We show that parity-time (PT) symmetry can be spontaneously broken in the recently reported energy level attraction of magnons and cavity photons. In the 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 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.
@article{arxiv.1905.11117,
title = {Steady Bell state generation via magnon-photon coupling},
author = {H. Y. Yuan and Peng Yan and Shasha Zheng and Q. Y. He and Ke Xia and Man-Hong Yung},
journal= {arXiv preprint arXiv:1905.11117},
year = {2020}
}