Single-atom quantum gate for light

We propose a scheme for an optical quantum gate, i.e., a mirror in a quantum superposition state of total transmission and reflection. The model consists of a high-$Q$ optical cavity strongly coupled to a single atom or ion in a quantum superposition state of two long lived internal states. By including spontaneous emission, dissipation, measurement back-action, and the time dependence of the atom-field coupling, we numerically simulate a system very close to experimental reality. The switching and quantum coherence properties are confirmed separately in two steps by using the gate directly and as an entrance beam splitter to an interferometer.