Thursday, August 14, 2008

Testing The Speed Of "Spooky Action At A Distance"

Another extremely important and elegant experiment on quantum entanglement out of the University of Geneva. This time, Nicolas Gisin group[1] tried to put a lower limit on the speed of any possible signal (still undetected, if any) that communicates between a pair of entangled photon.

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, and loopholes related to locality and detection 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.

Don't miss a News and Views article on this in the same issue of Nature. You can read the PhysicsWorld review of this work here, and the news report on it by Science. The comments from the community so far as been unanimous:

Although the research doesn't demonstrate spooky action at a distance directly, it does provide "a lower boundary for the speed" necessary for the phenomenon, says theoretical physicist Martin Bojowald of Pennsylvania State University in State College. Cosmologist Sean Carroll of the California Institute of Technology in Pasadena says that it's "yet another experiment that tells us quantum mechanics is right" and that there "really is an intrinsic connection between entangled particles, not that some signal passes quickly between them when an observation is performed." And physicist Lorenza Viola of Dartmouth College says there's much more to be determined. "I am sure we are not finished unveiling what the quantum [effects] due to entanglement really are and how powerful they can be."


This is another one of the example where the more they test it, the most convincing it becomes.

Zz.

[1] D. Salart et al., Nature v.454, p.861 (2008).

1 comment:

Peter Morgan said...

We're convinced already that states and linear operators in a Hilbert space formalism lets us construct good models.

We're convinced already that classical particle property models do not work (the loopholes can be jumped through, but the models we get are too ad-hoc, nonlocal, contextual, or lack Lorentz invariance; just not nice).

I'm not yet convinced that interesting random field models in which there just are correlations in the initial conditions (called entanglement if we're being open-minded, or conspiracy if we're not) are impossible. Because there are no particles in such a model, there is nothing to send signals, so this present focus of attention in Nature is not much to the point.

The opening sentences are striking in that they do not mention particles at all, "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 is somewhat remarkable in its form, 1 causes 2, 3 causes 1 and 2, or 1 and 2 just happen (which is generally called entanglement, but we could also call it delicately tuned experimental construction, caused by the experimenter).

Later paragraphs in the paper (after your selection) talk about sending photons down the fiber optics, business as usual. That they are entangled photons explains everything nicely.