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Physical Review Letters

Violation of Bell's Inequality and Classical Probability in a Two-Photon Correlation Experiment

Z. Y. Ou, L. Mandeļ

Published Jul 4, 1988• 878 citations
Publisher

Abstract

Correlation measurements of mixed signal and idler photons produced in the process of parametric down-conversion have been performed as a function of two linear polarizer settings. It is found that the Bell inequality for two separated particles is violated by about 6 standard deviations, and that classical probability for light waves is violated substantially also.

DOI
10.1103/physrevlett.61.50

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Correlation measurements of mixed signal and idler photons produced in the process of parametric down-conversion have been performed as a function of two linear polarizer settings. It is found that the Bell inequality for two separated particles is violated by about 6 standard deviations, and that classical probability for light waves is violated substantially also.
A number of optical experiments in which quantum mechanics is tested and the laws of classical probability are violated will be described. These all make use of the phenomenon in non-linear optics known as parametric down-conversion, in which an incident pump photon effectively splits into two correlated, lower frequency signal and idler photons in an entangled quantum state. We show that two-photon coincidence measurements of signal and idler photons violate classical probability by at least 600 standard deviations. Other measurements involving two down-converters lead to the realization of a ″quantum eraser″ and suggest a reinterpretation of the usual quantum wave function.
We report a new two-photon polarization correlation experiment for realizing the Einstein-Podolsky-Rosen-Bohm (EPRB) state and for testing Bell-type inequalities. We use the pair of orthogonally-polarized light quanta generated in Type 2 parametric down conversion. Using 1 nm interference filters in front of our detectors, we observe from the output of a 0.5mm beta - BaB2O4 (BBO) crystal the EPRB correlations in coincidence counts, and measure an associated Bell inequality violation of 22 standard deviations. The quantum state of the photon pair is a polarization analog of the spin-1/2 singlet state.
We report a new two-photon polarization correlation experiment for realizing the Einstein-Podolsky-Rosen-Bohm (EPRB) state and for testing Bell-type inequalities. We use the pair of orthogonally polarized light quanta generated in type-II parametric down-conversion. In a 1 nm bandwidth, we observe from the output of a 0.5 mm \ensuremath{\beta}-BaB2{\mathrm{BaB}}_{2}BaB2​O4{\mathrm{O}}_{4}O4​ crystal the EPRB correlations in coincidence counts, and measure an associated Bell inequality violation of 22 standard deviations.
We demonstrate hybrid entanglement of photon pairs via the experimental violation of a Bell inequality with two different degrees of freedom (DOF), namely, the path (linear momentum) of one photon and the polarization of the other photon. Hybrid entangled photon pairs are created by spontaneous parametric down conversion and coherent polarization to path conversion for one photon. For that photon, path superposition is analyzed, and polarization superposition for its twin photon. The correlations between these two measurements give an SSS parameter of $S=2.653\ifmmode\pm\else\textpm\fi{}0.027$ in a Clauser-Horne-Shimony-Holt inequality and thus violate local realism for two different DOF by more than 24 standard deviations. This experimentally supports the idea that entanglement is a fundamental concept which is indifferent to the specific physical realization of Hilbert space.
A pair of correlated light quanta of 532-nm wavelength with the same linear polarization but divergent directions of propagation was produced by nonlinear optical parametric down conversion. Each light quantum was converted to a definite polarization eigenstate and was reflected by a turning mirror to superpose with the other at a beam splitter. For coincident detection at separated detectors, polarization correlations of the Einstein-Podolsky-Rosen-Bohm type were observed. We also observed a violation of Bell's inequality by 3 standard deviations.