In the 1982 Aspect experiment testing Bell’s theorem, entangled photons displayed quantum correlations that violated Bell inequalities.
Alain Aspect and collaborators generated pairs of polarization-correlated photons and measured them with analyzers whose settings changed during the experiment. The results showed correlations stronger than any local hidden-variable theory satisfying Bell’s assumptions could produce. They agreed with the predictions of quantum mechanics.
The experiment addressed a central debate raised by Albert Einstein, Boris Podolsky, and Nathan Rosen, who had questioned whether quantum mechanics gave a complete description of physical reality. Bell’s theorem transformed that philosophical dispute into an experimentally testable one.
The result does not allow usable faster-than-light communication. Quantum correlations can be stronger than classical ones while still preventing observers from controlling the random individual outcomes needed to send a message. Later experiments improved on important loopholes, including detector and locality issues. Aspect’s work was a landmark test, not a demonstration that photons transmit ordinary signals instantaneously.