Nonreciprocal Genuine Microwave Entanglement via Magnon Kerr Nonlinearity

We present a utilization of the magnon Kerr effect to generate nonreciprocal genuine microwave entanglement in a hybrid system consisting of a yttrium iron garnet (YIG) sphere and three microwave cavities. Based on the quantum Langevin theory and linearization method under the condition of strong magnon driving, the system dynamics and covariance evolution are deduced and then applied to determinate the quantum correlations. It is found that three microwave cavities entangle with each other at the steady state. The basic root is that the Kerr nonlinearity can not only induce the enhanced parametric amplification of magnon but also cause the magnon frequency shift. Naturally, when the direction of the externally applied bias magnetic field is changed, switching of the magnon Kerr coefficient from positive to negative occurs and nonreciprocal tripartite entanglement among three microwave photons can be achieved. This may provide a fundamental resource for practical applications in quantum information processing and quantum networks.