Optimal signal ensembles

Classical messages can be sent via a noisy quantum channel in various ways, corresponding to various choices of ensembles of signal states of the channel. Previous work by Holeva and by Schumacher and Westmoreland relates the capacity of the channel to the properties of the signal ensemble. Here we describe some properties characterizing the ensemble that maximizes the capacity, using the relative entropy distance between density operators to give the results a geometric flavor.