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 = [ ħ ωk / ( 2 ε0 V )]1/2 is the electric field per photon, and < p.ε > is the dipole matrix element.