Nonreciprocal Quantum Optical Devices Based on Chiral Interaction of Confined Light with Spin-Polarized Atoms
The confinement of light in nanophotonic structures results in an inherent link between the light’s local polarization and its propagation direction . Remarkably, this leads to chiral, i.e., propagation-direction-dependent effects in the emission and absorption of light by quantum emitters . For example, when coupling atoms to an evanescent field, the emission rates into counter-propagating optical modes can become asymmetric . In our group, we became aware of this chiral light–emitter coupling when studying the interaction of single rubidium atoms with the evanescent part of a light field that is confined by continuous total internal reflection in a whispering-gallery-mode (WGM) microresonator . In the following, we employed this effect to demonstrate an integrated optical isolator  as well as an integrated optical circulator  which operate at the single-photon level and which exhibit low loss. The latter are the first two examples of a new class of nonreciprocal nanophotonic devices which exploit the chiral interaction of quantum emitters with transversally confined photons.
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