Optimal signal ensembles - art. no. 022308

Classical messages can be sent via a noisy quantum channel in various ways, corresponding to various choices of ensembles of signal states of the chan nel. 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 cap acity, using the relative entropy "distance" between density operators to g ive the results a geometric flavor.