  The nonclassical effect of photon anti-bunching is observed in the mixed
field of a narrow band two-photon source and a coherent field under certain
condition. A variety of different features in photon statistics are found to be
the consequence of a two-photon interference effect with dependence on the
relative phase of the fields. Besides the anti-bunching effect, we find another
one of the features to be also nonclassical. These features emphasize the
importance of quantum entanglement.
