Nanophotonic components allow the control of the flow of light in integrated optical environments. Thereby, the light’s strong confinement leads to an inherent link between its local polarization and propagation direction which fundamentally alters the physics of light-matter interaction and gives rise to phenomena such as directional emission and direction-dependent coupling strengths .
I will present the underlying principles of this chiral light-matter interaction and its consequences for integrated applications . In particular, I will show how we employ this effect to control the direction of spontaneous emission  and to realize low-loss nonreciprocal transmission at the single-photon level through a silica nanofiber . We use two different approaches where either an ensemble of spin-polarized atoms is weakly coupled to a nanofiber or a single atom is strongly coupled to the nanofiber via a whispering-gallery-mode resonator. The resulting optical isolators show a strong imbalance between the transmissions in forward and reverse direction and, at the same time, a forward transmissions exceeding 70%. We extended this system to a 4-port device, where a single atom routes photons nonreciprocally from one fiber port to the next. This realizes a quantum optical circulator  which can even be prepared in a superposition of its operational modes.
The demonstrated systems exemplify a new class of (quantum) nanophotonic devices that are ideally suited for photonic quantum information processing and quantum simulation.
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Juergen Volz and Arno Rauschenbeutel, "Quantum nonreciprocal devices based on chiral light-matter coupling (Conference Presentation)," Proc. SPIE 10358, Quantum Photonic Devices, 103580B (Presented at SPIE Nanoscience + Engineering: August 06, 2017; Published: 29 September 2017); https://doi.org/10.1117/12.2276946.5593158168001.
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