  We show that a thermal reservoir can effectively act as a squeezed reservoir
on atoms that are subject to energy-level modulation. For sufficiently fast and
strong modulation, for which the rotating-wave-approximation is broken, the
resulting squeezing persists at long times. These effects are analyzed by a
master equation that is valid beyond the rotating wave approximation. As an
example we consider a two-level-atom in a cavity with Lorentzian linewidth,
subject to sinusoidal energy modulation. A possible realization of these
effects is discussed for Rydberg atoms.
