  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) by considering the Kerr
nonlinear interaction in one of the paths. Interestingly, 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 novel approach to observe purely quantum
nonreciprocal effects.
