Coherence of entangled-photon pairs generated from separate crystals using a highly monochromatic laser pump

Summary form only given. We have experimentally demonstrated that the two-photon probability amplitudes of photon pairs generated using spontaneous parametric down conversion from separate nonlinear crystals, under the conditions outlined above, remain independent and cannot be used to engineer more complex quantum states. The spontaneous nature of the down-conversion process prevents pairs of photons generated in unconnected regions of interaction from having a stable phase relationship between components of their probability amplitudes. The experiment has been performed using two BaB/sub 2/O/sub 4/ nonlinear crystals, of lengths 0.5 mm and 1.5 mm respectively. They have been cut to generate phase-matched type-II collinear down conversion.