High-visibility two-photon interference in femtosecond pulse pumped type-II SPDC
By far the most efficient source of two-particle entangled state is spontaneous parametric down-conversion (SPDC). Especially, cw laser pumped type-II SPDC has been widely treated as a good source of entangled photon pairs, since one can easily prepare a polarization Bell state . However, femtosecond pulse pumped type-II SPDC shows poor quantum interference visibility when a thick crystal is used. To restore the visibility, one has to use narrow-band spectral filters at the expense of available photon flux.