In Alain Aspect’s 1982 Bell-test experiment, what did the measurements violate?

Alain Aspect’s 1982 Bell-test experiment found measurements that violated Bell’s inequality.

Bell’s theorem, proposed by physicist John Stewart Bell in 1964, showed that certain statistical limits must hold if hidden variables explain quantum outcomes through local influences. Aspect and his colleagues tested these limits using pairs of correlated photons produced in atomic transitions. Polarizing filters were rapidly changed while photons traveled toward separate detectors.

The observed correlations exceeded the bound set by Bell’s inequality and agreed with quantum-mechanical predictions. This did not allow messages to travel faster than light, but it ruled out a broad class of local hidden-variable theories. The result is commonly described as evidence for quantum entanglement and the nonclassical correlations between separated particles.

Aspect’s work was especially influential because the measurement settings were changed during the photons’ flight, addressing an important loophole in earlier tests. Later experiments improved the tests further, and the 2022 Nobel Prize in Physics recognized Aspect, John Clauser, and Anton Zeilinger for work on entangled photons and Bell inequalities.