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Physical Review A

Comment on "Reliability of Bell-inequality measurements using polarization correlations in parametric-down-conversion photon sources"

Paul G. Kwiat

Published Oct 1, 1995• 13 citations
Publisher

Abstract

In a recent article [Phys. Rev. A 50, R2803 (1994)] De Caro and Garuccio state that many parametric-down-conversion sources cannot be used in tests of Bell's inequalities because they produce quantum-mechanical product states. Here we point out that not only have suitable sources already been proposed in the literature, but experiments not suffering from the problems cited by the authors have also been reported.

DOI
10.1103/physreva.52.3380

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In a recent article [Phys. Rev. A 50, R2803 (1994)] De Caro and Garuccio state that many parametric-down-conversion sources cannot be used in tests of Bell's inequalities because they produce quantum-mechanical product states. Here we point out that not only have suitable sources already been proposed in the literature, but experiments not suffering from the problems cited by the authors have also been reported.
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.
The nonlocality inherent in quantum mechanics has been tested experimentally, but the nonlocal interpretation of all the relevant experiments can be challenged. All past tests of Bell's inequalities have required supplementary assumptions, without which the experimental results satisfy the inequalities, in agreement with the notion of local realism. There are basically three loopholes that need to be closed; neither the cascade sources nor the parametric down-conversion sources of correlation photons used to date are capable of closing all of them, even if the detectors used had been 100% efficient. We propose a two-crystal down-conversion source, relying on type-II collinear phase matching, which should permit a violation of Bell's inequalities without the need for supplementary assumptions. As the source can produce a true singletlike state, it is also relevant for quantum cryptographic applications.