Thermodynamics of a Non-Equilibrium Quantum System — Resonance Fluorescence

Abstract

The problem of resonance fluorescence from a two-level atom is the subject of intense current research activity [1]. One aspect that has so far received little comment is that it provides a most interesting example of nonequilibrium thermodynamics in a quantum system. Conceptually the situation is simple. The two-level atom is subject to two interactions (a) a coherent interaction with a driving field, (b) an incoherent interaction with a thermal reservoir (spontaneous emission into the radiation field). If the driving field were absent the atom would merely relax into equilibrium with the reservoir and the usual Wigner Weisskopf theory for a damped atom would apply. The effect of the driving field is to maintain the atom away from equilibrium in a new nonequilibrium steady state.