QLunch:Dephrasure channel and superadditivity of coherent information

Speaker: Graeme Smith from University of Colorado

Title:
Dephrasure channel and superadditivity of coherent information

Abstract:
The quantum capacity of a quantum channel captures its capability for noiseless quantum communication. It lies at the hears of quantum information theory. Unfortunately, our poor understanding of nonadditivity of coherent information makes it hard to understand the quantum capacity of all but very special channels. In this paper, we consider the dephrasure channel, which is the concatenation of a dephasing channel and an erasure channel. This very simple channel displays remarkably rich and exotic properties: we find nonadditivity of coherent information at the two-letter level, a substantial gap between the threshold for zero quantum capacity and zero single-letter coherent information, a big gap between single-letter coherent and private informations. Its clean form simplifies the evaluation of coherent information substantially and, as such, we hope that the ephrasure channel will provide a much-needed laboratory for the testing of new ideas about nonadditivty.