A solvable open quantum system: The strongly driven micromaser

A micromaser, where an additional classical field strongly drives the atoms crossing the cavity, provides an example of a solvable open quantum system with pumping and dissipation. The coherent time evolution consists in cavity field displacements correlated with atomic internal states, a different entanglement than the usual Jaynes-Cummings interaction. We provide analytical. results and phase space descriptions via the quantum characteristic function. We illustrate bunching and antibunching effects at steady state in atom pair correlation measurements, where the control parameter is simply the atomic velocity.