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Daniel Salart Subils
Publication date
2008
fields
Physics
and research's language is
English
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This is the reply to the comment arXiv:0810.4452 by Kofler, Ursin, Brukner, and Zeilinger.
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In science, one observes correlations and invents theoretical models that describe them. In all sciences, besides quantum physics, all correlations are described by either of two mechanisms. Either a first event influences a second one by sending some information encoded in bosons or molecules or other physical carriers, depending on the particular science. Or the correlated events have some common causes in their common past. Interestingly, quantum physics predicts an entirely different kind of cause for some correlations, named entanglement. This new kind of cause reveals itself, e.g., in correlations that violate Bell inequalities (hence cannot be described by common causes) between space-like separated events (hence cannot be described by classical communication). Einstein branded it as spooky action at a distance. A real spooky action at a distance would require a faster than light influence defined in some hypothetical universally privileged reference frame. Here we put stringent experimental bounds on the speed of all such hypothetical influences. We performed a Bell test during more than 24 hours between two villages separated by 18 km and approximately east-west oriented, with the source located precisely in the middle. We continuously observed 2-photon interferences well above the Bell inequality threshold. Taking advantage of the Earths rotation, the configuration of our experiment allowed us to determine, for any hypothetically privileged frame, a lower bound for the speed of this spooky influence. For instance, if such a privileged reference frame exists and is such that the Earths speed in this frame is less than 10^-3 that of the speed of light, then the speed of this spooky influence would have to exceed that of light by at least 4 orders of magnitude.
We call a probabilistic theory complete if it cannot be further refined by no-signaling hidden-variable models, and name a theory spooky if every equivalent hidden-variable model violates Shimonys Outcome Independence. We prove that a complete theory is spooky if and only if it admits a pure steering state in the sense of Schrodinger. Finally we show that steering of complementary states leads to a Schrodingers cat-like paradox.
We stand by our findings in Phys. Rev A. 96, 022126 (2017). In addition to refuting the invalid objections raised by Peleg and Vaidman, we report a retrocausation problem inherent in Vaidmans definition of the past of a quantum particle.
In this Reply we propose a modified security proof of the Quantum Dense Key Distribution protocol detecting also the eavesdropping attack proposed by Wojcik in his Comment.
This reply contains a brief response to the comment by R. Howl, D. Ratzel, and I. Fuentes [arXiv:1811.10306]
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