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=The+road+to+reality%3A+a+complete+guide+to+the+laws+of+the+universe&rft.place=London&rft.pages=603&rft.pub=Jonathan+Cape&rft.date=2004&rft.isbn=978-0-224-04447-9&rft.aulast=Penrose&rft.aufirst=Roger&rfr_id=info%3Asid%2Fen.wikipedia.org%3AQuantum+entanglement" class="Z3988"></span></span> </li> <li id="cite_note-Griffiths2004-10"><span class="mw-cite-backlink"><b><a href="#cite_ref-Griffiths2004_10-0">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFGriffiths,_David_J.2004" class="citation book cs1">Griffiths, David J. (2004). <i>Introduction to Quantum Mechanics</i> (2nd ed.). Prentice Hall. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-0-13-111892-8" title="Special:BookSources/978-0-13-111892-8"><bdi>978-0-13-111892-8</bdi></a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=Introduction+to+Quantum+Mechanics&rft.edition=2nd&rft.pub=Prentice+Hall&rft.date=2004&rft.isbn=978-0-13-111892-8&rft.au=Griffiths%2C+David+J.&rfr_id=info%3Asid%2Fen.wikipedia.org%3AQuantum+entanglement" class="Z3988"></span>.</span> </li> <li id="cite_note-11"><span class="mw-cite-backlink"><b><a href="#cite_ref-11">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFSiegel" class="citation web cs1">Siegel, Ethan. <a rel="nofollow" class="external text" href="https://www.forbes.com/sites/startswithabang/2020/01/02/no-we-still-cant-use-quantum-entanglement-to-communicate-faster-than-light/">"No, We Still Can't Use Quantum Entanglement To Communicate Faster Than Light"</a>. <i>Forbes</i><span class="reference-accessdate">. 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Retrieved <span class="nowrap">29 October</span> 2018</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=Scientific+American&rft.atitle=%22Schr%C3%B6dinger%27s+Bacterium%22+Could+Be+a+Quantum+Biology+Milestone+%E2%80%93+A+recent+experiment+may+have+placed+living+organisms+in+a+state+of+quantum+entanglement&rft.date=2018-10-29&rft.aulast=O%27Callaghan&rft.aufirst=Jonathan&rft_id=https%3A%2F%2Fwww.scientificamerican.com%2Farticle%2Fschroedingers-bacterium-could-be-a-quantum-biology-milestone%2F&rfr_id=info%3Asid%2Fen.wikipedia.org%3AQuantum+entanglement" class="Z3988"></span></span> </li> <li id="cite_note-141"><span class="mw-cite-backlink"><b><a href="#cite_ref-141">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFKrisnandaMarlettoVedralPaternostro2018" class="citation journal cs1">Krisnanda, T.; Marletto, C.; Vedral, V.; Paternostro, M.; Paterek, T. 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Cambridge, England: Cambridge University Press.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=Geometry+of+Quantum+States%3A+An+Introduction+to+Quantum+Entanglement&rft.place=Cambridge%2C+England&rft.pub=Cambridge+University+Press&rft.date=2006&rft.aulast=Bengtsson&rft.aufirst=I.&rft.au=%C5%BByczkowski%2C+K.&rfr_id=info%3Asid%2Fen.wikipedia.org%3AQuantum+entanglement" class="Z3988"></span> <a rel="nofollow" class="external text" href="http://chaos.if.uj.edu.pl/~karol/geometry.htm">second, revised edition (2017)</a></li> <li><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFBub2019" class="citation book cs1">Bub, Jeffrey (2019). <a rel="nofollow" class="external text" href="https://plato.stanford.edu/entries/qt-entangle/">"Quantum Entanglement and Information"</a>. <a href="/wiki/Stanford_Encyclopedia_of_Philosophy" title="Stanford Encyclopedia of Philosophy"><i>Stanford Encyclopedia of Philosophy</i></a>. Stanford, California: Stanford University.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=bookitem&rft.atitle=Quantum+Entanglement+and+Information&rft.btitle=Stanford+Encyclopedia+of+Philosophy&rft.place=Stanford%2C+California&rft.pub=Stanford+University&rft.date=2019&rft.aulast=Bub&rft.aufirst=Jeffrey&rft_id=https%3A%2F%2Fplato.stanford.edu%2Fentries%2Fqt-entangle%2F&rfr_id=info%3Asid%2Fen.wikipedia.org%3AQuantum+entanglement" class="Z3988"></span></li> <li><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFCramer2015" class="citation book cs1">Cramer, J. G. (2015). <i>The Quantum Handshake: Entanglement, Nonlocality and Transactions</i>. Springer Verlag. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-3-319-24642-0" title="Special:BookSources/978-3-319-24642-0"><bdi>978-3-319-24642-0</bdi></a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=The+Quantum+Handshake%3A+Entanglement%2C+Nonlocality+and+Transactions&rft.pub=Springer+Verlag&rft.date=2015&rft.isbn=978-3-319-24642-0&rft.aulast=Cramer&rft.aufirst=J.+G.&rfr_id=info%3Asid%2Fen.wikipedia.org%3AQuantum+entanglement" class="Z3988"></span></li> <li><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFDuarte2019" class="citation book cs1"><a href="/wiki/F._J._Duarte" title="F. J. Duarte">Duarte, F. J.</a> (2019). <i>Fundamentals of Quantum Entanglement</i>. Bristol, United Kingdom: Institute of Physics. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-0-7503-2226-3" title="Special:BookSources/978-0-7503-2226-3"><bdi>978-0-7503-2226-3</bdi></a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=Fundamentals+of+Quantum+Entanglement&rft.place=Bristol%2C+United+Kingdom&rft.pub=Institute+of+Physics&rft.date=2019&rft.isbn=978-0-7503-2226-3&rft.aulast=Duarte&rft.aufirst=F.+J.&rfr_id=info%3Asid%2Fen.wikipedia.org%3AQuantum+entanglement" class="Z3988"></span></li> <li><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFGühneTóth2009" class="citation journal cs1">Gühne O, Tóth G (2009). "Entanglement detection". <i><a href="/wiki/Physics_Reports" title="Physics Reports">Physics Reports</a></i>. <b>474</b> (1–6): 1–75. <a href="/wiki/ArXiv_(identifier)" class="mw-redirect" title="ArXiv (identifier)">arXiv</a>:<span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://arxiv.org/abs/0811.2803">0811.2803</a></span>. <a href="/wiki/Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/2009PhR...474....1G">2009PhR...474....1G</a>. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1016%2Fj.physrep.2009.02.004">10.1016/j.physrep.2009.02.004</a>. <a href="/wiki/S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:119288569">119288569</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=Physics+Reports&rft.atitle=Entanglement+detection&rft.volume=474&rft.issue=1%E2%80%936&rft.pages=1-75&rft.date=2009&rft_id=info%3Aarxiv%2F0811.2803&rft_id=https%3A%2F%2Fapi.semanticscholar.org%2FCorpusID%3A119288569%23id-name%3DS2CID&rft_id=info%3Adoi%2F10.1016%2Fj.physrep.2009.02.004&rft_id=info%3Abibcode%2F2009PhR...474....1G&rft.aulast=G%C3%BChne&rft.aufirst=O&rft.au=T%C3%B3th%2C+G&rfr_id=info%3Asid%2Fen.wikipedia.org%3AQuantum+entanglement" class="Z3988"></span></li> <li><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFBhaskaraPanigrahi2017" class="citation journal cs1">Bhaskara VS, Panigrahi PK (2017). "Generalized concurrence measure for faithful quantification of multiparticle pure state entanglement using Lagrange's identity and wedge product". <i>Quantum Information Processing</i>. <b>16</b> (5): 118. <a href="/wiki/ArXiv_(identifier)" class="mw-redirect" title="ArXiv (identifier)">arXiv</a>:<span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://arxiv.org/abs/1607.00164">1607.00164</a></span>. <a href="/wiki/Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/2017QuIP...16..118B">2017QuIP...16..118B</a>. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1007%2Fs11128-017-1568-0">10.1007/s11128-017-1568-0</a>. <a href="/wiki/S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:43754114">43754114</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=Quantum+Information+Processing&rft.atitle=Generalized+concurrence+measure+for+faithful+quantification+of+multiparticle+pure+state+entanglement+using+Lagrange%27s+identity+and+wedge+product&rft.volume=16&rft.issue=5&rft.pages=118&rft.date=2017&rft_id=info%3Aarxiv%2F1607.00164&rft_id=https%3A%2F%2Fapi.semanticscholar.org%2FCorpusID%3A43754114%23id-name%3DS2CID&rft_id=info%3Adoi%2F10.1007%2Fs11128-017-1568-0&rft_id=info%3Abibcode%2F2017QuIP...16..118B&rft.aulast=Bhaskara&rft.aufirst=VS&rft.au=Panigrahi%2C+PK&rfr_id=info%3Asid%2Fen.wikipedia.org%3AQuantum+entanglement" class="Z3988"></span></li> <li><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFSwainBhaskaraPanigrahi2022" class="citation journal cs1">Swain SN, Bhaskara VS, Panigrahi PK (2022). "Generalized entanglement measure for continuous-variable systems". <i>Physical Review A</i>. <b>105</b> (5): 052441. <a href="/wiki/ArXiv_(identifier)" class="mw-redirect" title="ArXiv (identifier)">arXiv</a>:<span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://arxiv.org/abs/1706.01448">1706.01448</a></span>. <a href="/wiki/Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/2022PhRvA.105e2441S">2022PhRvA.105e2441S</a>. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1103%2FPhysRevA.105.052441">10.1103/PhysRevA.105.052441</a>. <a href="/wiki/S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:239885759">239885759</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=Physical+Review+A&rft.atitle=Generalized+entanglement+measure+for+continuous-variable+systems&rft.volume=105&rft.issue=5&rft.pages=052441&rft.date=2022&rft_id=info%3Aarxiv%2F1706.01448&rft_id=https%3A%2F%2Fapi.semanticscholar.org%2FCorpusID%3A239885759%23id-name%3DS2CID&rft_id=info%3Adoi%2F10.1103%2FPhysRevA.105.052441&rft_id=info%3Abibcode%2F2022PhRvA.105e2441S&rft.aulast=Swain&rft.aufirst=SN&rft.au=Bhaskara%2C+VS&rft.au=Panigrahi%2C+PK&rfr_id=info%3Asid%2Fen.wikipedia.org%3AQuantum+entanglement" class="Z3988"></span></li> <li><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFJaeger2009" class="citation book cs1">Jaeger, G. (2009). <i>Entanglement, Information, and the Interpretation of Quantum Mechanics</i>. Heildelberg, Germany: Springer. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-3-540-92127-1" title="Special:BookSources/978-3-540-92127-1"><bdi>978-3-540-92127-1</bdi></a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=Entanglement%2C+Information%2C+and+the+Interpretation+of+Quantum+Mechanics&rft.place=Heildelberg%2C+Germany&rft.pub=Springer&rft.date=2009&rft.isbn=978-3-540-92127-1&rft.aulast=Jaeger&rft.aufirst=G.&rfr_id=info%3Asid%2Fen.wikipedia.org%3AQuantum+entanglement" class="Z3988"></span></li> <li><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFSteward2008" class="citation book cs1">Steward, E. G. (2008). <i>Quantum Mechanics: Its Early Development and the Road to Entanglement</i>. Imperial College Press. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-1-86094-978-4" title="Special:BookSources/978-1-86094-978-4"><bdi>978-1-86094-978-4</bdi></a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=Quantum+Mechanics%3A+Its+Early+Development+and+the+Road+to+Entanglement&rft.pub=Imperial+College+Press&rft.date=2008&rft.isbn=978-1-86094-978-4&rft.aulast=Steward&rft.aufirst=E.+G.&rfr_id=info%3Asid%2Fen.wikipedia.org%3AQuantum+entanglement" class="Z3988"></span></li></ul> </div> <div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Quantum_entanglement&action=edit&section=33" title="Edit section: External links"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1235681985">.mw-parser-output .side-box{margin:4px 0;box-sizing:border-box;border:1px solid #aaa;font-size:88%;line-height:1.25em;background-color:var(--background-color-interactive-subtle,#f8f9fa);display:flow-root}.mw-parser-output .side-box-abovebelow,.mw-parser-output .side-box-text{padding:0.25em 0.9em}.mw-parser-output .side-box-image{padding:2px 0 2px 0.9em;text-align:center}.mw-parser-output .side-box-imageright{padding:2px 0.9em 2px 0;text-align:center}@media(min-width:500px){.mw-parser-output .side-box-flex{display:flex;align-items:center}.mw-parser-output .side-box-text{flex:1;min-width:0}}@media(min-width:720px){.mw-parser-output .side-box{width:238px}.mw-parser-output .side-box-right{clear:right;float:right;margin-left:1em}.mw-parser-output .side-box-left{margin-right:1em}}</style><style data-mw-deduplicate="TemplateStyles:r1237033735">@media print{body.ns-0 .mw-parser-output .sistersitebox{display:none!important}}@media screen{html.skin-theme-clientpref-night .mw-parser-output .sistersitebox img[src*="Wiktionary-logo-en-v2.svg"]{background-color:white}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .sistersitebox img[src*="Wiktionary-logo-en-v2.svg"]{background-color:white}}</style><div class="side-box side-box-right plainlinks sistersitebox"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1126788409"> <div class="side-box-flex"> <div class="side-box-image"><span class="noviewer" typeof="mw:File"><span><img alt="" src="//upload.wikimedia.org/wikipedia/commons/thumb/f/fa/Wikiquote-logo.svg/34px-Wikiquote-logo.svg.png" decoding="async" width="34" height="40" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/f/fa/Wikiquote-logo.svg/51px-Wikiquote-logo.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/f/fa/Wikiquote-logo.svg/68px-Wikiquote-logo.svg.png 2x" data-file-width="300" data-file-height="355" /></span></span></div> <div class="side-box-text plainlist">Wikiquote has quotations related to <i><b><a href="https://en.wikiquote.org/wiki/Special:Search/Quantum_entanglement" class="extiw" title="q:Special:Search/Quantum entanglement">Quantum entanglement</a></b></i>.</div></div> </div> <ul><li><a rel="nofollow" class="external text" href="https://www.youtube.com/watch?v=xM3GOXaci7w">Explanatory video by <i>Scientific American</i> magazine</a></li> <li><a rel="nofollow" class="external text" href="https://web.archive.org/web/20121025073450/http://www.didaktik.physik.uni-erlangen.de/quantumlab/english/index.html">Entanglement experiment with photon pairs – interactive</a></li> <li>Audio – Cain/Gay (2009) <a rel="nofollow" class="external text" href="http://www.astronomycast.com/physics/ep-140-entanglement/">Astronomy Cast</a> Entanglement</li> <li><a rel="nofollow" class="external text" href="https://www.youtube.com/watch?v=ta09WXiUqcQ">"Spooky Actions at a Distance?": Oppenheimer Lecture, Prof. David Mermin (Cornell University) Univ. California, Berkeley, 2008.</a> Non-mathematical popular lecture on YouTube, posted Mar 2008</li> <li><a rel="nofollow" class="external text" href="https://demonstrations.wolfram.com/QuantumEntanglementVersusClassicalCorrelation/">"Quantum Entanglement versus Classical Correlation" (Interactive demonstration)</a></li></ul> <div class="navbox-styles"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1129693374"><style data-mw-deduplicate="TemplateStyles:r1236075235">.mw-parser-output .navbox{box-sizing:border-box;border:1px solid #a2a9b1;width:100%;clear:both;font-size:88%;text-align:center;padding:1px;margin:1em auto 0}.mw-parser-output .navbox .navbox{margin-top:0}.mw-parser-output .navbox+.navbox,.mw-parser-output .navbox+.navbox-styles+.navbox{margin-top:-1px}.mw-parser-output .navbox-inner,.mw-parser-output .navbox-subgroup{width:100%}.mw-parser-output .navbox-group,.mw-parser-output .navbox-title,.mw-parser-output .navbox-abovebelow{padding:0.25em 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.navbox-odd{background-color:transparent}.mw-parser-output .navbox .hlist td dl,.mw-parser-output .navbox .hlist td ol,.mw-parser-output .navbox .hlist td ul,.mw-parser-output .navbox td.hlist dl,.mw-parser-output .navbox td.hlist ol,.mw-parser-output .navbox td.hlist ul{padding:0.125em 0}.mw-parser-output .navbox .navbar{display:block;font-size:100%}.mw-parser-output .navbox-title .navbar{float:left;text-align:left;margin-right:0.5em}body.skin--responsive .mw-parser-output .navbox-image img{max-width:none!important}@media print{body.ns-0 .mw-parser-output .navbox{display:none!important}}</style></div><div role="navigation" class="navbox" aria-labelledby="Quantum_mechanics" style="padding:3px"><table class="nowraplinks hlist mw-collapsible autocollapse navbox-inner" style="border-spacing:0;background:transparent;color:inherit"><tbody><tr><th scope="col" class="navbox-title" colspan="2"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1129693374"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1239400231"><div class="navbar plainlinks hlist navbar-mini"><ul><li class="nv-view"><a href="/wiki/Template:Quantum_mechanics_topics" title="Template:Quantum mechanics topics"><abbr title="View this template">v</abbr></a></li><li class="nv-talk"><a href="/wiki/Template_talk:Quantum_mechanics_topics" title="Template talk:Quantum mechanics topics"><abbr title="Discuss this template">t</abbr></a></li><li class="nv-edit"><a href="/wiki/Special:EditPage/Template:Quantum_mechanics_topics" title="Special:EditPage/Template:Quantum mechanics topics"><abbr title="Edit this template">e</abbr></a></li></ul></div><div id="Quantum_mechanics" style="font-size:114%;margin:0 4em"><a href="/wiki/Quantum_mechanics" title="Quantum mechanics">Quantum mechanics</a></div></th></tr><tr><th scope="row" class="navbox-group" style="width:1%">Background</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Introduction_to_quantum_mechanics" title="Introduction to quantum mechanics">Introduction</a></li> <li><a href="/wiki/History_of_quantum_mechanics" title="History of quantum mechanics">History</a> <ul><li><a href="/wiki/Timeline_of_quantum_mechanics" title="Timeline of quantum mechanics">Timeline</a></li></ul></li> <li><a href="/wiki/Classical_mechanics" title="Classical mechanics">Classical mechanics</a></li> <li><a href="/wiki/Old_quantum_theory" title="Old quantum theory">Old quantum theory</a></li> <li><a href="/wiki/Glossary_of_elementary_quantum_mechanics" title="Glossary of elementary quantum mechanics">Glossary</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Fundamentals</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Born_rule" title="Born rule">Born rule</a></li> <li><a href="/wiki/Bra%E2%80%93ket_notation" title="Bra–ket notation">Bra–ket notation</a></li> <li><a href="/wiki/Complementarity_(physics)" title="Complementarity (physics)"> Complementarity</a></li> <li><a href="/wiki/Density_matrix" title="Density matrix">Density matrix</a></li> <li><a href="/wiki/Energy_level" title="Energy level">Energy level</a> <ul><li><a href="/wiki/Ground_state" title="Ground state">Ground state</a></li> <li><a href="/wiki/Excited_state" title="Excited state">Excited state</a></li> <li><a href="/wiki/Degenerate_energy_levels" title="Degenerate energy levels">Degenerate levels</a></li> <li><a href="/wiki/Zero-point_energy" title="Zero-point energy">Zero-point energy</a></li></ul></li> <li><a class="mw-selflink selflink">Entanglement</a></li> <li><a href="/wiki/Hamiltonian_(quantum_mechanics)" title="Hamiltonian (quantum mechanics)">Hamiltonian</a></li> <li><a href="/wiki/Wave_interference" title="Wave interference">Interference</a></li> <li><a href="/wiki/Quantum_decoherence" title="Quantum decoherence">Decoherence</a></li> <li><a href="/wiki/Measurement_in_quantum_mechanics" title="Measurement in quantum mechanics">Measurement</a></li> <li><a href="/wiki/Quantum_nonlocality" title="Quantum nonlocality">Nonlocality</a></li> <li><a href="/wiki/Quantum_state" title="Quantum state">Quantum state</a></li> <li><a href="/wiki/Quantum_superposition" title="Quantum superposition">Superposition</a></li> <li><a href="/wiki/Quantum_tunnelling" title="Quantum tunnelling">Tunnelling</a></li> <li><a href="/wiki/Scattering_theory" class="mw-redirect" title="Scattering theory">Scattering theory</a></li> <li><a href="/wiki/Symmetry_in_quantum_mechanics" title="Symmetry in quantum mechanics">Symmetry in quantum mechanics</a></li> <li><a href="/wiki/Uncertainty_principle" title="Uncertainty principle">Uncertainty</a></li> <li><a href="/wiki/Wave_function" title="Wave function">Wave function</a> <ul><li><a href="/wiki/Wave_function_collapse" title="Wave function collapse">Collapse</a></li> <li><a href="/wiki/Wave%E2%80%93particle_duality" title="Wave–particle duality">Wave–particle duality</a></li></ul></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Formulations</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Mathematical_formulation_of_quantum_mechanics" title="Mathematical formulation of quantum mechanics">Formulations</a></li> <li><a href="/wiki/Heisenberg_picture" title="Heisenberg picture">Heisenberg</a></li> <li><a href="/wiki/Interaction_picture" title="Interaction picture">Interaction</a></li> <li><a href="/wiki/Matrix_mechanics" title="Matrix mechanics">Matrix mechanics</a></li> <li><a href="/wiki/Schr%C3%B6dinger_picture" title="Schrödinger picture">Schrödinger</a></li> <li><a href="/wiki/Path_integral_formulation" title="Path integral formulation">Path integral formulation</a></li> <li><a href="/wiki/Phase-space_formulation" title="Phase-space formulation">Phase space</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Equations</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Klein%E2%80%93Gordon_equation" title="Klein–Gordon equation">Klein–Gordon</a></li> <li><a href="/wiki/Dirac_equation" title="Dirac equation">Dirac</a></li> <li><a href="/wiki/Weyl_equation" title="Weyl equation">Weyl</a></li> <li><a href="/wiki/Majorana_equation" title="Majorana equation">Majorana</a></li> <li><a href="/wiki/Rarita%E2%80%93Schwinger_equation" title="Rarita–Schwinger equation">Rarita–Schwinger</a></li> <li><a href="/wiki/Pauli_equation" title="Pauli equation">Pauli</a></li> <li><a href="/wiki/Rydberg_formula" title="Rydberg formula">Rydberg</a></li> <li><a href="/wiki/Schr%C3%B6dinger_equation" title="Schrödinger equation">Schrödinger</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Interpretations_of_quantum_mechanics" title="Interpretations of quantum mechanics">Interpretations</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Quantum_Bayesianism" title="Quantum Bayesianism">Bayesian</a></li> <li><a href="/wiki/Consistent_histories" title="Consistent histories">Consistent histories</a></li> <li><a href="/wiki/Copenhagen_interpretation" title="Copenhagen interpretation">Copenhagen</a></li> <li><a href="/wiki/De_Broglie%E2%80%93Bohm_theory" title="De Broglie–Bohm theory">de Broglie–Bohm</a></li> <li><a href="/wiki/Ensemble_interpretation" title="Ensemble interpretation">Ensemble</a></li> <li><a href="/wiki/Hidden-variable_theory" title="Hidden-variable theory">Hidden-variable</a> <ul><li><a href="/wiki/Local_hidden-variable_theory" title="Local hidden-variable theory">Local</a> <ul><li><a href="/wiki/Superdeterminism" title="Superdeterminism">Superdeterminism</a></li></ul></li></ul></li> <li><a href="/wiki/Many-worlds_interpretation" title="Many-worlds interpretation">Many-worlds</a></li> <li><a href="/wiki/Objective-collapse_theory" title="Objective-collapse theory">Objective collapse</a></li> <li><a href="/wiki/Quantum_logic" title="Quantum logic">Quantum logic</a></li> <li><a href="/wiki/Relational_quantum_mechanics" title="Relational quantum mechanics">Relational</a></li> <li><a href="/wiki/Transactional_interpretation" title="Transactional interpretation">Transactional</a></li> <li><a href="/wiki/Von_Neumann%E2%80%93Wigner_interpretation" title="Von Neumann–Wigner interpretation">Von Neumann–Wigner</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Experiments</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Bell_test" title="Bell test">Bell test</a></li> <li><a href="/wiki/Davisson%E2%80%93Germer_experiment" title="Davisson–Germer experiment">Davisson–Germer</a></li> <li><a href="/wiki/Delayed-choice_quantum_eraser" title="Delayed-choice quantum eraser">Delayed-choice quantum eraser</a></li> <li><a href="/wiki/Double-slit_experiment" title="Double-slit experiment">Double-slit</a></li> <li><a href="/wiki/Franck%E2%80%93Hertz_experiment" title="Franck–Hertz experiment">Franck–Hertz</a></li> <li><a href="/wiki/Mach%E2%80%93Zehnder_interferometer" title="Mach–Zehnder interferometer">Mach–Zehnder interferometer</a></li> <li><a href="/wiki/Elitzur%E2%80%93Vaidman_bomb_tester" title="Elitzur–Vaidman bomb tester">Elitzur–Vaidman</a></li> <li><a href="/wiki/Popper%27s_experiment" title="Popper's experiment">Popper</a></li> <li><a href="/wiki/Quantum_eraser_experiment" title="Quantum eraser experiment">Quantum eraser</a></li> <li><a href="/wiki/Stern%E2%80%93Gerlach_experiment" title="Stern–Gerlach experiment">Stern–Gerlach</a></li> <li><a href="/wiki/Wheeler%27s_delayed-choice_experiment" title="Wheeler's delayed-choice experiment">Wheeler's delayed choice</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Quantum_nanoscience" class="mw-redirect" title="Quantum nanoscience">Science</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Quantum_biology" title="Quantum biology">Quantum biology</a></li> <li><a href="/wiki/Quantum_chemistry" title="Quantum chemistry">Quantum chemistry</a></li> <li><a href="/wiki/Quantum_chaos" title="Quantum chaos">Quantum chaos</a></li> <li><a href="/wiki/Quantum_cosmology" title="Quantum cosmology">Quantum cosmology</a></li> <li><a href="/wiki/Quantum_differential_calculus" title="Quantum differential calculus">Quantum differential calculus</a></li> <li><a href="/wiki/Quantum_dynamics" title="Quantum dynamics">Quantum dynamics</a></li> <li><a href="/wiki/Quantum_geometry" title="Quantum geometry">Quantum geometry</a></li> <li><a href="/wiki/Measurement_problem" title="Measurement problem">Quantum measurement problem</a></li> <li><a href="/wiki/Quantum_mind" title="Quantum mind">Quantum mind</a></li> <li><a href="/wiki/Quantum_stochastic_calculus" title="Quantum stochastic calculus">Quantum stochastic calculus</a></li> <li><a href="/wiki/Quantum_spacetime" title="Quantum spacetime">Quantum spacetime</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Quantum_technology" class="mw-redirect" title="Quantum technology">Technology</a></th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Quantum_algorithm" title="Quantum algorithm">Quantum algorithms</a></li> <li><a href="/wiki/Quantum_amplifier" title="Quantum amplifier">Quantum amplifier</a></li> <li><a href="/wiki/Quantum_bus" title="Quantum bus">Quantum bus</a></li> <li><a href="/wiki/Quantum_cellular_automaton" title="Quantum cellular automaton">Quantum cellular automata</a> <ul><li><a href="/wiki/Quantum_finite_automaton" title="Quantum finite automaton">Quantum finite automata</a></li></ul></li> <li><a href="/wiki/Quantum_channel" title="Quantum channel">Quantum channel</a></li> <li><a href="/wiki/Quantum_circuit" title="Quantum circuit">Quantum circuit</a></li> <li><a href="/wiki/Quantum_complexity_theory" title="Quantum complexity theory">Quantum complexity theory</a></li> <li><a href="/wiki/Quantum_computing" title="Quantum computing">Quantum computing</a> <ul><li><a href="/wiki/Timeline_of_quantum_computing_and_communication" title="Timeline of quantum computing and communication">Timeline</a></li></ul></li> <li><a href="/wiki/Quantum_cryptography" title="Quantum cryptography">Quantum cryptography</a></li> <li><a href="/wiki/Quantum_electronics" class="mw-redirect" title="Quantum electronics">Quantum electronics</a></li> <li><a href="/wiki/Quantum_error_correction" title="Quantum error correction">Quantum error correction</a></li> <li><a href="/wiki/Quantum_imaging" title="Quantum imaging">Quantum imaging</a></li> <li><a href="/wiki/Quantum_image_processing" title="Quantum image processing">Quantum image processing</a></li> <li><a href="/wiki/Quantum_information" title="Quantum information">Quantum information</a></li> <li><a href="/wiki/Quantum_key_distribution" title="Quantum key distribution">Quantum key distribution</a></li> <li><a href="/wiki/Quantum_logic" title="Quantum logic">Quantum logic</a></li> <li><a href="/wiki/Quantum_logic_gate" title="Quantum logic gate">Quantum logic gates</a></li> <li><a href="/wiki/Quantum_machine" title="Quantum machine">Quantum machine</a></li> <li><a href="/wiki/Quantum_machine_learning" title="Quantum machine learning">Quantum machine learning</a></li> <li><a href="/wiki/Quantum_metamaterial" title="Quantum metamaterial">Quantum metamaterial</a></li> <li><a href="/wiki/Quantum_metrology" title="Quantum metrology">Quantum metrology</a></li> <li><a href="/wiki/Quantum_network" title="Quantum network">Quantum network</a></li> <li><a href="/wiki/Quantum_neural_network" title="Quantum neural network">Quantum neural network</a></li> <li><a href="/wiki/Quantum_optics" title="Quantum optics">Quantum optics</a></li> <li><a href="/wiki/Quantum_programming" title="Quantum programming">Quantum programming</a></li> <li><a href="/wiki/Quantum_sensor" title="Quantum sensor">Quantum sensing</a></li> <li><a href="/wiki/Quantum_simulator" title="Quantum simulator">Quantum simulator</a></li> <li><a href="/wiki/Quantum_teleportation" title="Quantum teleportation">Quantum teleportation</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Extensions</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Quantum_fluctuation" title="Quantum fluctuation">Quantum fluctuation</a></li> <li><a href="/wiki/Casimir_effect" title="Casimir effect">Casimir effect</a></li> <li><a href="/wiki/Quantum_statistical_mechanics" title="Quantum statistical mechanics">Quantum statistical mechanics</a></li> <li><a href="/wiki/Quantum_field_theory" title="Quantum field theory">Quantum field theory</a> <ul><li><a href="/wiki/History_of_quantum_field_theory" title="History of quantum field theory">History</a></li></ul></li> <li><a href="/wiki/Quantum_gravity" title="Quantum gravity">Quantum gravity</a></li> <li><a href="/wiki/Relativistic_quantum_mechanics" title="Relativistic quantum mechanics">Relativistic quantum mechanics</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Related</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Schr%C3%B6dinger%27s_cat" title="Schrödinger's cat">Schrödinger's cat</a> <ul><li><a href="/wiki/Schr%C3%B6dinger%27s_cat_in_popular_culture" title="Schrödinger's cat in popular culture">in popular culture</a></li></ul></li> <li><a href="/wiki/Wigner%27s_friend" title="Wigner's friend">Wigner's friend</a></li> <li><a href="/wiki/Einstein%E2%80%93Podolsky%E2%80%93Rosen_paradox" title="Einstein–Podolsky–Rosen paradox">EPR paradox</a></li> <li><a href="/wiki/Quantum_mysticism" title="Quantum mysticism">Quantum mysticism</a></li></ul> </div></td></tr><tr><td class="navbox-abovebelow" colspan="2"><div> <ul><li><span class="noviewer" typeof="mw:File"><span title="Category"><img alt="" src="//upload.wikimedia.org/wikipedia/en/thumb/9/96/Symbol_category_class.svg/16px-Symbol_category_class.svg.png" decoding="async" width="16" height="16" class="mw-file-element" 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