Spatial Interference Effect of Two-Photon in Femtosecond-PulsePumped Spontaneous Parametric Down-Conversion

We have used the transverse correlated properties of the entangled photon pairs generated in the process of spontaneous parametric down-conversion, which is pumped by a femtosecond pulse laser, to perform Young's interference experiment. Different from the case of continuous wave laser pump, broadband pulse laser pump can submerge an interference pattern. In order to obtain high visibility interference pattern, we used a lens with tunable focal length and two interference filters to eliminate the effects of broadband pump laser. It is proved that the process of two-photon direct interference is a post-selection process.