Entanglement-enhanced classical capacity of two-qubit quantum channels with memory: the exact solution

摘要

The maximal amount of information which is reliably transmitted over two usesof general Pauli channels with memory is proven to be achieved by maximallyentangled states beyond some memory threshold. In particular, this proves aconjecture on the depolarizing channel by Macchiavello and Palma [Phys. Rev. A{\bf 65}, 050301(R) (2002)]. Below the memory threshold, for arbitrary Paulichannels, the two-use classical capacity is only achieved by a particular typeof product states.