Fiber optic switches play a vital role in optical communication networks, enabling optical signals to be rerouted to different fiber output ports. Making these switches small enough to manipulate light would enable quantum communication and information technology. A new demonstration of such a switch shows that a single atom can direct light from one fiber optic cable to a second one. The quantum light switch can occupy both possible states (on and off) at the same time. Courtesy of TU Vienna. While a number of recent experiments have demonstrated strong coupling of single atoms and solid-state quantum emitters to whispering gallery mode (WGM) microresonators, professor Arno Rauschenbeutel and his team at the Vienna University of Technology's Vienna Center for Quantum Science and Technology say that their method is unique because it uses a novel type of silica WGM known as a "bottle resonator." In the monolithic dielectric structures known as WGMs, light is guided near the surface by continuous total internal reflection, resulting in a superior kind of optical resonator that couples light in and out with near 100 percent efficiency using tapered fiber couplers. The Vienna team's bottle resonator is conceptually similar to other WGM microresonators but has the advantage of being fully tunable, they said. The system, consisting of a bottle microresonator coupled to a single atom and interfaced by two tapered fiber couplers, is extremely sensitive. "When the circumference of the resonator matches the wavelength of the light, we can make one hundred percent of the light from the glass fiber go into the bottle resonator – and from there it can move on into a second glass fiber," Rauschenbeutel said. "When we take a single rubidium atom and bring it into contact with the resonator, the behavior of the system can change dramatically." A bottle resonator with access fiber (right) and output fiber (left). The atom (green) determines whether the light pulse remains in the access fiber and then enters the bottle resonator and, from there, the output fiber. Courtesy of Vienna University of Technology. If the light is in resonance with the atom, the atom acts as a switch that redirects light to one fiber or the other. It is also possible to keep all of the light in the original glass fiber, with none of it transferring to the bottle resonator or the outgoing fiber. For a standard light switch at home, occupying both the "on" and "off" states simultaneously would be impossible, but for a quantum light switch, this dual state is feasible, so this mechanism is a very powerful new tool for quantum information and quantum communication. Light in a bottle: An optical fiber with a captured beam of light. Courtesy of TU Vienna. Next, the scientists plan to make use of the fact that the rubidium atom can occupy different quantum states, only one of which interacts with the resonator. If the atom occupies the non-interacting quantum state, the light behaves as if the atom is not there. So, depending on the quantum state of the atom, light is sent into either of the two glass fibers. This opens up the possibility of exploiting some of the most remarkable properties of quantum mechanics, Rauschenbeutel said. "We are planning to deterministically create quantum entanglement between light and matter," Rauschenbeutel said. "For that, we will no longer need any exotic machinery which is only found in laboratories. Instead, we can now do it with conventional glass fiber cables which are available everywhere." A paper on the work appears in Physical Review Letters (doi: 10.1103/PhysRevLett.111.193601). For more information, visit: www.tuwien.ac.at