Editing Quantum entanglement (section)

== History ==
{{hatnote| Background: [[History of quantum mechanics]]}}
[[File:NYT May 4, 1935.jpg|right|thumb|268x268px|Article headline regarding the [[EPR paradox|Einstein–Podolsky–Rosen (EPR) paradox]] paper, in the 4 May 1935 issue of ''[[The New York Times]]'']]
Albert Einstein and Niels Bohr engaged in a long-running collegial dispute about the meaning of quantum mechanics, now known as the [[Bohr–Einstein debates]]. During these debates, Einstein introduced a [[thought experiment]] 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.<ref>{{cite book|first=Don |last=Howard |chapter=''Nicht Sein Kann Was Nicht Sein Darf'', or The Prehistory of EPR, 1909–1935: Einstein's Early Worries About The Quantum Mechanics of Composite Systems |title=Sixty-Two Years of Uncertainty |editor-first=A. I. |editor-last=Miller |publisher=Plenum Press |location=New York |year=1990 |chapter-url=http://www.ub.edu/hcub/hfq/sites/default/files/Howard1990-1.pdf |pages=61–111}}</ref> That same year, [[Hermann Weyl]] observed in his textbook on [[group theory]] and quantum mechanics that quantum systems made of multiple interacting pieces exhibit a kind of ''[[Gestalt psychology|Gestalt]],'' in which "the whole is greater than the sum of its parts".<ref>{{cite book|first=Hermann |last=Weyl |author-link=Hermann Weyl |title=Gruppentheorie und Quantenmechanik |title-link=Gruppentheorie und Quantenmechanik |trans-title=Group Theory and Quantum Mechanics |translator-first=H. P. |translator-last=Robertson |translator-link=Howard P. Robertson |year=1931 |edition=2nd |pages=92–93}}</ref><ref>{{cite journal|first=Adrian |last=Heathcote |title=Multiplicity and indiscernability |doi=10.1007/s11229-020-02600-8 |journal=Synthese |volume=198 |pages=8779–8808 |year=2021 |issue=9 |quote=For Weyl clearly anticipated entanglement by noting that the pure state of a coupled system need not be determined by the states of the composites [...] Weyl deserves far more credit than he has received for laying out the basis for entanglement — more than six years before Schrödinger coined the term.}}</ref> In 1932, [[Erwin Schrödinger]] wrote down the defining equations of quantum entanglement but set them aside, unpublished.<ref>{{cite thesis |last=Christandl |first=Matthias |date=2006 |degree=PhD |publisher=University of Cambridge |title=The Structure of Bipartite Quantum States – Insights from Group Theory and Cryptography |journal= |pages=vi, iv |arxiv=quant-ph/0604183 |bibcode=2006PhDT.......289C }}</ref> In 1935, [[Grete Hermann]] studied the mathematics of an electron interacting with a photon and noted the phenomenon that would come to be called entanglement.<ref>{{cite book |first=Thomas |last=Filk |chapter=Carl Friedrich von Weizsäcker's 'Ortsbestimmung eines Elektrons' and its Influence on Grete Hermann |doi=10.1007/978-94-024-0970-3_5 |title=Grete Hermann – Between Physics and Philosophy |publisher=Springer |series=Studies in History and Philosophy of Science |volume=42 |editor-first1=Elise |editor-last1=Crull |editor-first2=Guido |editor-last2=Bacciagaluppi |year=2016 |isbn=978-94-024-0968-0 |page=76}}</ref> Later that same year, Einstein, [[Boris Podolsky]] and [[Nathan Rosen]] published a paper on what is now known as the [[Einstein–Podolsky–Rosen paradox|Einstein–Podolsky–Rosen (EPR) paradox]], a thought experiment that attempted to show that "the [[quantum-mechanical]] description of physical reality given by wave functions is not complete".<ref name="Einstein1935" /> Their thought experiment had two systems interact, then separate, and they showed that afterwards quantum mechanics cannot describe the two systems individually.

Shortly after this paper appeared, [[Erwin Schrödinger]] wrote a letter to Einstein in [[German language|German]] in which he used the word ''Verschränkung'' (translated by himself as ''entanglement'') to describe situations like that of the EPR scenario.<ref name=MK>{{cite book |last=Kumar |first=Manjit |title=Quantum: Einstein, Bohr, and the Great Debate about the Nature of Reality |publisher=W. W. Norton & Company |year=2010 |page=313 |isbn=978-0-393-07829-9}}</ref> Schrödinger followed up with a full paper defining and discussing the notion of ''entanglement'',<ref name="Schroeder-2017">{{cite journal |last=Schroeder |first=Daniel V. |date=1 November 2017 |title=Entanglement isn't just for spin |url=https://pubs.aip.org/ajp/article/85/11/812/1057936/Entanglement-isn-t-just-for-spin |journal=American Journal of Physics |volume=85 |issue=11 |pages=812–820 |arxiv=1703.10620 |doi=10.1119/1.5003808 |bibcode=2017AmJPh..85..812S |issn=0002-9505}}</ref> saying "I would not call [entanglement] ''one'' but rather ''the'' characteristic trait of quantum mechanics, the one that enforces its entire departure from [[Classical mechanics|classical]] lines of thought."<ref name="Schrödinger1935"/> 
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 [[theory of relativity]].<ref>{{cite book|editor-first1=Alisa |editor-last1=Bokulich |editor-first2=Gregg |editor-last2=Jaeger |title=Philosophy of Quantum Information and Entanglement |publisher=Cambridge University Press |year=2010 |isbn=9780511676550 |chapter=Introduction |page=xv}}</ref> Einstein later referred to the effects of entanglement as "''spukhafte Fernwirkung''"<ref name="spukhafte">Letter from Einstein to Max Born, 3 March 1947; ''The Born-Einstein Letters; Correspondence between Albert Einstein and Max and Hedwig Born from 1916 to 1955'', Walker, New York, 1971. Cited in {{cite journal |author=Hobson |first=M. P. |display-authors=etal |year=1998 |title=Quantum Entanglement and Communication Complexity |journal=SIAM J. Comput. |volume=30 |issue=6 |pages=1829–1841 |citeseerx=10.1.1.20.8324}})</ref> or "[[spooky action at a distance]]", meaning the acquisition of a value of a property at one location resulting from a measurement at a distant location.<ref name="MerminMoon-1985">{{Cite journal |last=Mermin |first=N. David |author-link=N. David Mermin |date=1985 |title=Is the Moon There When Nobody Looks? Reality and the Quantum Theory |url=https://archive.org/details/mermin_moon |journal=Physics Today |volume=38 |number=4 |pages=38–47 |doi=10.1063/1.880968|bibcode=1985PhT....38d..38M }}</ref>

In 1946, [[John Archibald Wheeler]] suggested studying the [[polarization (physics)|polarization]] of pairs of [[gamma-ray]] photons produced by electron–[[positron]] annihilation.<ref>{{cite journal|first=J. A. |last=Wheeler |author-link=John Archibald Wheeler |title=Polyelectrons |journal=Annals of the New York Academy of Sciences |volume=48 |number=3 |pages=219–238 |year=1946 |doi=10.1111/j.1749-6632.1946.tb31764.x}}</ref> [[Chien-Shiung Wu]] and I. Shaknov carried out this experiment in 1949,<ref name=":3">
{{cite journal
 |last1=Wu |first1=C. S.
 |last2=Shaknov |first2=I.
 |year=1950
 |title=The Angular Correlation of Scattered Annihilation Radiation
 |journal=[[Physical Review]]
 |volume=77 |issue= 1|pages=136
 |bibcode=1950PhRv...77..136W
 |doi=10.1103/PhysRev.77.136
}}</ref> thereby demonstrating that the entangled particle pairs considered by EPR could be created in the laboratory.<ref>
{{cite journal
 |last1=Duarte |first1=F. J. |author1-link=F. J. Duarte
 |year=2012
 |title=The origin of quantum entanglement experiments based on polarization measurements
 |journal=[[European Physical Journal H]]
 |volume=37 |issue=2 |pages=311–318
 |bibcode=2012EPJH...37..311D
 |doi=10.1140/epjh/e2012-20047-y
}}</ref> 

Despite Schrödinger's claim of its importance, little work on entanglement was published for decades after his paper was published.<ref name="Schroeder-2017"/> In 1964 [[John S. Bell]] demonstrated an upper limit, seen in [[Bell's inequality]], regarding the strength of correlations that can be produced in any theory obeying [[local realism]], and showed that quantum theory predicts violations of this limit for certain entangled systems.<ref name=":4">{{cite journal |author=Bell |first=J. S. |author-link=John Stewart Bell |year=1964 |title=On the Einstein Poldolsky Rosen paradox |journal=[[Physics Physique Физика]] |volume=1 |issue=3 |pages=195–200 |doi=10.1103/PhysicsPhysiqueFizika.1.195 |doi-access=free}}</ref><ref>{{cite journal |last=Mermin |first=N. David |date=1981 |title=Quantum Mysteries for Anyone |url=https://www.jstor.org/stable/2026482 |journal=The Journal of Philosophy |volume=78 |issue=7 |pages=397–408 |doi=10.2307/2026482 |jstor=2026482 |issn=0022-362X}}</ref>{{rp|405}} His inequality is experimentally testable, and there have been numerous [[Bell test experiments|relevant experiments]], starting with the pioneering work of [[Stuart Freedman]] and [[John Clauser]] in 1972<ref name="Clauser">{{cite journal|doi=10.1103/PhysRevLett.28.938|last1=Freedman|first1=Stuart J.|last2=Clauser|first2=John F.|title=Experimental Test of Local Hidden-Variable Theories|journal=Physical Review Letters |volume=28 |issue=14 |pages=938–941|year=1972 |bibcode=1972PhRvL..28..938F|url=https://escholarship.org/uc/item/2f18n5nk|doi-access=free}}</ref> and [[Alain Aspect]]'s experiments in 1982.<ref name="Aspect1982">
{{cite journal
 | last1 = Aspect | first1 = Alain
 | last2 = Grangier | first2 = Philippe
 | last3 = Roger | first3 = Gérard
 | title = Experimental Realization of Einstein–Podolsky–Rosen–Bohm Gedankenexperiment: A New Violation of Bell's Inequalities
 | journal = Physical Review Letters
 | volume = 49
 | issue = 2
 | pages = 91–94
 | year = 1982
 | doi = 10.1103/PhysRevLett.49.91 | doi-access = free
 | bibcode=1982PhRvL..49...91A
}}</ref><ref name="hanson">{{cite journal|last1=Hanson|first1=Ronald|title=Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometres|journal=Nature|volume=526|issue=7575|pages=682–686|doi=10.1038/nature15759|arxiv=1508.05949|bibcode = 2015Natur.526..682H|pmid=26503041|year=2015|s2cid=205246446}}</ref><ref>{{cite journal |last=Aspect |first=Alain |date=16 December 2015 |title=Closing the Door on Einstein and Bohr's Quantum Debate |journal=[[Physics (magazine)|Physics]] |volume=8 |pages=123 |bibcode=2015PhyOJ...8..123A |doi=10.1103/Physics.8.123 |doi-access=free}}</ref>

While Bell actively discouraged students from pursuing work like his as too esoteric, after a talk at Oxford a student named [[Artur Ekert]] suggested that the violation of a Bell inequality could be used as a resource for communication.<ref name=Gilder2009/>{{rp|315}} Ekert followed up by  publishing a [[quantum key distribution]]  protocol called [[E91 protocol|E91]] based on it.<ref name=ekert91/><ref name="horodecki2007"/>{{rp|874|q=The first discovery within quantum information theory, which involves entanglement, is due to Ekert 1991.}}

In 1992, the entanglement concept was leveraged to propose [[quantum teleportation]],<ref name="BBCJPW93">{{cite journal |last1=Bennett |first1=Charles H. |author-link1=Charles H. Bennett (computer scientist) |last2=Brassard |first2=Gilles |author-link2=Gilles Brassard |last3=Crépeau |first3=Claude |author-link3=Claude Crépeau |last4=Jozsa |first4=Richard |author-link4=Richard Jozsa |last5=Peres |first5=Asher |author-link5=Asher Peres |last6=Wootters |first6=William K. |author-link6=William Wootters |date=29 March 1993 |title=Teleporting an Unknown Quantum State via Dual Classical and Einstein–Podolsky–Rosen Channels |journal=[[Physical Review Letters]] |volume=70 |issue=13 |pages=1895–1899 |doi=10.1103/PhysRevLett.70.1895 |pmid=10053414 |bibcode=1993PhRvL..70.1895B |doi-access=free |citeseerx=10.1.1.46.9405}}</ref> an effect that was realized experimentally in 1997.<ref>{{cite journal |last=Lindley |first=David |date=8 January 2010 |title=Landmarks: Teleportation is not Science Fiction |url=https://physics.aps.org/story/v25/st1 |volume=25 |journal=[[Physical Review Focus|Physics (Physical Review Focus)]]}}</ref><ref name="Bouwmeester-1997">{{cite journal |last1=Bouwmeester |first1=Dik |author-link1=Dirk Bouwmeester |last2=Pan |first2=Jian-Wei |last3=Mattle |first3=Klaus |last4=Eibl |first4=Manfred |last5=Weinfurter |first5=Harald |last6=Zeilinger |first6=Anton |date=1 December 1997 |title=Experimental quantum teleportation |journal=Nature |volume=390 |issue=6660 |pages=575–579 |doi=10.1038/37539 |arxiv=1901.11004 |bibcode=1997Natur.390..575B |s2cid=4422887}}</ref><ref name="Rome1998">
{{cite journal
 |last1=Boschi |first1=D. |last2=Branca |first2=S. |last3=De Martini |first3=F. |last4=Hardy |first4=L. |last5=Popescu |first5=S.
 |journal=[[Physical Review Letters]]
 |volume=80
 |issue=6
 |pages=1121–1125
 |doi= 10.1103/PhysRevLett.80.1121
 |title=Experimental Realization of Teleporting an Unknown Pure Quantum State via Dual Classical and Einstein–Podolsky–Rosen Channels
 |date=9 February 1998
 |arxiv = quant-ph/9710013 |bibcode = 1998PhRvL..80.1121B |s2cid=15020942
}}</ref>

Beginning in the mid-1990s, [[Anton Zeilinger]] used the generation of entanglement via [[Spontaneous parametric down-conversion| parametric down-conversion]] to develop [[entanglement swapping]]<ref name=Gilder2009>{{cite book |last=Gilder |first=Louisa |title=The age of entanglement: when quantum physics was reborn |date=2009 |publisher=Vintage Books |isbn=978-1-4000-9526-1 |edition=1. Vintage Book |location=New York, NY}}</ref>{{rp|317}} and demonstrate [[quantum cryptography]] with entangled photons.<ref>{{cite journal|first1=T. |last1=Jennewein |first2=C. |last2=Simon |first3=G. |last3=Weihs |first4=H. |last4=Weinfurter |first5=A. |last5=Zeilinger |author-link5=Anton Zeilinger |title=Quantum Cryptography with Entangled Photons |journal=Physical Review Letters |volume=84 |pages=4729–4732 |year=2000 |issue=20 |doi=10.1103/PhysRevLett.84.4729|pmid=10990782 |arxiv=quant-ph/9912117 |bibcode=2000PhRvL..84.4729J }}</ref><ref>{{cite journal|last1=Del Santo |first1=F |last2=Schwarzhans |first2=E. |year=2022 |title="Philosophysics" at the University of Vienna: The (Pre-) History of Foundations of Quantum Physics in the Viennese Cultural Context |journal=Physics in Perspective |volume=24 |number=2–3 |pages=125–153 |doi=10.1007/s00016-022-00290-y |pmid=36437910 |pmc=9678993 |arxiv=2011.11969|bibcode=2022PhP....24..125D }}</ref>

In 2022, the [[Nobel Prize in Physics]] was awarded to Aspect, Clauser, and Zeilinger "for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science".<ref name="NobelPrize">{{cite press release |url=https://www.nobelprize.org/prizes/physics/2022/press-release/ |title=The Nobel Prize in Physics 2022 |date=4 October 2022 |publisher=[[The Royal Swedish Academy of Sciences]] |access-date=5 October 2022}}</ref>