Editing Bell test (section)

{{Short description|Experiments to test Bell's theorem in quantum mechanics}}{{Redirect|Bell experiment|the KEK particle physics experiment|Belle experiment}}
{{Quantum mechanics|cTopic=Experiments}}
A '''Bell test''', also known as '''Bell inequality test''' or '''Bell experiment''', is a real-world [[physics]] experiment designed to test the theory of [[quantum mechanics]] in relation to [[Albert Einstein]]'s concept of [[local realism]]. Named for [[John Stewart Bell]], the experiments test whether or not the real world satisfies local realism, which requires the presence of some [[local hidden-variable theory|additional local variables]] (called "hidden" because they are not a feature of quantum theory) to explain the behavior of particles like [[photon]]s and [[electron]]s. The test empirically evaluates the implications of [[Bell's theorem]]. {{asof|2015}}, all Bell tests have found that the hypothesis of local hidden variables is inconsistent with the way that physical systems behave.<ref name="Markoff">{{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>

Many types of Bell tests have been performed in physics laboratories, often with the goal of ameliorating problems of experimental design or set-up that could in principle affect the validity of the findings of [[Aspect's experiment|earlier Bell tests]]. This is known as "closing [[loopholes in Bell tests]]".<ref name="Markoff"/>

Bell inequality violations are also used in some [[quantum cryptography]] [[Cryptographic protocol|protocols]], whereby a spy's presence is detected when Bell's inequalities ''cease'' to be violated.