Parametric amplification induced giant nonreciprocal unconventional photon blockade in a single microring resonator

We theoretically propose an all-optical scheme to implement a giant nonreciprocal unconventional photon blockade (UPB) in a single photonic device. By considering the backscattering coupling between clockwise (CW) and counterclockwise (CCW) waves with the same frequency, we show that in the single microring χ2-nonlinear resonator, UPB happens when the resonator is pumped to induce a parametric amplification nonlinear process in one direction but not the other. This originates from the induced nonreciprocal transmission for the driving mode's resonance by the unidirectionally pumping, leading to different quantum interference effects between distinct driven-dissipative excitation paths for the CW and CCW modes. We analytically give the optimal conditions for achieving the nonreciprocal UPB. Our work provides a way to achieve single quantum nonreciprocal devices without moving parts, which greatly simplifies its experimental implementation.