Efficient conversion between photons and between photon and atom by stimulated emission

Quantum-state conversion among different information-carrying media is essential in a quantum network. It is found that the many schemes of quantum-state conversion can be unified under one simple process---that is, the parametric process of three-wave mixing. These schemes include quantum-state storage in atomic media, where a photonic state is converted to an atomic Raman state and vice versa, the frequency up- and down-conversion in photonic states, and photonic-state conversion to Brillouin phononic states and vice versa. The efficiency of conversion can in principle reach 100%, and the conversion process is noiseless. When the same idea is applied to a four-wave mixing process, unit conversion efficiency between a two-photon state and a single-photon state can be achieved. When applied to a two-photon entangled state in parametric down-conversion, entangled states of multiple photons can be generated.