Second-order correlation and squeezing of photons in cavities with ultrastrong magnon-photon interactions

We investigate the second-order photon correlation function in cavity-magnon systems, focusing on ferromagnetic and antiferromagnetic cavities within the ultrastrong coupling regime, and extending beyond the rotating-wave approximation. By deriving exact integral solutions for the second-order correlation function, we demonstrate that counter-rotating magnon-photon interactions induce quadrature squeezing in the cavity mode. Furthermore, we show that tuning the anisotropic magnon-cavity couplings enhances the squeezing effect by changing the level repulsion of the magnon-cavity photon hybrid mode without increasing the cavity photon occupation number. Our study reveals distinct quantum correlation behaviors in ferromagnetic and antiferromagnetic cavities: For ferromagnetic cavities, we show that squeezing increases with coupling strength asymmetry, whereas in the antiferromagnetic case, magnon modes with opposite chirality suppress quantum effects and impose a lower bound on correlation functions. These findings provide a pathway to optimize photon blockade for quantum information technology in magnon-cavity systems in the ultrastrong coupling limit.