The Fermi problem in discrete systems

The Fermi two-atom problem illustrates an apparent causality violation in quantum field theory that has to do with the nature of the built-in correlations in the vacuum. It has been a constant subject of theoretical debate and discussion in the last few decades. Nevertheless, although the issues at hand could in principle be tested experimentally, the smallness of such apparent violations of causality in quantum electrodynamics have prevented the observation of the predicted effect. In this paper, we show that the problem can be simulated within the framework of discrete systems that can be manifested, for instance, by trapped atoms in optical lattices or trapped ions. Unlike the original continuum case, the causal structure is no longer sharp. Nevertheless, as we show, it is possible to distinguish between 'trivial' effects due to 'direct' causality violations and the effects associated with Fermi's problem, even in such discrete settings. The ability to control externally the strength of the atom–field interactions enables us also to study both the original Fermi problem with 'bare atoms' and correction in the scenario that involves 'dressed' atoms. Finally, we show that, in principle, the Fermi effect can be detected using trapped ions.