  We propose a scheme for the ultrafast control of the emitter-field coupling
rate in cavity quantum electrodynamics. This is achieved by the control of the
vacuum field seen by the emitter through a modulation of the optical modes in a
coupled-cavity structure. The scheme allows the on/off switching of the
coupling rate without perturbing the emitter and without introducing frequency
chirps on the emitted photons. It can be used to control the shape of
single-photon pulses for high-fidelity quantum state transfer, to control Rabi
oscillations and as a gain-modulation method in lasers. We discuss two possible
experimental implementations based on photonic crystal cavities and on
microwave circuits.
