Building A Single-Atom Quantum Optical Circulator

By Jof Enriquez,
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Scientists at the Technische Universität Wien (TU Wien) in Vienna, Austria have devised a new method to control the flow of light along perpendicular and intersecting glass fibers using a single atom. Building such a nanoscale optical element for controlling light, including single photons, can be important for optical signal processing of quantum information.

Traditional optical circulators – components that direct the flow of light – are controlled through the Faraday effect: a strong magnetic field applied to a transparent material between two polarization beam splitters, which are rotated with respect to each other. The direction of the magnetic field breaks the symmetry and determines the direction in which light goes, explains Prof. Arno Rauschenbeutel, from the Vienna Center for Quantum Science and Technology at the Institute of Atomic and Subatomic Physics of TU Wien, in a news release.

However, for technical reasons, using the Faraday effect for circulators is not feasible at the nanoscale. An alternative method of controlling the light path was devised by Rauschenbeutel's team through the coupling of a single rubidium atom to the light field of a so-called "bottle resonator" – a microscopic bulbous glass object on the surface of which the light circulates.

Without the atom, the resonator can couple to two adjacent ultrathin glass fibers, and light changes between the fibers via the bottle resonator. But no sense of circulation is defined: light can flow symmetrically either in a clockwise direction or a counterclockwise direction.

However, if the rubidium atom is correctly prepared and coupled to the resonator, one can make its interaction with the light differ for the two directions of circulation. "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," says Rauschenbeutel.

This 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 desired sense of circulation can be adjusted via the internal state of the atom.

The ability of the research team's circulator to let light pass in both a clockwise direction and a counterclockwise direction simultaneously is deemed impossible in classical physics. In quantum physics, however, the superposition of two possible states or combinations are indeed attainable. The phenomenon holds true even for individual photons routed in circulators of nanoscale dimensions in a quantum device, according to the TU Wien scientists.

"Rather than a magnetic field or a temporal modulation, it is the internal quantum state of the atom that controls the operation direction of the circulator. This working principle is compatible with preparing the circulator in a coherent superposition of its operational states. Such a quantum circulator may thus become a key element for routing and processing quantum information in scalable integrated optical circuits," the researchers write in their scientific paper, published in arXiv.