An ultrathin glass fiber system could facilitate the interaction of pairs of photons. The technique, developed by a team at the Vienna University of Technology, could become an important tool in furthering quantum computing. To establish the photon-photon interaction, an ultrathin glass fiber was coupled with a tiny bottle-like light resonator, allowing light to partially enter it, move in circles and then return to the fiber. This detour through the resonator led to the phase of the photon being inverted, according to the researchers, meaning a wave crest appeared where a wave trough would have been expected. The light in a glass fiber is coupled to a bottle resonator. Courtesy of Vienna University of Technology. “It is like a pendulum which should actually swing to the left, but due to coupling with a second pendulum, it swings to the right,” said professor Dr. Arno Rauschenbeutel. “There cannot be a more extreme change in the pendulum's oscillation. We achieve the strongest possible interaction with the smallest possible intensity of light.” In quantum mechanics, two separate photons can only be understood as a joint wave-like object, which is located in the resonator and in the glass fiber at the same time. The photons being absorbed and those passing through are indistinguishable, the researchers said. When these photons hit the resonator at the same time, both of them together experience a phase shift by 180 degrees. “That way, a maximally entangled photon state can be created,” Rauschenbeutel said. “Such states are required in all fields of quantum optics — in quantum teleportation, or for light-transistors which could potentially be used for quantum computing.” Glass fiber-based technology is already being used in online communications, making the new system compatible with some existing technologies. The newly established strong photon-photon interaction presents an important step toward a broader, worldwide quantum network for data transmission, the researchers said. The research was published in Nature Photonics (doi: 10.1038/nphoton.2014.253). For more information, visit www.tuwien.ac.at.