CLASSICAL INFORMATION CAPACITY OF A QUANTUM CHANNEL

We consider the transmission of classical information over a quantum c hannel. The channel is defined by an ''alphabet'' of quantum states, e .g., certain photon polarizations, together with a specified set of pr obabilities with which these states must be sent. If the receiver is r estricted to making separate measurements on the received ''letter'' s tates, then the Kholevo theorem implies that the amount of information transmitted per letter cannot be greater than the von Neumann entropy H of the letter ensemble. In fact the actual amount of transmitted in formation will usually be significantly less than H. We show, however, that if the sender uses a block coding scheme consisting of a choice of code words that respects the a priori probabilities of the letter s tates, and the receiver distinguishes whole words rather than individu al letters, then the information transmitted per letter can be made ar bitrarily close to H and never exceeds H. This provides a precise info rmation-theoretic interpretation of von Neumann entropy in quantum mec hanics. We apply this result to ''superdense'' coding, and we consider its extension to noisy channels.