The Experiment That Put Einstein's Quantum Objection to the Test

Alain Aspect turned one of quantum mechanics’ deepest arguments into a decisive laboratory experiment. Building on John Bell’s theorem and earlier photon tests, Aspect and his collaborators created increasingly stringent measurements of entangled photons at Orsay. In 1982 they rapidly switched analyzer settings while photons were in flight and observed correlations violating Bell inequalities by five standard deviations. The experiments strengthened the case against local hidden-variable explanations and helped open.

Aspect's story shows how an argument once dismissed as philosophy became experimental physics. Bell had shown that local hidden-variable theories and quantum mechanics predict measurably different correlations; Aspect then built the technology needed for a stronger test. His rapidly switched 1982 experiment agreed with quantum mechanics while violating Bell's bound, yet still left loopholes for later generations to close. Following that progression reveals how entanglement moved from Einstein's objection to a practical.