A Note on Quantum Key Channel Efficiency and Security using Correlated   Photon Beam Transmitters
  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. 
PJ Edwards, W N Cheung, H van Pham, G Ganesharajah, P Lynam, and L   Barbopoulos