Enhancement of Microwave Entanglement via Coherent Quantum Feedback with Cavity Magnonics

Entangled microwave fields are a crucial resource for quantum information processing and quantum computation. Here a scheme is presented to enhance quantum entanglement between two microwave cavity modes using coherent quantum feedback in cavity magnonics. The system consists of a planar cross‐line cavity that includes two microwave cavity modes, with a ferrimagnetic yttrium‐iron‐garnet (YIG) sphere positioned at the intersection of these modes. The magnon mode confined in the YIG sphere can simultaneously be coupled to both microwave cavity modes via magnetic dipole interaction. Additionally, a coherent feedback loop is integrated into one of the cavity modes to modulate its frequency detuning and dissipation. These results demonstrate that feedback control can effectively reduce the dissipation and effective environmental temperature, thereby relaxing the stringent requirement of quantum state generation on extremely low bath temperatures. Furthermore, by enhancing the feedback signal to reduce the high power requirement for activating magnon Kerr effect, the feasibility of the experiment is significantly improved. The scheme finds broad applications in many quantum tasks that require microwave entanglement and other research field that needs to cool quantum systems.