Perfect chirality constructed by position-dependent backscattering in a whispering gallery mode microresonator

Abstract

Unidirectional propagation of photons originated from perfect chirality meets the critical requirement for building a high-performance quantum network. However, it not only requires that the circular dipole emitter is precisely located at points of circularly polarized electric fields, which leads to non-reciprocal interactions for photons with opposite propagation directions, but also the light-emitter interaction strength should be strong enough to guarantee a π phase shift. Unfortunately, these perfect chirality points are scarce and accessible points with elliptically polarized fields result in non-ideal photon-emitter chiral interactions and emitters radiating photons bidirectionally. Meanwhile, reflection properties, phase shifts, and non-reciprocal interactions are sensitive to frequency detunings and dissipations. Here, without engineering the dipole and optimizing the distribution of the field, a scatter such as a nanotip placed at the evanescent field of a whispering gallery mode resonator (WGMR) is adopted to control the transporting properties of single photons under non-ideal chiral interactions. By properly adjusting the relative position between the nanotip and the atom or the overlap between the nanotip and the mode volume of the WGMR, amplitudes of reflected photons in different pathways are changed. Consequently, complete destructive interference appears and thus no photons are reflected. The corresponding phase shifts of π and non-reciprocal interactions are guaranteed simultaneously. Significantly, the perfect chirality reconstructed here is robust against frequency detunings and dissipations. Therefore, the atom-WGMR-nanotip structure can be regarded as a compound chiral atom with radiating photons in only one direction.