# Steady Bell state generation via magnon-photon coupling

## TL;DR

This paper demonstrates that in a magnon-photon system, spontaneous parity-time symmetry breaking leads to stable, high-fidelity Bell state entanglement, challenging conventional expectations of instability in such broken-symmetry phases.

## Contribution

It reveals that $ ext{PT}$-symmetry breaking in magnon-photon coupling produces steady entanglement, offering a new approach for quantum resource generation.

## Key findings

- High-fidelity Bell state formed in $ ext{PT}$-broken phase
- Entanglement remains steady and robust against environmental perturbations
- No steady entanglement observed in normal level repulsion case

## Abstract

We show that parity-time ($\mathcal{PT}$) symmetry can be spontaneously broken in the recently reported energy level attraction of magnons and cavity photons. In the $\mathcal{PT}$-broken phase, magnon and photon form a high-fidelity Bell state with maximum entanglement. This entanglement is steady and robust against the perturbation of environment, in contrast to the general wisdom that expects instability of the hybridized state when the symmetry is broken. This anomaly is further understood by the compete of non-Hermitian evolution and particle number conservation of the hybridized system. As a comparison, neither $\mathcal{PT}$-symmetry broken nor steady magnon-photon entanglement is observed inside the normal level repulsion case. Our results may open a novel window to utilize magnon-photon entanglement as a resource for quantum technologies.