Strongly interacting photons

A fine marriage between atomic and optical physics has produced a medium that is transparent to single photons but opaque to multiple photons. The finding heralds the development of devices such as single-photon switches. See Letter p.57 Single-photon quantum nonlinear optics In conventional optical materials, nonlinear interactions between single photons are negligibly weak. This paper demonstrates that a cold, dense atomic gas can be nonlinear at the level of individual quanta, exhibiting strong absorption of photon pairs, while remaining transparent to single photons. The approach opens up possibilities for quantum-by-quantum control of light fields, including single-photon switching and deterministic quantum logic. The authors suggest that it could also be extended to other material systems with strong interactions between their constituents that can be coupled to light.