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Quantum correlations in Newtonian space and time: faster than light communication or nonlocality

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Mathematical Challenges in Quantum Information

Using a theorem that allows one to assert the presence of nonlocal correlations between parties that are never measured in the same run of an experiment (using marginal), we study the following question: Experimental violations of Bell inequalities using space-like separated measurements precludes the explanation of quantum correlations through causal influences propagating at subluminal speed. Yet, everything looks as if the two parties somehow communicate behind the scene. We investigate the assumption that they do so at a speed faster than light, though finite. Such an assumption doesnt respect the spirit of Einstein relativity. However, it is not crystal clear that such communication behind the scene would contradict relativity. Indeed, one could imagine that this communication remains for ever hidden to humans, i.e. that it could not be controlled by humans, only Nature exploits it to produce correlations that cant be explained by usual common causes. To define faster than light hidden communication requires a universal privileged reference frame in which this faster than light speed is defined. Again, such a universal privilege d frame is not in the spirit of relativity, but it is also clearly not in contradiction: for example the reference frame in which the cosmic microwave background radiation is isotropic defines such a privileged frame. Hence, a priori, a hidden communication explanation is not more surprising than nonlocality. We prove that for any finite speed, such models predict correlations that can be exploited for faster-than-light communication. This superluminal communication doesnt require access to any hidden physical quantities, but only the manipulation of measurement devices at the level of our present-day description of quantum experiments. Consequently, all possible explanations of quantum correlations that satisfy the principle of continuity, which states that everything propagates gradually and continuously through space and time, or in other words, all combination of local common causes and direct causes that reproduce quantum correlations, lead to faster than light communication, which can be exploited by humans, at least in principle. Accordingly, either there is superluminal communication or the conclusion that Nature is nonlocal (i.e. discontinuous) is unavoidable.

1. Jean-Daniel Bancal, Stefano Pironio, Antonio Acin, Yeong-Cherng Liang, Valerio Scarani and Nicolas Gisin, Quantum non-locality based on finite-speed causal influences leads to superluminal signalling, Nature Physics, 8, 867-70, 2012. 2. N. Gisin, arXiv:1210.7308 3. Tomer Jack Barnea, Jean-Daniel Bancal, Yeong-Cherng Liang, Nicolas Gisin, A tripartite quantum state violating the hidden influence constraints, quant-ph/1304.1812, PRA in press.

This talk is part of the Isaac Newton Institute Seminar Series series.

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