Laser-Induced Line Shifts and Double-Quantum Lamb Dips

There has been recently considerable interest in light-induced resonance frequency shifts of atoms and molecules, since they can give rise to systematic errors in high-resolution spectroscopy. Light shifts occur when atomic or molecular oscillators interact with off-resonant light fields. It is useful to think of these shifts as being generated by the exchange of virtual quanta between the light field and the oscillator [1J. From this point of view one may acknowledge that they constitute a class of fundamental phenomena of intrinsic importance rather than a mere experimental nuisance.