Phys. Rev. Lett. 111, 193601 (2013)
A fibre-optic switch with a photon-number-dependent routing capability has been demonstrated by researchers at the Vienna University of Technology in Austria. Danny O'Shea and co-workers made their four-port switch by interfacing two optical fibre tapers (named bus and drop) with a high-Q-factor whispering-gallery bottle resonator that is coupled to a nearby trapped rubidium atom. In the switch's on state, all the light from the bus fibre is coupled to the drop fibre via the resonator. In contrast, in the off state, strong coupling between the atom and the resonator prevents light from entering the resonator; instead, the light remains in the bus fibre. When operating in its off state, the switch's routing depends on the number of photons, with the incoming stream of light sorted into single photons and pairs at the two output ports. In its current design, the switch had a fidelity of 0.62, but the researchers say that it should be possible to improve this to 0.95 by using a microresonator with a higher Q-factor and trapping the atoms closer to the resonator surface.
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