Communication between observers in a relativistic scenario has proved to be
a setting for a fruitful dialogue between quantum field theory and quantum
information theory. A state that an inertial observer in Minkowski space
perceives to be the vacuum will appear to an accelerating observer to be a
thermal bath of radiation. We study the impact of this Davies-Fulling-Unruh
noise on communication, particularly quantum communication from an inertial
sender to an accelerating observer and private communication between two
inertial observers in the presence of an accelerating eavesdropper. In both
cases, we establish compact, tractable formulas for the associated
communication capacities assuming encodings that allow a single excitation
in one of a fixed number of modes per use of the communications channel.