We prove additivity of the minimum output entropy and the Holevo capacity for rotationally invariant quantum channels acting on spin-1/2 and spin-1 systems. The physical significance of these channels and their relations to other known channels is also discussed.
Additivity of the Holevo capacity is proved for product channels, under the condition that one of the channels is a unital qubit channel, with the other completely arbitrary. As a byproduct this proves that the Holevo bound is the classical information capacity of such qubit channels, and provides an explicit formula for this classical capacity. Additivity of minimal entropy and multiplicativity of p-norms are also proved under the same assumptions. The proof relies on a new bound for the p-norm of an output state from the half-noisy phase-damping channel.
Additivity of the minimal output entropy for the family of transpose depolarizing channels introduced by Fannes et al.5 is considered. It is shown that using the method of Ref. 3 allows us to prove the additivity for the range of the parameter values for which the problem was left open in Ref. 5. Together with the result of Ref. 5, this covers the whole family of transpose depolarizing channels. In addition, the multiplicativity of the maximal p-norm for this family of channels is proved for all 1 ≤ p ≤ 2.