Editing Bell's theorem (section)

==Experiments==
[[Image:Bell-test-photon-analyer.png|thumb|upright=2|'''Scheme of a "two-channel" Bell test'''<br />The source S produces pairs of "photons", sent in opposite directions. Each photon encounters a two-channel polariser whose orientation (a or b) can be set by the experimenter. Emerging signals from each channel are detected and coincidences of four types (++, −−, +− and −+) counted by the coincidence monitor.]]
{{main|Bell test}}

In 1967, the unusual title ''Physics Physique Физика'' caught the attention of [[John Clauser]], who then discovered Bell's paper and began to consider how to perform a [[Bell test]] in the laboratory.<ref>{{Cite web|url=https://www.scientificamerican.com/article/how-the-hippies-saved-physics-science-counterculture-and-quantum-revival-excerpt/|title=How the Hippies Saved Physics: Science, Counterculture, and the Quantum Revival [Excerpt]|last=Kaiser|first=David|author-link=David Kaiser (physicist)|date=2012-01-30|website=[[Scientific American]]|language=en|access-date=2020-02-11}}</ref> Clauser and [[Stuart Freedman]] would go on to perform a Bell test in 1972.<ref>{{cite journal|last1=Freedman|first1=S. J.|author-link=Stuart Freedman|last2=Clauser|first2=J. F.|author-link2=John Clauser|year=1972|title=Experimental test of local hidden-variable theories|url=https://www.rpi.edu/dept/phys/Courses/PHYS4100/S06/BellsInequ1972.pdf|journal=[[Physical Review Letters]]|volume=28|issue=938|pages=938–941|bibcode=1972PhRvL..28..938F|doi=10.1103/PhysRevLett.28.938}}</ref><ref>{{cite thesis|url=https://escholarship.org/content/qt2f18n5nk/qt2f18n5nk.pdf?t=p2au19 |title=Experimental test of local hidden-variable theories |first=Stuart Jay |last=Freedman |date=1972-05-05 |type=PhD |publisher=University of California, Berkeley}}</ref> This was only a limited test, because the choice of detector settings was made before the photons had left the source. In 1982, [[Alain Aspect]] and collaborators performed the [[Aspect's experiment|first Bell test]] to remove this limitation.<ref>{{cite journal |first1=Alain |last1=Aspect |author-link1=Alain Aspect |first2=Jean |last2=Dalibard |first3=Gérard |last3=Roger |year=1982 |title=Experimental Test of Bell's Inequalities Using Time-Varying Analyzers |journal=[[Physical Review Letters]] |volume=49 |issue=25 |pages=1804–7 |doi=10.1103/PhysRevLett.49.1804|bibcode = 1982PhRvL..49.1804A|doi-access=free }}</ref> This began a trend of progressively more stringent Bell tests. The GHZ thought experiment was implemented in practice, using entangled triplets of photons, in 2000.<ref name="GHZ2000">{{cite journal |first1=Jian-Wei |last1=Pan |first2=D. |last2=Bouwmeester |first3=M. |last3=Daniell |first4=H. |last4=Weinfurter |first5=A. |last5=Zeilinger |author-link5=Anton Zeilinger |year=2000 |title=Experimental test of quantum nonlocality in three-photon GHZ entanglement |journal=[[Nature (journal)|Nature]] |volume=403 |issue=6769 |pages=515–519 |bibcode=2000Natur.403..515P |doi=10.1038/35000514 |pmid=10676953|s2cid=4309261 }}</ref> By 2002, testing the CHSH inequality was feasible in undergraduate laboratory courses.<ref>{{cite journal|title=Entangled photons, nonlocality, and Bell inequalities in the undergraduate laboratory |first1=Dietrich |last1=Dehlinger |first2=M. W. |last2=Mitchell |journal=[[American Journal of Physics]] |volume=70 |pages=903–910 |year=2002 |issue=9 |doi=10.1119/1.1498860|arxiv=quant-ph/0205171 |bibcode=2002AmJPh..70..903D |s2cid=49487096 }}</ref>

In Bell tests, there may be problems of experimental design or set-up that affect the validity of the experimental findings. These problems are often referred to as "loopholes". The purpose of the experiment is to test whether nature can be described by [[local hidden-variable theory]], which would contradict the predictions of quantum mechanics.

The most prevalent loopholes in real experiments are the ''detection'' and ''locality'' loopholes.<ref name=larsson14>{{cite journal |last1=Larsson |first1=Jan-Åke |title=Loopholes in Bell inequality tests of local realism |journal=Journal of Physics A: Mathematical and Theoretical |date=2014 |volume=47 |issue=42 |page=424003 |doi=10.1088/1751-8113/47/42/424003 |arxiv=1407.0363 |bibcode=2014JPhA...47P4003L |s2cid=40332044 }}</ref> The detection loophole is opened when a small fraction of the particles (usually photons) are detected in the experiment, making it possible to explain the data with local hidden variables by assuming that the detected particles are an unrepresentative sample. The locality loophole is opened when the detections are not done with a [[Spacetime#Spacetime interval|spacelike separation]], making it possible for the result of one measurement to influence the other without contradicting relativity. In some experiments there may be additional defects that make local-hidden-variable explanations of Bell test violations possible.<ref>{{cite journal|first1=I. |last1=Gerhardt |first2=Q. |last2=Liu |first3=A. |last3=Lamas-Linares |first4=J. |last4=Skaar |first5=V. |last5=Scarani |first6=V. |last6=Makarov |first7=C. |last7=Kurtsiefer |display-authors=5|year=2011 |title=Experimentally faking the violation of Bell's inequalities |journal=[[Physical Review Letters]] |volume=107 |issue=17 |page=170404 |arxiv=1106.3224 |doi=10.1103/PhysRevLett.107.170404 |bibcode=2011PhRvL.107q0404G |pmid=22107491|s2cid=16306493 }}</ref>

Although both the locality and detection loopholes had been closed in different experiments, a long-standing challenge was to close both simultaneously in the same experiment. This was finally achieved in three experiments in 2015.<ref>{{cite journal|title=Quantum 'spookiness' passes toughest test yet|journal=[[Nature News]] |date=27 August 2015|first=Zeeya|last=Merali|volume=525 |issue=7567|pages=14–15|doi=10.1038/nature.2015.18255 |pmid=26333448|bibcode=2015Natur.525...14M |s2cid=4409566|doi-access=free}}</ref><ref name="NYT-20151021">{{cite news |last=Markoff |first=Jack |title=Sorry, Einstein. Quantum Study Suggests 'Spooky Action' Is Real. |url=https://www.nytimes.com/2015/10/22/science/quantum-theory-experiment-said-to-prove-spooky-interactions.html |date=21 October 2015 |work=[[New York Times]] |accessdate=21 October 2015 }}</ref><ref name="NTR-20151021">{{cite journal |author=Hensen, B. |title=Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometres |date=21 October 2015 |journal=[[Nature (journal)|Nature]] |doi=10.1038/nature15759 |display-authors=etal |volume=526 |issue=7575 |pages=682–686 |bibcode=2015Natur.526..682H |pmid=26503041|arxiv=1508.05949 |s2cid=205246446 }}</ref><ref name="PRL115-250402">{{cite journal |last=Shalm |first=L. K. |title=Strong Loophole-Free Test of Local Realism|date=16 December 2015|journal=[[Physical Review Letters]] |display-authors=etal |volume=115|issue=25|page= 250402| doi=10.1103/PhysRevLett.115.250402 |bibcode=2015PhRvL.115y0402S |pmid=26722906|pmc=5815856|arxiv=1511.03189}}</ref><ref name="PRL115-250401">{{cite journal |last=Giustina |first=M. |title=Significant-Loophole-Free Test of Bell's Theorem with Entangled Photons|date=16 December 2015|journal=[[Physical Review Letters]] |display-authors=etal |volume=115|issue=25|page= 250401| doi=10.1103/PhysRevLett.115.250401 |pmid=26722905|arxiv=1511.03190|bibcode=2015PhRvL.115y0401G|s2cid=13789503}}</ref>
Regarding these results, [[Alain Aspect]] writes that "no experiment ... can be said to be totally loophole-free," but he says the experiments "remove the last doubts that we should renounce" local hidden variables, and refers to examples of remaining loopholes as being "far fetched" and "foreign to the usual way of reasoning in physics."<ref>{{cite journal |last=Aspect |first=Alain |author-link=Alain Aspect |date=December 16, 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>

These efforts to experimentally validate violations of the Bell inequalities would later result in Clauser, Aspect, and [[Anton Zeilinger]] being awarded the 2022 [[Nobel Prize in Physics]].<ref>{{Cite news |last1=Ahlander |first1=Johan |last2=Burger |first2=Ludwig |last3=Pollard |first3=Niklas |date=2022-10-04 |title=Nobel physics prize goes to sleuths of 'spooky' quantum science |language=en |work=Reuters |url=https://www.reuters.com/world/aspect-clauser-zeilinger-win-2022-nobel-prize-physics-2022-10-04/ |access-date=2022-10-04}}</ref>