The capacity of the quantum channel with general signal states
Alexander Semenovich Holevo · IEEE Transactions on Information Theory · 1998
It is shown that the capacity of a classical-quantum channel with arbitrary (possibly mixed) states equals the maximum of the entropy bound with respect to all a priori distributions. This completes the recent result of Hausladen, Jozsa, Schumacher, Westmoreland, and Wootters (1996), who proved the equality for the pure state channel.