  We describe an approach to optical non-reciprocity that exploits the local
helicity of evanescent electric fields in axisymmetric resonators. By
interfacing an optical cavity to helicity-sensitive transitions, such as Zeeman
levels in a quantum dot, light transmission through a waveguide becomes
direction-dependent when the state degeneracy is lifted. Using a linearized
quantum master equation, we analyze the configurations that exhibit
non-reciprocity, and we show that reasonable parameters from existing cavity
QED experiments are sufficient to demonstrate a coherent non-reciprocal optical
isolator operating at the level of a single photon.
