Cloud of virtual photons surrounding a two-level atom driven by an external field
G. Compagno, S. Vivirito, Franco Persico · Physical Review A · 1992
The distortions of the cloud of virtual photons surrounding a ground-state two-level atom, induced by an external field driving the atom, are investigated. It is first shown that if the driving field is coupled to the atom via diagonal dipole matrix elements, then a region of quasistability for the virtual cloud exists. Then the number of photons in this virtual cloud is evaluated by perturbation theory, as a function of the intensity of the external driving field. Finally, the analytical expression obtained is discussed using approximations appropriate to different ranges of the intensity of the driving field.