  A single photon source (SPS) is very important for quantum computation. In
particular, it is essential for secured quantum cryptography. But there is no
perfect SPS in reality. Therefore, probabilistic SPS where probability of
simultaneous emission of two, three, four and more photon is less than the
emission of a single photon are used. Since classical photon always comes in
bunch, the required single photon source must be nonclassical. In the
well-known antibunched state the rate of simultaneous emission of two photon is
less than that of single photon. But the requirement of quantum cryptography is
a multiphoton version of the antibunched state or the higher order antibunched
state. Recently we have reported a mathematical criterion for higher order
antibunching. Here we have shown that any proposal for SPS to be used in
quantum cryptography should satisfy this criterion. We have studied four wave
mixing as a possible candidate of single photon source.
