Entanglement-Assisted Communication of Classical and Quantum Information

In this paper, we consider the problem of transmitting classical and quantum information reliably over an entanglement-assisted quantum (EAQ) channel. Our main result is a capacity theorem that gives a 3-D achievable rate region. Points in the region are rate triples, consisting of the classical communication rate, the quantum communication rate, and the entanglement consumption rate of a particular coding scheme. The crucial protocol in achieving the boundary points of the capacity region is a protocol that we name the classically enhanced father (CEF) protocol. The CEF protocol is more general than other protocols in the family tree of quantum Shannon theoretic protocols, in the sense that several previously known quantum protocols are now child protocols of it. The CEF protocol also shows an improvement over a timesharing strategy for the case of a qubit dephasing channel-this result justifies the need for simultaneous coding of classical and quantum information over an EAQ channel. Our capacity theorem is of a multiletter nature (requiring a limit over many uses of the channel), but it reduces to a single-letter characterization for at least three channels: the completely depolarizing channel, the quantum erasure channel, and the qubit dephasing channel.