A new approach for a loophole-free Bell experiment

by Prof. Adan Cabello (University of Sevilla)

Wednesday 13 Dec 2006, 11:00 → 12:00 Europe/Stockholm

FB31 (AlbaNova)

Experiments to test Bell inequalities have fallen within quantum mechanics and, under additional assumptions, exclude local realistic theories. However, so far, any performed Bell experiment still admits an interpretation in terms of local hidden-variable theories. 42 years after Bell's original paper, we still do not have a loophole-free Bell experiment. A particularly relevant loophole in experiments with photons is the detection loophole. It arises because only a small subset of all the created pairs are actually detected, so we need to assume that the undetected photons behave like the detected ones. However, this fair sampling assumption excludes hidden variables a priori. Avoiding the fair sampling assumption using the standard Clauser-Horne-Shimony-Holt Bell inequality would require an overall photo-detection efficiency higher than 0.75. However, the best of currently available photo-detection efficiency is 0.33, and is expected to go up to 0.60 in near future. We present a proposal for a loophole-free experiment with these near future photo-detection efficiencies. It is based on the observation that the premise of the original Bell inequality is not locality, but Einstein, Podolsky, and Rosen's (EPR's) criterion for the existence of elements of reality. It turns out to be that there are bipartite Bell inequalities for EPR-type theories requiring lower minimum detection efficiencies. For example, if we can prepare pairs of photons entangled in four degrees of freedom, then photo-detection efficiencies in the range 0.40-0.57 would suffice for closing the detection loophole.