We propose a scheme based on spatiotemporal modulation to realize nonreciprocal photon blockade in a Kerr-type whispering-gallery-mode microcavity and enable the generation of nonreciprocal entangled photon pairs. In particular, spatiotemporal modulation breaks the time-reversal and space-inversion symmetries of the system, leading to distinct response behaviors for input signals injected from different ports. Moreover, it introduces extra degrees of freedom, which allow for the simultaneous control of both the frequency of single photons and the direction of single-photon flux. Under optimal photon blockade conditions, the optical paths from port 1 to ports 3 and 4 are entangled; correspondingly, the emitted single-photon pairs exhibit high synchronization and strong correlation. Notably, a photon detected at one port can act as a herald for photon emission at the other port, which significantly enhances the collection efficiency of single photons. As the first magnet-free, static, and nonreciprocal single-photon generation scheme, this work opens up a novel pathway for the development of integrated and multifunctional single-photon sources.
Li et al. (Mon,) studied this question.