Quantum signalling in cavity QED

TL;DR

This paper investigates quantum signalling between two-level systems in a cavity, revealing phase modulation's superiority over amplitude modulation in early-time communication and analyzing how mode truncations affect causality and modeling accuracy.

Contribution

It demonstrates that phase modulation outperforms amplitude modulation for quantum communication in cavity QED and establishes the necessary scaling of UV cutoffs to preserve causality in mode truncations.

Findings

Phase modulation surpasses amplitude modulation in early-time quantum signalling.

UV cutoff must scale polynomially with desired accuracy to preserve causality.

Amplitude modulation is generally sub-optimal for quantum communication in this setting.

Abstract

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.