With just a single atom, light can be switched between two fiber optic cables at the Vienna University of Technology. Such a switch enables quantum phenomena to be used for information and communication technology.
Fiber optic cables are turned in to a quantum lab: scientists are trying to build optical switches at the smallest possible scale in order to manipulate light. At the Vienna University of Technology, this can now be done using a single atom. Conventional glass fibre cables, which are used for internet data transfer, can be interconnected by tiny quantum systems.
Professor Arno Rauschenbeutel and his team at the Vienna University of Technology capture light in so-called "bottle resonators". At the surface of these bulgy glass objects, light runs in circles. If such a resonator is brought into the vicinity of a glass fibre which is carrying light, the two systems couple and light can cross over from the glass fibre into the bottle resonator.
"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", explains Arno Rauschenbeutel.
This system, consisting of the incoming fiber, the resonator and the outgoing fiber, is extremely sensitive: "When we take a single Rubidium atom and bring it into contact with the resonator, the behaviour of the system can change dramatically", says Rauschenbeutel. If the light is in resonance with the atom, it is even possible to keep all the light in the original glass fiber, and none of it transfers to the bottle resonator and the outgoing glass fiber. The atom thus acts as a switch which redirects light one or the other fiber.
In the next step, 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 was not there. Thus, depending on the quantum state of the atom, light is sent into either of the two glass fibers. This opens up the possibility to exploit some of the most remarkable properties of quantum mechanics: "In quantum physics, objects can occupy different states at the same time", says Arno Rauschenbeutel. The atom can be prepared in such a way that it occupies both switch states at once. As a consequence, the states "light" and "no light" are simultaneously present in each of the two glass fiber cables.