What are the “loopholes” in past Bell's theorem experiments?

Question

I am intrigued by the following Phys.org 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?

Answer 1

The main loopholes were the detection (efficiency) loophole and the locality (or communication) loophole (http://en.wikipedia.org/wiki/Loopholes_in_Bell_test_experiments ). I don't know why or if this time it's going to be different.

Answer 2

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.