Nonreciprocal optical-microwave entanglement induced by Barnett effect

We propose a scheme for realizing nonreciprocal bipartite and tripartite entanglement in a hybrid cavity optomagnonic system. The system consists of a tunable Fabry-Perot (FP) cavity and a microwave cavity, an yttrium iron garnet slab (YIG1) and a rotating yttrium iron garnet sphere (YIG2). The magnon modes in both YIG1 and YIG2 are strongly coupled to the microwave cavity mode, and the magnon mode in YIG1 also has a three-mode resonant interaction with the optical modes in the FP cavity. The Barnett effect causes a frequency shift of the magnon mode in YIG2. By reversing the external bias magnetic field of YIG2 to control the sign of the Barnett frequency shift, one can not only switch the entanglement partner of the optical mode between the magnon and microwave modes, but also achieve perfect on-off control of bipartite optical-microwave entanglement and tripartite optical-magnon-microwave entanglement. The study presents a flexible control scheme to realize nonreciprocal bipartite and tripartite entanglement in quantum nodes that integrate diverse physical systems such as optics, magnons, and microwaves, and may have potential applications in entanglement-based quantum networks.