  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 ".
