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Signaling loophole in experimental Bell tests

Adam Bednorz

Published Nov 11, 2015• 2 citations
OpenAlex

Abstract

The recent Bell test performed in Delft [B. Hensen et al. Nature 526, 682 (2015), arXiv:1508.05949], claimed as loophole-free, has shown violation of local realism with 96% confidence level. However, the raw data indicate violation of no-signaling principle (microcausality) around 99% confidence level. While violation of local realism is consistent with quantum mechanics, it is not the case of no-signaling. Violation of the latter would be a groundbreaking revolution in physics but also invalidate the conclusion of the Bell theorem, where it no-signaling assumed and must be verified or at least not falsified. If the signature of signaling is believed to be a statistical error, a fair calculation of the Bell parameter gives S = 1.86 < 2, in accordance with local realism. Therefore further experimental tests are urgent to clarify with better confidence level whether local realism or no-signaling is violated or none of them.

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The recent Bell test performed in Delft [B. Hensen et al. Nature 526, 682 (2015), arXiv:1508.05949], claimed as loophole-free, has shown violation of local realism with 96% confidence level. However, the raw data indicate violation of no-signaling principle (microcausality) around 99% confidence level. While violation of local realism is consistent with quantum mechanics, it is not the case of no-signaling. Violation of the latter would be a groundbreaking revolution in physics but also invalidate the conclusion of the Bell theorem, where it no-signaling assumed and must be verified or at least not falsified. If the signature of signaling is believed to be a statistical error, a fair calculation of the Bell parameter gives S = 1.86 < 2, in accordance with local realism. Therefore further experimental tests are urgent to clarify with better confidence level whether local realism or no-signaling is violated or none of them.
G. Adenier and A.Y. Khrennikov (2016) show that a recent ``loophole free'' CHSH Bell experiment violates no-signaling equalities, contrary to the expected impossibility of signaling in that experiment. We show that a local realism setup, in which nature sets hidden variables based on forecasts, and which can violate a Bell Inequality, can also give the illusion of signaling where there is none. This suggests that the violation of the CHSH Bell inequality, and the puzzling no-signaling violation in the CHSH Bell experiment may be explained by hidden variables based on forecasts as well.
Quantum mechanics is strictly incompatible with local realism. It has been shown by Bell and others that it is possible, in principle, to experimentally differentiate between local realism and quantum mechanics. Numerous experiments have attempted to falsify local realism; however, they have consistently failed to close the detection loophole under strict locality conditions, thereby allowing local realistic explanations for their observations. In 2015, three experiments took place that tested local realism without the impediments of these significant loopholes. Between these three experiments, a substantial data set was collected. All of the collected data show a strong violation of local realism and strong support for quantum mechanics. This article reviews the theoretical basis of Bell tests and the affiliated loopholes, as well as the methods employed by these recent experiments and the implications of the results they observed. La mécanique quantique est strictement incompatible avec le réalisme local. Bell et d’autres scientifiques ont montré qu’il est possible, en théorie, de trouver la différence entre le réalisme local et la mécanique quantique expérimentalement. De nombreuses expériences ont tenté de falsifier le réalisme local; cependant, elles ont toujours échoué à combler la faille de détection dans des conditions de localité strictes, permettant ainsi des explications réalistes locales pour leurs observations. En 2015, trois expériences ont testé le réalisme local sans les entraves de ces failles importantes. Entre ces trois expériences, des données substantielles ont été recueillies. Toutes les données recueillies ont montré une forte déviation du réalisme local et un appui solid pour la mécanique quantique. Cet article examine les bases théoriques des tests de Bell et les failles af liées, ainsi que les méthodes employées par ces expériences récentes et les implications de leurs résultats.
Local realism in recent experiments is excluded on condition of freedom or randomness of choice combined with no signaling between observers by implementations of simple quantum models. Both no signaling and the underlying quantum model can be directly checked by analysis of experimental data. For particular tests performed on the data, it is shown that two of these experiments give the probability of the data under no-signaling (or choice independence in one of them) hypothesis at the level of 5%, accounting for the look-elsewhere effect, moderately suggesting that no signaling is violated with 95% confidence. On the other hand, the data from the two other experiments violate the assumption of the simple quantum model. Further experiments are necessary to clarify these issues and freedom and randomness of choice.
Bell's theorem shows that local realistic theories place strong restrictions on observable correlations between different systems, giving rise to Bell's inequality which can be violated in experiments using entangled quantum states. Bell's theorem is based on the assumptions of realism, locality, and the freedom to choose between measurement settings. In experimental tests, "loopholes" arise which allow observed violations to still be explained by local realistic theories. Violating Bell's inequality while simultaneously closing all such loopholes is one of the most significant still open challenges in fundamental physics today. In this paper, we present an experiment that violates Bell's inequality while simultaneously closing the locality loophole and addressing the freedom-of-choice loophole, also closing the latter within a reasonable set of assumptions. We also explain that the locality and freedom-of-choice loopholes can be closed only within nondeterminism, i.e., in the context of stochastic local realism.
Local realism is the worldview in which physical properties of objects exist independently of measurement and where physical influences cannot travel faster than the speed of light. Bell's theorem states that this worldview is incompatible with the predictions of quantum mechanics, as is expressed in Bell's inequalities. Previous experiments convincingly supported the quantum predictions. Yet, every experiment requires assumptions that provide loopholes for a local realist explanation. Here, we report a Bell test that closes the most significant of these loopholes simultaneously. Using a well-optimized source of entangled photons, rapid setting generation, and highly efficient superconducting detectors, we observe a violation of a Bell inequality with high statistical significance. The purely statistical probability of our results to occur under local realism does not exceed 3.74×10^{-31}, corresponding to an 11.5 standard deviation effect.
Abstract Recently, a group of experiments tested local realism with random choices prepared by humans. These various tests were subject to additional assumptions, which lead to loopholes in the interpretations of almost all of the experiments. Among these assumptions are fair sampling, no signaling, and faithful reproduction of a Bell‐type quantum model. The data from 9 of 13 experiments were examined and occurring anomalies were analyzed in view of the above assumptions. It is concluded that further tests of local realism need better setup calibration to avoid apparent signaling or necessity of the complicated underlying quantum model.