Spontaneous emission as a bridge from Lindbladian to nonreciprocal reservoirs

Abstract

We study an out-of-equilibrium quantum system in which a state connecting two reservoirs is also coupled by stimulated and spontaneous emission of photons to an antitrapped state, thus implementing particle loss. After revisiting the spontaneous emission process, we show that the proper effective description of such a system requires one to go beyond the usual Lindbladian formalism and includes a nonreciprocal (``non-Hermitian'') coupling to the reservoir modeling the untrapped state. The presence of both, the reservoirs and the nonreciprocal coupling, have observable consequences that we compute, for example, by looking at the quantum Zeno effect in the loss current. We discuss the connection of our findings to possible experiments in cold atomic gases.