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AIP conference proceedings

There may be more to entangled photon experiments than we have appreciated so far

Arnt Inge Vistnes, Guillaume Adenier

Published Jan 1, 2012• 8 citations
Publisher

Abstract

Nowadays entangled photons are often produced by spontaneous parametric down conversion in nonlinear birefringent crystals. Entangled photon pairs can show a very convincing break of the Bell inequality, usually tested through the CHSH (Clauser, Horne, Shimony, Holt) procedure. However, a "compensating crystal", placed after the non-linear crystal, is necessary for the traditional setup. The purpose of this crystal has been explained using the concept of indistinguishability. In this paper we argue that indistinguishability is an unsatisfactory explanation. We would prefer an explanation based on physical mechanisms and not on an abstract and not easily testable concept like indistinguishability. Experimental details seems to reveal that there may be some interesting physics going on that we have not appreciated so far. Such details may be of great help in order to find a more satisfactory explanation of the process leading to entanglement.

DOI
10.1063/1.4773143

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Nowadays entangled photons are often produced by spontaneous parametric down conversion in nonlinear birefringent crystals. Entangled photon pairs can show a very convincing break of the Bell inequality, usually tested through the CHSH (Clauser, Horne, Shimony, Holt) procedure. However, a "compensating crystal", placed after the non-linear crystal, is necessary for the traditional setup. The purpose of this crystal has been explained using the concept of indistinguishability. In this paper we argue that indistinguishability is an unsatisfactory explanation. We would prefer an explanation based on physical mechanisms and not on an abstract and not easily testable concept like indistinguishability. Experimental details seems to reveal that there may be some interesting physics going on that we have not appreciated so far. Such details may be of great help in order to find a more satisfactory explanation of the process leading to entanglement.
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We use polarization-entangled photon pairs to demonstrate quantum nonlocality in an experiment suitable for advanced undergraduates. The photons are produced by spontaneous parametric downconversion using a violet diode laser and two nonlinear crystals. The polarization state of the photons is tunable. Using an entangled state analogous to that described in the Einstein-Podolsky-Rosen ``paradox,'' we demonstrate strong polarization correlations of the entanged photons. Bell's idea of a hidden variable theory is presented by way of an example and compared to the quantum prediction. A test of the Clauser, Horne, Shimony and Holt version of the Bell inequality finds S=2.307pm0.035S = 2.307 \\pm 0.035S=2.307pm0.035, in clear contradiciton of hidden variable theories. The experiments described can be performed in an afternoon.
Many future directions of scientific endeavors depend on quantum theory and the precise interpretation and significance of the entanglement of quantum-particles. This interpretation depends in turn on the physical meaning of so called Bell-tests that are mostly performed using entangled photons and randomly switched polarizers to measure their polarization at distant locations. This paper presents a detailed critique of the well known theory of Bell tests given by Clauser-Horne-Shimony-Holt (CHSH). It is demonstrated that several important steps of the CHSH derivations contain serious inaccuracies of the underlying physics and probability theory and even a calculus error. As a consequence, the Bell-CHSH theory cannot be used to demonstrate extreme and opposite interpretations of entanglement such as super-luminal influences or alternatively super-determinism that cast aspersions on Einstein’s concepts of locality and separability.
We use polarization-entangled photon pairs to demonstrate quantum nonlocality in an experiment suitable for advanced undergraduates. The photons are produced by spontaneous parametric downconversion using a violet diode laser and two nonlinear crystals. The polarization state of the photons is tunable. Using an entangled state analogous to that described in the Einstein–Podolsky–Rosen paradox, we demonstrate strong polarization correlations of the entangled photons. Bell’s idea of a hidden variable theory is presented by way of an example and compared to the quantum prediction. A test of the Clauser, Horne, Shimony, and Holt version of the Bell inequality finds S=2.307±0.035, in clear contradiction of hidden variable theories. The experiments described can be performed in an afternoon.
Inspired by previous studies and pioneers of the field, we present new results on an extensive Einstein, Podolsky, and Rosen (EPR)–Bell experiment using photons generated by parametric down-conversion, where one of the photons is deliberately phase-shifted. Our experiments show some surprising results for particular angles of this phase shift, where, for some states, the Clauser, Horne, Shimony, and Holt inequality shows no violations, requiring us to review our understanding of “entangled” states vis-à-vis the correlations obtained via experimental coincidence counts in agreement with quantum mechanics theoretical predictions. We give some possible explanations about the correlated states that show no violations of these inequalities.
Quantum nonlocality is presented often as the most remarkable and inexplicable phenomenon known to modern science. It has been known already for a long time that the probabilistic models used to prove Bell and Clauser-Horn-Shimony-Holt inequalities (BI-CHSH) for spin polarization correlation experiments (SPCE) are incompatible with the experimental protocols of SPCE. In particular these models use the same common probability space, joint probability distributions and/or conditional independence to describe coincidence experiments in incompatible experimental settings. Strangely enough these results are not known or simply neglected. This is why we will once again reanalyze Bell locality assumptions and show that they have nothing to do with the notion of Einsteinian locality therefore their violation should not be called quantum nonlocality but rather quantum non-Kolmogorovness or quantum contextuality. Moreover if local variables describing the measuring instruments are correctly taken into account then BI-CHSH can no longer be proven and one can easily construct non-signaling probabilistic models able to reproduce the predictions of QT. The violation of BI-CHSH is considered usually as a proof that a quantum state is entangled. Since BI-CHSH are violated also in some experiments from outside the domain of quantum physics therefore the entanglement is not exclusively a quantum phenomenon. In order to further demystify these notions we show that one can prepare two macroscopic systems in such a way that simple realizable local experiments on these systems violate BI. In view of these arguments the further testing of BI-CHSH inequalities in search for the loopholes does not seem to be necessary.