Long distance Bell experiment

The discussion about the "paradox" argued by Einstein, Podolsky and Rosen in 1935 and Bell's inequalities are still alive even after many experimental tests of increasing accuracy. (Check out the Physics FAQ for a more thorough description of the problem) Of all these tests the one performed by Aspect and coworkers is the most famous one because it tackled the question of timing and locality (also known as Einstein locality). In one of his later works on the problem John Bell expressed his interest in this issue:

"Of more importance, in my opinion, is the complete absence of the vital time factor in existing experiments. The analyzers are not rotated during the flight of the particles. Even if one is obliged to admit some long-range influence, it need not travel faster than light -- and so would be much less indigestible."

Because of the technical limitations at that time, however, Aspect and coworkers could not fully enforce the locality condition, mainly because they used periodic, predictable switching. Our goal in the experiment presented here was to exclude any possibility of classical (slower than the speed of light) communication between the observers. The criterion is that the measurement processes on each side (choosing an analyzer direction, setting the analyzer and detecting a photon) on both sides of the experiment should be spacelike separated.

We are have achieved this condition by a new experiment that uses type-II parametric down-conversion as a source and fast random switching of the analyzers, which are separated by about 400 meters across the natural sciences campus in Innsbruck. 

G. Weihs, T. Jennewein, C. Simon, H. Weinfurter and A. Zeilinger

Violation of Bell's inequality under strict Einstein locality conditions
PRL 81 (21), 5039-5043 (1998)