  We present an interaction scheme that exhibits a dynamical consequence of
virtual photons carried by a vacuum-field dressed two-level atom in the
ultrastrong coupling regime. We show that, with the aid of an external driving
field, virtual photons provide a transition matrix element that enables the
atom to evolve coherently and reversibly to an auxiliary level accompanied by
the emission of a real photon. The process corresponds to a type of vacuum Rabi
oscillation, and we show that the effective vacuum Rabi frequency is
proportional to the amplitude of a single virtual photon in the ground state.
Therefore the interaction scheme could serve as a probe of ground state
structures in the ultrastrong coupling regime.
