We consider the scenario in which Alice transmits private classical messages
to Bob via a classical-quantum channel, part of whose output is intercepted by
an eavesdropper, Eve. We prove the existence of a universal coding scheme under
which Alice's messages can be inferred correctly by Bob, and yet Eve learns
nothing about them. The code is universal in the sense that it does not depend
on specific knowledge of the channel. Prior knowledge of the probability
distribution on the input alphabet of the channel, and bounds on the
corresponding Holevo quantities of the output ensembles at Bob's and Eve's end
suffice.Comment: 31 pages, no figures. Published versio
