The Transmission Rate for Certain Quantum Communications Channels

The author considers a block diagram for a quantum communications channel consisting of both classical and specifically quantum elements. The way in which the density matrix is transformed in the transmission line under the influence of attenuation and additive noise is examined. The conditional probabilities $w(r/s)$ formed in the receiver as a result of quantum measurement of $r$ are determined. For two particular forms of modulation-amplitude modulation and coherent modulation-and the corresponding methods of reception, the author computes the information capacity for fixed signal power at the channel input in the presence of thermal noise. Coherent modulation is found to have advantages over amplitude modulation.