Quantum nonlinear optics with single photons enabled by strongly interacting atoms

A cold, dense atomic gas is found to be optically nonlinear at the level of individual quanta, thereby opening possibilities for quantum-by-quantum control of light fields, including single-photon switching and deterministic quantum logic. 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. The realization of strong nonlinear interactions between individual light quanta (photons) is a long-standing goal in optical science and engineering 1 , 2 , being of both fundamental and technological significance. In conventional optical materials, the nonlinearity at light powers corresponding to single photons is negligibly weak. Here we demonstrate a medium that is nonlinear at the level of individual quanta, exhibiting strong absorption of photon pairs while remaining transparent to single photons. The quantum nonlinearity is obtained by coherently coupling slowly propagating photons 3 , 4 , 5 to strongly interacting atomic Rydberg states 6 , 7 , 8 , 9 , 10 , 11 , 12 in a cold, dense atomic gas 13 , 14 . Our approach paves the way for quantum-by-quantum control of light fields, including single-photon switching 15 , all-optical deterministic quantum logic 16 and the realization of strongly correlated many-body states of light 17 .