An experiment conducted by the French physicist Alain Aspect (1947– ) and his colleagues in the early 1980s to test Bell's inequality (see Bell's theorem). The experiment involves producing pairs of photons from a source of excited calcium ions. The photons have different wavelengths and filters are used to ensure that the photons in a pair travel to different detectors in different directions. The photons are circularly polarized and the net angular momentum of the pair is zero. Two polarizing filters are used, each placed at an angle in the path of the photons. These filters either reflect or transmit photons of different linear polarization to one of four detectors (two for each polarizing filter). Coincidence measurements are made using these detectors and the experiment is organized so that the measurements apply only to photons separated to a point at which they cannot communicate by sending a signal at the speed of light. The results are generally believed to show that there are no local hidden variables in quantum mechanics. See also quantum entanglement (Feature).
http://www.drchinese.com/David/Aspect.pdf The original 1982 paper in Physical Review Letters