  In analogy to transistors in classical electronic circuits, a quantum optical
switch is an important element of quantum circuits and quantum networks.
Operated at the fundamental limit where a single quantum of light or matter
controls another field or material system, it may enable fascinating
applications such as long-distance quantum communication, distributed quantum
information processing and metrology, and the exploration of novel quantum
states of matter. 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
where 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 that is nonlinear at the two-photon level, a photon number
router that separates individual photons and photon pairs into different output
modes, and a single-photon switch where a single "gate" photon controls the
propagation of a subsequent probe field. These techniques pave the way towards
integrated quantum nanophotonic networks involving multiple atomic nodes
connected by guided light.
