The prospect of quantum networks, in which quantum information is carried bysingle photons in photonic circuits, has long been the driving force behind theeffort to achieve all-optical routing of single photons. Here we realize themost 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 twooutputs. Our device is based on a single 87Rb atom coupled to a fiber-coupled,chip-based microresonator, and is completely all-optical, requiring no otherfields beside the in-fiber single-photon pulses. Nonclassical statistics of thecontrol pulse confirm that a single reflected photon toggles the switch fromhigh reflection (65%) to high transmission (90%), with average of ~1.5 controlphotons per switching event (~3 including linear losses). The fact that thecontrol and target photons are both in-fiber and practically identical makesthis scheme compatible with scalable architectures for quantum informationprocessing.
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