Nonreciprocal photon blockade via chiral cavity-atom interaction

We introduce a method to achieve nonreciprocal photon blockade in an optical resonator that is chirally coupled to a two-level atom. This approach enables single-excitation and two-excitation resonant conditions to be simultaneously satisfied. The atom-uncoupled optical mode can be blocked with eliminating its two-photon excitation by judiciously selecting parameters, e.g., the optical coupling strength. The chiral interaction between the resonator mode and the atom leads to different optimal parameter conditions for generating photon blockade in the two opposite drivings. This offers a natural way to realize nonreciprocal photon blockade. The nonreciprocity achieved by the chiral cavity-atom interaction diminishes the experimental difficulty by eliminating the need to rotate the resonator. The blockade mode achieves very high purity of single-photon excitation, as characterized by a second-order correlation function of ${g}^{(2)}(0)\ensuremath{\sim}{10}^{\ensuremath{-}4}$. In particular, in the nonreciprocal regime, the mean number of photons is significantly higher where photon blockade occurs compared to the general Jaynes-Cummings case. Our findings are particularly relevant for the development of quantum nonreciprocal devices and their applications in quantum information processing, including chiral networks.