A vacuum Rabi oscillation is a damped oscillation of an initially excited atom coupled to an electromagnetic resonator or cavity in which the atom alternately emits photon(s) into a single-mode electromagnetic cavity and reabsorbs them. The atom interacts with a single-mode field confined to a limited volume V in an optical cavity. Spontaneous emission results as a consequence of coupling between the atom and the vacuum fluctuations of the cavity field. The vacuum Rabi frequency is given by
where R is the location of the atom, f ( R ) = exp ( i k.R) for plane-wave fields, ε is the field polarization,