  We investigate the transmission of both classical and quantum information
between two arbitrary observers in globally hyperbolic spacetimes using a
quantum field as a communication channel. The field is supposed to be in some
arbitrary quasifree state and no choice of representation of its canonical
commutation relations is made. Both sender and receiver possess some localized
two-level quantum system with which they can interact with the quantum field to
prepare the input and receive the output of the channel, respectively. The
interaction between the two-level systems and the quantum field is such that
one can trace out the field degrees of freedom exactly and thus obtain the
quantum channel in a nonperturbative way. We end the paper determining the
unassisted as well as the entanglement-assisted classical and quantum channel
capacities.
