We investigate the transmission of classical and quantum information between
three observers in a general globally hyperbolic spacetime using a quantum
scalar field as a communication channel. We build a model for a quantum
broadcast channel in which one observer (sender) wishes to transmit (classical
and quantum) information to two other observers (receivers). They possess some
localized two-level quantum system (a qubit) that can interact with the quantum
field in order to prepare an input or receive the output of this channel. The
field is supposed to be in an arbitrary quasifree state, the three observers
may be in arbitrary states of motion, and no choice of representation of the
field canonical commutation relations is made. The interaction of the field and
qubits is such that it allows us to obtain the map that describes this channel
in a non-perturbative manner. We conclude by analyzing the rates at which
information can be transmitted through this channel and by investigating
relativistic causality effects on such rates.Comment: 15 pages, 1 figur