Dispersion spreading of polarization-entangled states of light and two-photon interference

We study the interference structure of the second-order intensity correlation function for polarization-entangled two-photon light obtained from type-II collinear frequency-degenerate spontaneous parametric down-conversion (SPDC). The structure is visualised due to the spreading of the two-photon amplitude as two-photon light propagates through optical fibre with group-velocity dispersion (GVD). Because of the spreading, polarization-entangled Bell states can be obtained without any birefringence compensation at the output of the nonlinear crystal; instead, proper time selection of the intensity correlation function is required. A birefringent material inserted at the output of the nonlinear crystal (either reducing the initial o-e delay between the oppositely polarized twin photons or increasing this delay) leads to a more complicated interference structure of the correlation function.