Experimental test of quantum nonlocality in three-photon Greenberger-Horne-Zeilinger entanglement

Bell's theorem(1) states that certain statistical correlations predicted by quantum physics for measurements on two-particle systems cannot be underst ood within a realistic picture based on local properties of each individual particle-even if the two particles are separated by large distances. Einst ein, Podolsky and Rosen first recognized(2) the fundamental significance of these quantum correlations (termed 'entanglement' by Schrodinger(3)) and t he two-particle quantum predictions have found ever-increasing experimental support(4). A more striking conflict between quantum mechanical and local realistic predictions (for perfect correlations) has been discovered(5,6); but experimental verification has been difficult, as it requires entangleme nt between at least three particles. Here we report experimental confirmati on of this conflict, using our recently developed method: to observe three- photon entanglement, or 'Greenberger-Horne-Zeilinger' (GHZ) states. The res ults of three specific experiments, involving-measurements of polarization correlations between three photons, lead to predictions for a fourth experi ment; quantum physical predictions are mutually contradictory with expectat ions based on local realism. We find the results of the fourth experiment t o be in agreement with the quantum prediction and in striking conflict with local realism.