Quantum interference inside a nonlinear crystal with spontaneous parametric down-conversion

Abstract We report the experimental observation of two-photon interference which takes place inside a type-I nonlinear crystal with spontaneous parametric down-conversion. The interference originates from the non-collinear configuration of two down-converted photons for both frequency degenerate and non-degenerate cases. The symmetry of the two-photon frequency spectrum and spatial spectrum dominates the interference visibility. The present work could provide more experimental evidence of quantum properties, which is originally formed inside a two-photon entangled source.