The phase operator in quantum information processing

It is shown that unitary depolarizers, which play an important role in quantum information processing, can be constructed in terms of the Pegg-Barnett phase operator. By using the result, the classical information capacity of quantum dense coding with unitary encoding in a finite-dimensional Hilbert space is derived. Furthermore, the relation between the capacity of quantum dense coding and the coherent information of a noisy quantum channel is obtained.