Science 354, 1577–1580 (2016)

Non-reciprocal optical components such as isolators and circulators are useful in photonic circuits for signal routing and processing tasks. Quantum optical applications also require such elements, but with the additional constraints that they need to work with low light levels and exhibit low loss. Now, Michael Scheucher and collaborators report the realization of a fibre-integrated optical circulator based on a single rubidium atom resonantly coupled to the evanescent field of a whispering-gallery-mode microresonator. Two coupling fibres interfaced with the microresonator identify the four-port circulator. Owing to strong transverse confinement, the evanescent fields of the clockwise and anticlockwise propagating resonator modes are almost fully circularly polarized (albeit in opposite directions). The internal state of the atom determines whether circularly polarized light in the clockwise or anticlockwise resonator mode interacts strongly with the single atom, thus determining whether the incident light couples to the microresonator and switches fibre on its exit (as opposed to staying in its initial fibre). Therefore, the internal state of the rubidium atom effectively controls the operation direction of the circulator. The average operation fidelity of the device is above 0.7, and the average insertion loss is 1.4 dB.