Quantum incoherence and entanglement in cascaded-crystal parametric down-conversion

Summary form only given. A great deal of research has been carried out over the years with the help of entangled photon states. Studies range from the examination of quantum paradoxes to practical applications in optical measurements and quantum information. A standard source of entangled photons consists of a nonlinear crystal without a center of symmetry pumped by a highly coherent laser. This combination provides a medium for the generation of photon pairs that can be entangled in any number of variables: time, frequency, direction of propagation, and polarization. We present an experimental and theoretical study of the fourth-order coherence properties of entangled-photon states generated in a nonlinear optical medium comprising two separate nonlinear crystals pumped by the same highly coherent and monochromatic laser source. We demonstrate that the nature of the down-conversion process insures the existence of nonclassical correlations and stable phase relations (coherence) between the photons only in each pair and nor between pairs generated from physically distinct regions of nonlinear interaction. This result has been verified in a set of quantum-interference experiments without invoking any spectral filtering.