  In this work we study nonperturbatively the transmission of classical and
quantum information in globally hyperbolic spacetimes, where the communication
channel is between two qubit detectors interacting with a quantized massless
scalar field via delta-coupling interaction. This interaction approximates very
rapid detector-field interaction, effectively occurring at a single instant in
time for each detector. We show that when both detectors interact via
delta-coupling, one can arrange and tune the detectors so that the channel
capacity is (at least) as good as the quantum channel constructed
nonperturbatively using \textit{gapless detectors} by Landulfo [PRD 93,
104019]. Furthermore, we prove that this channel capacity is in fact optimal,
i.e., both nonperturbative methods give essentially the same channel capacity,
thus there is a sense in which the two methods can be regarded as equivalent as
far as relativistic quantum communication is concerned.
