Two photon interference and optical free induction decay
  The two photon interference phenomenon is theoretically investigated for the general situations with an arbitrary input two photon state with and without photon polarization. For the case without polarization, the necessary-sufficient condition for the destructive interference of coincidence counting is given as the symmetric pairing of photons in the light pulses. For both case it is shown that the "dip" in coincidence curve can be understood in terms of the free induction decay mechanism. This observation predicts the destructive interference phenomenon to occur even for certain cases with separable input two photon state, but it can only be explained in terms of "the two photon (not two photons)interference ". 
P.Zhang, C.P.Sun