Virtual photons and causality in the dynamics of a pair of two-level atoms

The dynamics of an initially bare pair of two-level atoms at distance R is investigated. The pair is coupled to the vacuum radiation field in the multipolar scheme and in the dipole approximation. The Heisenberg equations of motion for various atomic operators are obtained neglecting terms O(${\mathit{e}}^3$) and for t smaller than the spontaneous relaxation time. A rigorous proof of causality in the atom-atom interaction is given. Interatomic correlations, however, are shown to develop for t<R/c. This early development is shown to be compatible with the principle of relativistic causality, and to be related to the peculiar form of the vacuum zero-point fluctuations. The techniques adopted are applied to study dynamics and correlations of a superradiant state.