  We construct a relativistic quantum communication channel between two
localized qubit systems, mediated by a relativistic quantum field, that can
achieve the theoretical maximum for the quantum capacity in arbitrary curved
spacetimes using the Unruh-DeWitt detector formalism. Using techniques from
algebraic quantum field theory, we express the quantum capacity of the quantum
communication channel purely in terms of the correlation functions of the field
and the causal propagator for the wave equation. Consequently, the resulting
quantum channel, and hence the quantum capacity, are by construction manifestly
diffeomorphism-invariant, respect the causal structure of spacetime, and are
independent of the details of the background geometry, topology, and the choice
of Hilbert space (quasifree) representations of the field.
