# Testing the speed of 'spooky action at a distance'

 Authors: D Salart, A Baas, C Branciard, N Gisin, H Zbinden Nature 454, 861–864 (2008) http://dx.doi.org/10.1038/nature07121 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 performed(1), and loopholes related to locality(2-4) and detection(5,6) 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. salartbellnature.pdf

# BibTeX Source

@Article{Salart2008,
author =       "D. Salart and A. Baas and C. Branciard and N. Gisin and H. Zbinden",
title =        "Testing the speed of 'spooky action at a distance'",
journal =      "Nature",
year =         "2008",
volume =       "454",
pages =        "861--864",
number =       "7206",
abstract =     "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 performed(1), and loopholes related to
locality(2-4) and detection(5,6) 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.",
doi =          "10.1038/nature07121",
owner =        "cc",
sn =           "0028-0836",
timestamp =    "2010.08.20",
ut =           "WOS:000258398600030",
}