Aspect experiment
The Aspect experiments are a celebrated series of Bell tests carried out by Alain Aspect and his collaborators in Orsay, France, in the early 1980s. Using pairs of photons emitted by excited calcium atoms, they measured the polarisation correlations predicted by quantum mechanics and compared them against the CHSH bound that any local hidden-variable theory must respect. Their results clearly violated that bound, in striking agreement with quantum theory.
Their signature advance, in 1982, was to switch the measurement settings while the photons were already in flight, using fast acoustic-optical devices. Because the choice of what to measure on one side was made too late to influence the distant side by any signal travelling at or below light speed, the experiment closed — at least in spirit — the so-called locality loophole that earlier tests had left open. It made the conflict with local realism much harder to explain away.
Aspect's work is rightly remembered as the moment Bell's abstract theorem became a decisive laboratory fact, and it inspired a generation of ever more rigorous tests. For these contributions he shared the 2022 Nobel Prize in Physics with John Clauser and Anton Zeilinger. Honesty requires noting the limits of the era: the photon detectors were inefficient, leaving a detection loophole that only the loophole-free experiments of 2015 would finally close.
Switching settings in mid-flight, Aspect's photons broke the local-realist bound — a landmark Bell test.
Aspect's tests addressed the locality loophole but, like all photon experiments of their day, still suffered from low detector efficiency. They were a decisive step, not the final word; the loophole-free tests of 2015 closed the remaining gaps.