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Physical Review A

Critical analysis of the empirical tests of local hidden-variable theories

Emilio Santos

Published Oct 1, 1992• 138 citations

Abstract

A local hidden-variable model is exhibited for the experiments by Aspect, Grangier, and Roger [Phys. Rev. Lett. 47, 460 (1981); 49, 91 (1982)] and Aspect, Dalibard, and Roger [Phys. Rev. Lett. 49, 1804 (1982)] measuring polarization correlation of optical-photon pairs. The model agrees with quantum-mechanical predictions for all measurable quantities even with ideal polarizers and detectors, and emphasizes the need of a high degree of directional correlation, besides the correlation of spin (or polarization or other quantities), in any test of locality. It is proved that homogeneous inequalities, involving only coincidence detection rates, cannot discriminate between quantum mechanics and local theories, which invalidates all previously used empirical tests. The role of supplementary assumptions, like the so-called no enhancement, for the derivation of Bell's inequalities is discussed. Finally it is conjectured that quantum mechanics might be compatible with local realism, if we assume that not all self-adjoint operators represent observables and not all density operators represent states.

DOI: 10.1103/physreva.46.3646

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A local hidden-variable model is exhibited for the experiments by Aspect, Grangier, and Roger [Phys. Rev. Lett. 47, 460 (1981); 49, 91 (1982)] and Aspect, Dalibard, and Roger [Phys. Rev. Lett. 49, 1804 (1982)] measuring polarization correlation of optical-photon pairs. The model agrees with quantum-mechanical predictions for all measurable quantities even with ideal polarizers and detectors, and emphasizes the need of a high degree of directional correlation, besides the correlation of spin (or polarization or other quantities), in any test of locality. It is proved that homogeneous inequalities, involving only coincidence detection rates, cannot discriminate between quantum mechanics and local theories, which invalidates all previously used empirical tests. The role of supplementary assumptions, like the so-called no enhancement, for the derivation of Bell's inequalities is discussed. Finally it is conjectured that quantum mechanics might be compatible with local realism, if we assume that not all self-adjoint operators represent observables and not all density operators represent states.

In recent years parametric-down-conversion photon sources have been proposed and used to test local realism via Bell-type inequalities. This study shows that this technique fails to discriminate between quantum mechanics and local realism in all those tests in which polarization correlations are measured, since dichotomic observables cannot be associated satisfactorily with the state produced in these experiments. A more adequate description of these photon sources leads to an alternative inequality for Einstein locality, which is always satisfied by quantum-mechanical predictions. The result can be extended to all the experiments in which only one-half of the emitted pairs are detected.

This paper furthers the long historical examination of and debate on the foundations of quantum mechanics (QM) by presenting two local hidden variable (LHV) rules in the context of the EPRB experiment which violate Bell's inequality, but which are nevertheless local and deterministic under reasonable definitions of the terms, and coincide approximately with the conventional QM prediction. The theories are based on the general idea of probabilistic detection of particles depending on an interaction of hidden variables within the measuring device and particle, and relate mathematically to Fourier analysis. The crucial discrepancy of variations in the hidden variable distribution based on relative polarizer orientations is isolated which invalidates assumptions in Bell-type theorems. The first theory can be analyzed completely symbolically whereas the second was analyzed using numerical methods. The properties of the second in particular are shown to be approximately consistent with the reported results and uncertainties in all three Aspect experiments. Variation in the total photon pairs detected over orientations is shown to be a basic characteristic of these theories. Some comments on the relevance of active vs. passive locality are made. Two sections consider these ideas relative to energy conservation and the measurement problem (collapse of the wavefunction). One section proposes new experiments.

A family of theories called "deterministic local hidden variable theories" (DLHV) is characterized and their consequences are investigated for an experimentally realistic situation. An experiment on the polarization correlation of optical photon pairs emitted in certain atomic cascades is proposed as a test of the family of DLHV theories. The model-independent hidden variable predictions applicable to the realistic experimental arrangement are obtained by a generalization of a previous theorem of Bell. Quantum mechanical predictions for the same experimental situation are calculated for two cases: a J = 0 + J = 1 + J = 0 and a J = 1 + J = 1 + J = 0 electric dipole cascade. For both cascades, it is shown that the quantum mechanical and the DLHV predictions are in sufficient disagreement for an actual experimental test to be possible. Restrictions on detector solid angle and polarizer efficiency, necessary to obtain a decisive experiment, are explicitly given. The Einstein, Podolsky, and Rosen argument for the existence of a DLHV theory is reviewed.