Information theory for key distribution systems secured by mesoscopic coherent states

This work analyzes a key distribution system presented earlier [Phys. Rev. A 68, 052307 (2003)] using elements of information theory. Mutual information functions describing the information content shared by Alice and Bob, the legitimate users, and between Alice and Eve, the attacker, are obtained. These results support the original claims that the overall security level to be obtained with the M-ry key distribution system can be made arbitrarily high and thus secure for any practical application. Effects of losses and amplifiers in the optical channel are also analyzed