Cut-off rate performance of quantum communication channels with symmetric signal states

The cut-off rate performance of quantum communication channels is considere d when the signal quantum states are pure and symmetric. The properties of the quantum cut-off rate which is obtained by the entangled measurement of the received signal are investigated for communication channels in which th e symmetric quantum states are both linearly independent and linearly depen dent. It is found that the quantum cut-off rate becomes independent of the number of the signal quantum states when they are linearly dependent. Furth ermore the properties of the semiclassical cut-off rate which is obtained b y the disentangled measurement of the received signal are considered. The n ecessary and sufficient condition for the positive operator-valued measure of the disentangled measurement to maximize the semiclassical cut-off rate is derived. For binary quantum communication channels with pure signal stat es, the several parameters, such as the channel capacity and the cut-off ra te, which characterize the quantum communication channel are compared.