  Quantum-correlated photon sources provide a means of suppressing multiple
photon emission and thus improving the security and efficiency of quasi
single-photon quantum key distribution systems. We present illustrative
photon-counting statistics for a Poissonian source conditioned by photon number
measurements on a fully correlated twin beam with a non-ideal photon counter.
We show that high photon counting efficiency is needed to obtain significant
improvement in channel efficiency (bits per symbol) and/or security (in the
form of reduced Shannon entropy leakage) over a single beam source.
