We reexamine the dispersion interaction between two atoms in external electromagnetic fields, provided the atoms are not in the equilibrium with the field. We show that the processes of absorption and spontaneous/stimulated emission should be taken into account in the case of non-equilibrium dynamics.
The van der Waals--Casimir-Polder potential is calculated for two atoms out of equilibrium with external thermal electromagnetic field. The effects of absorption and spontaneous or stimulated emission are shown to strongly modify the potential. We demonstrate that the potential decays like ${R}^{\ensuremath{-}2}$ at large distances (much slower than at the equilibrium) can be attractive, repulsive, and resonant depending on the initial states of the atoms and electromagnetic field.
We consider the Casimir-Polder interaction between two atoms, one in the ground state and the other in its excited state. The interaction is time dependent for this system, because of the dynamical self-dressing and the spontaneous decay of the excited atom. We calculate the dynamical Casimir-Polder potential between the two atoms using an effective Hamiltonian approach. The results obtained and their physical meaning are discussed and compared with previous results based on a time-independent approach, which uses a nonnormalizable dressed state for the excited atom.