  We propose how to achieve quantum nonreciprocity via unconventional photon
blockade (UPB) in a compound device consisting of an optical harmonic resonator
and a spinning optomechanical resonator. We show that, even with an extremely
weak single-photon nonlinearity, nonreciprocal UPB can emerge in this system,
i.e., strong photon antibunching can emerge only by driving the device from one
side, but not from the other side. This nonreciprocity results from the Fizeau
drag, leading to different splitting of the resonance frequencies for the
counter-circulating modes. Such nonreciprocal quantum UPB devices can be
particularly useful in achieving e.g., few-photon diodes or circulators, and
quantum chiral photonic engineering.
