Researchers at the Technische Universität Wien (TU Wien) in Vienna, Austria, have created traffic rules for light in optical signal processing. The rule is even valid for single photons.
The team couples two glass fibers at their intersection to an optical resonator, where the light circulates, much like cars in a roundabout on the street. The direction of circulation is defined by a single atom coupled to the resonator. The atom also ensures that the light always leaves the roundabout at the next exit.
Difference to previous optical signal processing approaches
The most common method to implement optical circulators is based on the Faraday Effect: a strong magnetic field is applied to a magneto-optical material, which is located between two polarization beam splitters that are rotated with respect to each other. The direction of the magnetic field breaks the symmetry and determines in which direction the light is redirected.
“For our approach, we use the fact that for nanoscale waveguides the polarization is inherently linked to the propagation direction,” explains Michael Scheucher from the Atominstitut at TU Wien, who is also the first author of the paper. “When a polarization-sensitive emitter, in our case a single atom, is coupled to these fields, we can break the symmetry of the light transmission.”
Today, scientists and engineers are building optical integrated circuits with similar functions as known from electronics. An outstanding challenge, according to Scheucher, is to control the flow of light in integrated structures. “A key element is a circulator that follows a simple protocol: If the four ports are numbered in ascending order, a signal that enters the circulator through port 1, 2, 3 or 4, exits it through port 2, 3, 4 or 1, respectively. One therefore speaks of a nonreciprocal component. In microwave technology, such roundabouts are essential for signal routing and multiplexing.”
Advantages of TU Wien’s novel method
Other methods for integrated optical circulators function only at very high light intensities or suffer from high optical losses, according to the expert. In nanotechnology, however, scientists want to be able to process very small light signals, ideally light pulses that consist solely of individual photons. “This renders our low-loss circulator the first realization of a circulator that is compatible with the requirements for distributing quantum information in integrated optical networks or circuits,” Scheucher says. “In addition, the operation direction is defined by the atomic state, which can be easily controlled, unlike external magnetic fields.”
The innovation coming out of TU Wien will possibly impact the future of optical processing of quantum information. “Our circulator is controlled by a single atom, which is a well controllable quantum system,” Scheucher notes. “This enables our new approach to also make use of different quantum effects, such as nonlinear response on the two-photon level.”
This functionality, for instance, can be useful for photon sorting, where single photons are sent to one port and pairs of photons sent to another. Furthermore, Scheucher says the circulator can be prepared in quantum superposition of routing light in one and the other direction. Thus, our circulator is a new quantum device capable of taking an active part in processing quantum information.
“In addition to its importance for quantum communication and quantum information processing, arrays of interconnected quantum circulators are potential candidates for implementing lattice-based quantum simulations to model complex quantum systems,” Scheucher explains. “This way, one can investigate properties that are not accessible experimentally or theoretically.”
The TU Wien had to overcome challenges: Thy needed an ultralight resonator to realize their low-loss device. In addition, this resonator had to be coupled to a waveguide as efficiently as possible. “In order to fulfill these requirements, our experiment employed a so-called whispering-gallery mode resonator that is coupled to two tapered optical nanofibers by overlapping their near fields,” Scheucher explains. “Furthermore, we had to realize coherent coupling between the light in the resonator and the atom. Therefore, the atoms had to be brought very close to the resonator surface to couple to the near field of the resonator.”
“While our system is inherently fiber-integrated for real scalability, we need to reduce the footprint and experimental overhead to integrate our device on a chip,” says the researcher. “Therefore we need to fabricate resonators of similar properties on a chip. In addition, the atom has to be replaced with a polarization dependent solid-state emitter.”
Written by Sandra Henderson, Research Editor, Novus Light Technologies Today