Letter

Nature 454, 861-864 (14 August 2008) | doi:10.1038/nature07121; Received 2 April 2008; Accepted 30 May 2008

Testing the speed of 'spooky action at a distance'

Daniel Salart1, Augustin Baas1, Cyril Branciard1, Nicolas Gisin1 & Hugo Zbinden1

  1. Group of Applied Physics, University of Geneva, 20 Rue de l'Ecole de Médecine, CH-1211 Geneva 4, Switzerland

Correspondence to: Daniel Salart1 Correspondence and requests for materials should be addressed to D.S. (Email: daniel.salart@physics.unige.ch).

Correlations are generally described by one of two mechanisms: either a first event influences a second one by sending information encoded in bosons or other physical carriers, or the correlated events have some common causes in their shared history. Quantum physics predicts an entirely different kind of cause for some correlations, named entanglement. This reveals itself in correlations that violate Bell inequalities (implying that they cannot be described by common causes) between space-like separated events (implying that they cannot be described by classical communication). Many Bell tests have been performed1, and loopholes related to locality2, 3, 4 and detection5, 6 have been closed in several independent experiments. It is still possible that a first event could influence a second, but the speed of this hypothetical influence (Einstein's 'spooky action at a distance') would need to be defined in some universal privileged reference frame and be greater than the speed of light. Here we put stringent experimental bounds on the speed of all such hypothetical influences. We performed a Bell test over 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 two-photon interferences well above the Bell inequality threshold. Taking advantage of the Earth's rotation, the configuration of our experiment allowed us to determine, for any hypothetically privileged frame, a lower bound for the speed of the influence. For example, if such a privileged reference frame exists and is such that the Earth's speed in this frame is less than 10-3 times that of the speed of light, then the speed of the influence would have to exceed that of light by at least four orders of magnitude.

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