Identifying entanglement using quantum ghost interference and imaging.

We report a quantum interference and imaging experiment which allows identifying the entanglement in momentum and position variables of a two-photon system. The measurements show indeed that the uncertainties in the sum of momenta and in the difference of positions of the entangled two-photon satisfy both EPR inequalities Delta(k(1)+k(2))<min((Delta k(1),Delta k(2)) and Delta(x(1)-x(2))<min((Delta x(1),Delta x(2)). These two inequalities, together, represent a nonclassicality condition. Our measurements provide a direct way to distinguish between quantum entanglement and classical correlation in continuous variables for systems of pairs of photons.