  The prospect of quantum networks, in which quantum information is carried by
single photons in photonic circuits, has long been the driving force behind the
effort to achieve all-optical routing of single photons. Here we realize the
most basic unit of such a photonic circuit: a single-photon activated switch,
capable of routing a photon from any of its two inputs to any of its two
outputs. Our device is based on a single 87Rb atom coupled to a fiber-coupled,
chip-based microresonator, and is completely all-optical, requiring no other
fields beside the in-fiber single-photon pulses. Nonclassical statistics of the
control pulse confirm that a single reflected photon toggles the switch from
high reflection (65%) to high transmission (90%), with average of ~1.5 control
photons per switching event (~3 including linear losses). The fact that the
control and target photons are both in-fiber and practically identical makes
this scheme compatible with scalable architectures for quantum information
processing.
