Aspect's experiment

Aspect's experiment was a series of 1980s tests of Bell inequalities using entangled photons and rapidly switched measurement settings. The results supported quantum mechanics over local hidden-variable theories.

Bell inequality: A mathematical bound on correlations predicted by local hidden-variable theories. Aspect’s measurements tested whether photon correlations exceeded this local bound.
John Stewart Bell: The physicist whose theorem showed that local hidden-variable theories obey limits on quantum correlations. Bell supplied the inequality that Aspect’s experiments tested.
Quantum entanglement: A quantum relationship in which the joint state of multiple systems cannot be reduced to independent states. Entanglement produced the strong photon correlations at the heart of Aspect’s tests.
Locality loophole: A possible weakness in a Bell test if measurement events could influence one another at light speed or below. Fast switching was intended to prevent communication between distant measurement choices and outcomes.
Bell test: An experiment that tests a Bell inequality using measurements on correlated systems. Aspect’s work established a practical design for testing Bell inequalities with photons.
EPR paradox: A thought experiment arguing that quantum mechanics leaves physical reality incomplete because of distant correlations. The paradox raised the conflict that Bell’s theorem and Aspect’s tests made experimentally addressable.
Quantum nonlocality: The feature of quantum theory whereby entangled systems exhibit correlations incompatible with local causal explanations. The observed Bell-inequality violations became evidence for this distinctive feature.
Detection loophole: A possible bias in a Bell test when too many particles go undetected for the sample to represent all trials. The photon detectors of the period did not register every emitted pair.
Polarization: The orientation-dependent property of a light wave’s electric field. The experiment encoded photon measurement settings and outcomes in polarization.
Freedman–Clauser experiment: A 1972 photon experiment that tested a Bell inequality using fixed analyzer settings. It preceded Aspect’s work but did not rapidly change measurement settings during photon flight.

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