Nanophotonic quantum phase switch with a single atom

By analogy to transistors in classical electronic circuits, quantum optical switches are important elements of quantum circuits and quantum networks(1-3). Operated at the fundamental limit where a single quantum of light or matter controls another field or material system(4), such a switch may enable applications such as long-distance quantum communication(5), distributed quantum information processing(2) and metrology(6), and the exploration of novel quantum states of matter(7). Here, by strongly coupling a photon to a single atom trapped in the near field of a nanoscale photonic crystal cavity, we realize a system in which a single atom switches the phase of a photon and a single photon modifies the atom's phase. We experimentally demonstrate an atom-induced optical phase shift(8) that is nonlinear at the two-photon level(9), a photon number router that separates individual photons and photon pairs into different output modes(10), and a single-photon switch in which a single 'gate' photon controls the propagation of a subsequent probe field(11,12). These techniques pave the way to integrated quantum nanophotonic networks involving multiple atomic nodes connected by guided light.