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Physical Review Letters

Experimental Test of Bell's Inequalities Using Time- Varying Analyzers

Alain Aspect, Jean Dalibard, Gérard Roger

Published Dec 20, 1982• 3,939 citations
Publisher

Abstract

Correlations of linear polarizations of pairs of photons have been measured with time-varying analyzers. The analyzer in each leg of the apparatus is an acousto-optical switch followed by two linear polarizers. The switches operate at incommensurate frequencies near 50 MHz. Each analyzer amounts to a polarizer which jumps between two orientations in a time short compared with the photon transit time. The results are in good agreement with quantum mechanical predictions but violate Bell's inequalities by 5 standard deviations.

DOI
10.1103/physrevlett.49.1804

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Correlations of linear polarizations of pairs of photons have been measured with time-varying analyzers. The analyzer in each leg of the apparatus is an acousto-optical switch followed by two linear polarizers. The switches operate at incommensurate frequencies near 50 MHz. Each analyzer amounts to a polarizer which jumps between two orientations in a time short compared with the photon transit time. The results are in good agreement with quantum mechanical predictions but violate Bell's inequalities by 5 standard deviations.
As a criterion between quantum mechanics and local hidden-variable theories, the so-called Einstein-Podolsky-Rosen paradox is mainly tested in the form of the statistical correlation between polarizations of photons issuing from a cascade transition. It has been stated more than once that an improved form of the test would make use of polarizers, the orientation of which would change randomly in a time comparable with the time of flight of the two photons; the Bell locality assumption could then be replaced by a weaker assumption also considered by Bell: The Einstein principle of separability. However, to our knowledge, no workable experimental scheme has yet been proposed, and we believe the one described in this paper to be a workable one. After explaining the difference between the Bell locality assumption and the Einstein principle of separability, we briefly discuss the theoretical implications of the modified experiment. The overall scheme of the apparatus we are proposing is described, and the generalized Bell inequalities, modified for our case, are derived. As in previous experiments, supplementary assumptions are made in order to derive experimentally testable inequalities. Finally, we describe the device we intend to use to carry out the proposed scheme.
In this thesis activity the candidate has explored the main aspects of Bell Inequalities theory and eventually executed an experimental Violation test. This work contains a brief overview of the theoretical aspects around Bell Inequalities: the doubts arisen by Einstein, Podolski and Rosen about Quantum Theory completeness; the hidden variable theories and the bounds on classical correlations; quantum correlations and violation of Bell inequalities; the response given by Bell to the EPR paradox by introducing its famous theorem. In addition, the entanglement phenomenon has been studied, in particular in those dof that are given an important role in the violation of Bell Inequalities. Some documentation work was also done on non linear optics in order to understand fundamental effects that is common practice to exploit in the photon entanglement generation: SHG for the pump photons, SPDC in non-linear crystals. Moreover, the candidate also explored in this thesis some issues more closely related to the experimental aspect:loopholes that could affect the integrity of Bell inequalities proofs; efficiency threshold estimates regarding detection loophole; loophole-free configurations. Eventually, here in this report are also showed, along with the experimental setup brief description and some comments on the apparatus and its setting, the results of an experimental test of the CHSH inequality violation using photons entangled at first in polarization and then in a time-bin entanglement configuration.
Experiments testing Bell's inequalities generally uses sources of entangled photons emitted in opposite direction, sent through polarizer of specific different orientation and finally correlated. But the correlation result would be the same if photons was emitted by only one source of 360° rotating polarization, separated in two flux by a semi-transparent mirror, and finally correlated as in the standard experiment. Therefore, it would demonstrate that the result is not due to any entangled state of the photons.