Entangled states maximize the two qubit channel capacity for some Pauli channels with memory

摘要

We prove that a general upper bound on the maximal mutual information ofquantum channels is saturated in the case of Pauli channels with an arbitrarydegree of memory. For a subset of such channels we explicitly identify theoptimal signal states. We show analytically that for such a class of channelsentangled states are indeed optimal above a given memory threshold. It isnoteworthy that the resulting channel capacity is a non-differentiable functionof the memory parameter.