  We consider quantum signalling between two-level quantum systems in a cavity,
in the pertubative regime of the earliest possible arrival times of the signal.
We present two main results: First we find that, perhaps surprisingly, the
analogue of amplitude modulated signalling (Alice using her energy eigenstates
|g>, |e>, as in the Fermi problem) is generally sub-optimal for communication.
Namely, e.g., phase modulated signalling (Alice using, e.g., |+>,|e>-states)
overcomes the quantum noise already at a lower order in perturbation theory.
Second, we study the effect of mode truncations that are commonly used in
cavity QED on the modelling of the communication between two-level atoms. We
show that, on general grounds, namely for causality to be preserved, the UV
cutoff must scale at least polynomially with the desired accuracy of the
predictions.
