  Recent studies show that hybrid quantum systems based on magnonics provide a
new and promising platform for generating macroscopic quantum states involving
a large number of spins. Here we show how to entangle two magnon modes in two
massive yttrium-iron-garnet (YIG) spheres using cavity optomagnonics, where
magnons couple to high-quality optical whispering gallery modes supported by
the YIG sphere. The spheres can be as large as 1 mm in diameter and each sphere
contains more than $10^{18}$ spins. The proposal is based on the asymmetry of
the Stokes and anti-Stokes sidebands generated by the magnon-induced Brillouin
light scattering in cavity optomagnonics. This allows one to utilize the Stokes
and anti-Stokes scattering process, respectively, for generating and verifying
the entanglement. Our work indicates that cavity optomagnonics could be a
promising system for preparing macroscopic quantum states.
