  We investigate the use of photon number states to identify eavesdropping
attacks on quantum key distribution (QKD) schemes. The technique is based on
the fact that different photon numbers traverse a channel with different
transmittivity. We then describe two QKD schemes that utilize this method, one
of which overcomes the upper limit on the key generation rate imposed by the
dead time of detectors when using a heralded source of photons.
