Stochastic resonance in a fundamental quantum system

We provide a detailed description of the response of an experimentally acce ssible, open, driven quantum system in its stationary state to a weak perio dic signal. The system, a single mode of the quantized radiation field in c oherent interaction with a sequence of two-level atoms and coupled to an en vironment of temperature T, is shown to exhibit stochastic resonance on out put, under changes of T. Furthermore, at vanishing temperature, the transit ion rates between the metastable states of the radiation field remain finit e and are set by the quantum noise level at T = 0 K. Consequently, signal e nhancement and synchronization can be achieved even in this very quantum li mit, for a properly tuned signal frequency.