Nonreciprocal unconventional magnon blockade via the Sagnac-Fizeau shift in an optomagnonic system

We propose a scheme to achieve nonreciprocal unconventional magnon blockade (NUMB) in an optomagnonic system composed of a spinning resonator with ${\ensuremath{\chi}}^{(2)}$ nonlinearity and one yttrium iron garnet sphere. When two-photon driving is input from different ends of the resonator, complete quantum destructive interference appears and disappears between different paths for two-magnon excitation due to the opposite Sagnac-Fizeau shifts induced by Fizeau drag, which is the physical origin for achieving NUMB. To intuitively understand the nonreciprocal phenomenon, the analytical solution of the equal-time second-order correlation function is approximately given through the Schr\"odinger equation and compared with the full numerical solution obtained from the Lindblad master equation. From the two almost identical solutions, we show that a giant NUMB can be observed in the absence of probe detuning. We also show that a switchable NUMB can be achieved by tuning the relative phase between the external laser fields. In addition, increasing the amplitude of the probe field appropriately can reduce the impact of ambient thermal noise on NUMB. Our work may have potential applications in the design of nonreciprocal single-magnon devices.