SOURCE OF PHOTONS WITH CORRELATED POLARIZATIONS AND CORRELATED DIRECTIONS

We propose a source of photon pairs which uses two nonlinear crystals to produce pairs of photons (by parametric down conversion) that have correlated polarisations with the useful property that one photon of the pair always goes in one specific direction and the other photon always goes in another specific direction. This source can be used instead of an atomic cascade in experiments to test Bell's inequalities.