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I am intrigued by the following article:

Researchers began using photons in 1980s to test Bell's theory and determine if Einstein's reasoning is right or wrong. Since then, researchers have used various quantum states to test the theory but continued to have loopholes in their methods, therefore falling short of a definitive result. Luo said the new collaboration would, for the first time, be using several different quantum systems—including photons, ions, quantum dots and solid-state ensembles—to test the theory across large distances and hopefully eliminate all possible loopholes, he said.

Physics researchers join effort to finally complete quantum theory

I am familiar with Bell's inequalities, and I would like to know: how did the experimental results fall short? Why is this time going to be different?

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2 Answers 2

The main loopholes were the detection (efficiency) loophole and the locality (or communication) loophole ( ). I don't know why or if this time it's going to be different.

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The experiments are getting better every year, and all that was needed to close these loopholes is to have a large enough distance separation and to increase the efficiency of your detectors and your photon transmission high enough. Previously, with large enough separation to close the locality loophole, the photon loss was too large to avoid the detection loophole. We've been approaching the point at which the loopholes will be closed for the past few years, and it's almost guaranteed we'll pass it soon. –  Peter Shor Feb 2 '13 at 17:37
@Peter Shor: Thank you for your assessment. Actually, the last phrase of my answer was about the specific experimental group mentioned in the question. Interestingly, their time frame is five years. On the other hand, I often hear and read predictions that a loophole-free experiment will be conducted in a year or two and that a few experimental groups are trying to do that. However, I am afraid I am less optimistic about loophole-free demonstration of violations of the Bell inequalities. Let us wait and see. –  akhmeteli Feb 2 '13 at 18:50

Most experiments that have tested Bell's inequalities, have been carried out using pairs of photons 'flying' in opposite directions. Althouhg the experiment done by Alain Aspect involved photons from cascade decay of excited atomic states, experiments done these days have overcome this problem by using photons coming from a single process. Photons or particles, like electron-positron pairs, that are coming from a single quantum process are said to be in entangled states. It is in such entangled pairs of particles that the EPR argument has been tested through Bell's inequality. Experiments have also been designed and performed, to limit any possible 'communication' between pairs of particles, by randomly altering of the conditions (orientations) of the polarisers, just a 'split' second before the arrival of the particles at the polarisers. So far the experimental measurements, of photon spin correlations done over distances of about 100Km, have violated Bell's inequality, adding credit to the power of quantum mechanical prediction. The new experiments you are refering to, make the bold step to entangle three or more particles, in which the task will be to test the three or more parties correlations. Also, a very interesting aspect of the new experiments, is that they plan to generate hibrid entabgled states such as electrons and atoms, for example. These are the new elements in testing Bell's theorem in these experiments. The results from these experiments could lead to new studies of the evolution of the early universe.

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This does not seem to answer the question. I mean, it's a concise summary of the experimental history here, but it simply does not mention any of the criticisms which are still raised concerning the degree to which the experimental realizations truly match the condition used to derive the result. –  dmckee Mar 18 '13 at 16:40

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