Virtual Field, Causal Photon Absorption and Photodetectors
Giuseppe Compagno, G. Massimo Palma, Roberto Passante, Franco Persico · Europhysics Letters (EPL) · 1989
The time-dependent electric-energy density surrounding a two-level atom fixed at r = 0 is studied, the atom being taken in its ground state at t = 0 and the field having initially only one photon in a delocalized mode. The atom-field coupling includes both rotating and counterrotating terms. The energy density in the rotating wave approximation is shown to behave noncausally, while in the presence of the complete coupling it is shown to be affected only within a sphere of radius r = ct centred on the atom. It is concluded that the counterrotating terms in the atom-field coupling are essential in order to ensure causality and cannot be neglected in any accurate treatment of photon absorption. Some consequences of this conclusion on the operation of photodetectors in one-photon absorption are discussed.