Quantum electrodynamics of accelerated atoms in free space and in confined cavities

We consider a gedanken experiment with a beam of atoms in their ground state that are accelerated through a single-mode microwave cavity. We show that taking into account of the ''counter-rotating'' terms in the interaction Hamiltonian leads to the excitation of an atom with simultaneous emission of a photon into a field mode. In the case of a slow switching on of the interaction, the ratio of emission and absorption probabilities is exponentially small and is described by the Unruh factor. In the opposite case of sharp cavity boundaries the above ratio is much greater and radiation is produced with an intensity which can exceed the intensity of Unruh acceleration radiation in free space by many orders of magnitude. In both cases real photons are produced, contrary to the opinion that a uniformly accelerated atom does not radiate. The cavity field at steady state is described by a thermal density matrix. However, under some conditions laser gain is possible. We present a detailed discussion of how the acceleration of atoms affects the generated cavity field in different situations, progressing from a simple physical picture of Unruh radiation to more complicated situations.