Physical Review Letters ( IF 8.385 ) Pub Date :
H. Y. Yuan, Peng Yan, Shasha Zheng, Q. Y. He, Ke Xia, and Man-Hong Yung

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 hybrid 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 an exciting window to utilize magnon-photon entanglement as a resource for quantum information science.

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