Identifying entanglement using quantum "ghost" interference and imaging

Abstract

We report a quantum interference and imaging experiment which quantitatively demonstrates that Einstein-Podolsky-Rosen (EPR) type entangled two-photon states exhibit both momentum-momentum and position-position correlations, stronger than any classical correlation. 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 D(k1+k2)<min(D(k1),D(k2)) and D(x1-x2)<min(D(x1),D(x2)). These two inequalities, together, represent a non-classicality condition. Our measurements provide a direct way to distinguish between quantum entanglement and classical correlation in continuous variables for two-photons/two photons systems.Comment: We have changed Eq.(2) from one inequality to two inequalities. The two expressions are actually consistent with each other, but the new one represents a more stringent condition for entanglement and, in our opinion, better explains the original idea of EPR. We have clarified this point in the paper. 4 pages; submitted to PR