Purely quantum nonreciprocity by a spatially separated transmission scheme

Nonreciprocal photon blockade is of particular interest due to its potential applications in chiral quantum technologies and topological photonics. In the regular cases, nonreciprocal transmission (classical nonreciprocity) and nonreciprocal photon blockade (quantum nonreciprocity) often appear simultaneously. Nevertheless, how to achieve purely quantum nonreciprocity (no classical nonreciprocity) remains largely unexplored. Here, we propose a spatially separated transmission scheme: that the photons transport in different directions take different paths, in an optical system consisting of two spinning cavities coupled indirectly by two common drop-filter waveguides. Based on the spatially separated transmission scheme, we demonstrate a purely quantum nonreciprocity (nonreciprocal photon blockade) that cannot be observed in the classical transmission spectra. In addition, we find that the nonreciprocal photon blockade is enhanced nonreciprocally, i.e., the nonreciprocal photon blockade is enhanced when the photons transport in one direction but suppressed in the reverse direction. We identify that the nonreciprocal enhancement of nonreciprocal photon blockade is induced by the destructive or constructive interference between two paths for two photons passing through the whole system. The spatially separated transmission scheme proposed in the work provides a different approach to observe purely quantum nonreciprocal effects.