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Routing one photon with a few others
Single particles of light, photons, are ideal carriers of quantum information because they can travel far and fast and don't interact much with each other. However, this behavior has hampered attempts to control the propagation of single photons using all-optical setups. Shomroni et al. coupled a trapped atom to an optic fiber. That allowed them to control the polarization and propagation direction of a single photon in the fiber by controlling the atom's state (see the Perspective by Rempe). Because a faint pulse containing between 1.5 and 3 photons can switch the atom's state, the scheme provides a route to develop all-optical quantum networks.
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. We realized 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 atom coupled to a fiber-coupled, chip-based microresonator. A single reflected control photon toggles the switch from high reflection (R ~ 65%) to high transmission (T ~ 90%), with an average of ~1.5 control photons per switching event (~3, including linear losses). No additional control fields are required. The control and target photons are both in-fiber and practically identical, making this scheme compatible with scalable architectures for quantum information processing.