A router capable of guiding single photons could overcome challenges associated with building quantum computers. A team from the Weizmann Institute of Science developed the device, which is based on a single atom connected to a fiber-coupled, chip-based microresonator. Depending on the atom’s state, it either transmits or reflects incoming photons. With the atom in one state, the researchers said, a photon originating from the right will continue on its path toward the left. Alternately, a photon coming from the left is reflected back; this also reverses the atomic state. The complete assembly of the new photonic router. Courtesy of Weizmann Institute of Science. In the reversed state, the atom allows photons coming from the left to continue toward the right, while any photons from the right are reflected back. This flips the atom back again. The researchers said the atom toggles from a state of high reflection (R ~ 65 percent) to high transmission ( T ~ 90 percent), with an average of ~ 1.5 photons per switching event, or ~ 3 including linear losses. “In a sense, the device acts as the photonic equivalent to electronic transistors, which switch electric currents in response to other electric currents,” said Dr. Barak Dayan, head of the Weizmann Institute's quantum optics group. He added that a single atom essentially functions as a transistor for the photons, while the photons control the router and also comprise the information it carries. Photons could serve as a router between quantum systems, the researchers said, as they do not interact with each other and their interaction with other particles is weak. “The road to building quantum computers is still very long, but the device we constructed demonstrates a simple and robust system, which should be applicable to any future architecture of such computers,” Dayan said. “In our future experiments, we hope to expand the kinds of devices that work solely on photons; for example, new kinds of quantum memory or logic gates.” The work was funded by the Benoziyo Endowment Fund for the Advancement of Science. The research was published in Science (doi: 10.1126/science.1254699). For more information, visit www.weizmann.ac.il.