Optical Circulator Could Be Used to Route Quantum Data in Integrated Optical Circuits

An advance in optical signal processing uses a fiber-integrated quantum optical circulator, operated by a single atom, to control the direction of light. The nonreciprocal behavior of the circulator arises from a chiral interaction between the atom and the transversally confined light. The circulator’s nonlinear response at the single-photon level enables photon number-dependent routing. Dubbed a “nano-roundabout,” the circulator could be used in integrated optical chips, and would be an advance for nanotechnology where the ability to process light pulses consisting of individual photons could be useful.

Functional principle of a nano-roundabout. Courtesy of TU Wien.

To develop a means of signal processing using light, researchers at TU Wien coupled two glass fibers at their intersection point to an optical resonator, in which the light circulated in a way similar to the movement of cars in a traffic circle. The team coupled a single atom to the light field of a “bottle resonator,” i.e., a microscopic glass with a surface on which the light could circulate.

The team found that when the bottle resonator was placed close to two ultrathin glass fibers, the fibers coupled. In the absence of an atom, the light changed from one glass fiber to the other via the bottle resonator; and no sense of direction was defined for the circulator, which meant that light could travel both clockwise and counter-clockwise.

To break the symmetry between forward and backward propagation direction, the team used the polarization properties of the light. They coupled an atom to the resonator, preventing the coupling of the light into the resonator and the overcoupling of light into the other glass fiber.

The direction of circulation and the polarization of light were “locked” together; direction of rotation depended on whether the light in the resonator traveled clockwise or counter-clockwise. When coupled to the resonator, the atom could interact differently with the light depending on the direction of circulation.

Arno Rauschenbeutel from the Vienna Center for Quantum Science and Technology at the Institute of Atomic and Subatomic Physics of TU Wien. Courtesy of Jacqueline Godany.

"The clockwise circulating light is not affected by the atom. The light in the opposite direction, on the other hand, strongly couples to the atom and therefore cannot enter the resonator," said researcher Arno Rauschenbeutel.

The asymmetry of the light-atom coupling with respect to the propagation direction of the light in the resonator allows control over the circulator operation. The circulation can be adjusted via the internal state of the atom.

"Because we use only a single atom, we can subtly control the process," said Rauschenbeutel. "The atom can be prepared in a state in which both traffic rules apply at the same time: All light particles then travel together through the circulator in both clockwise and counterclockwise direction."

Luckily, this is impossible according to the rules of classical physics, as it could result in chaos if applied to road traffic. In quantum physics, however, such superpositions of different states are permitted, opening up exciting possibilities for the optical processing of quantum information.

The research was published in Science (doi: 10.1126/science.aaj2118).