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Generalized Unruh effect and Lamb shift for atoms on arbitrary stationary trajectories

We study the spontaneous de-excitation and excitation of accelerated atoms on arbitrary stationary trajectories (`generalized Unruh effect'). We consider the effects of vacuum fluctuations and radiation reaction separately. We show that radiation reaction is generally not altered by stationary acceleration, whereas the contribution of vacuum fluctuations differs for all stationary accelerated trajectories from its inertial value. Spontaneous excitation from the ground state occurs for all accelerated stationary trajectories and is therefore the `normal case'. Furthermore, we show that the radiative energy shift (`Lamb shift') of a two-level atom is modified by acceleration for all stationary trajectories. Again only vacuum fluctuations give rise to the shift. Our results are illustrated for the special case of an atom in circular motion, which may be relevant experimentally.