Controlling light-matter interactions has persistently been pursued and is now actively explored. Understanding these interactions is not only of fundamental importance but also of interest for various applications. Recently, there has been an increasing number of studies on collective excitations of ferromagnetic spin system (i.e., magnons) coupled to microwave photons in a cavity (see, e.g., [1]–[5]). Owing to the strong coupling between magnons and cavity photons, a new type of bosonic quasiparticles called cavity magnon-polaritons can be created. While the damping rate of magnons is ﬁxed, by engineering the ports of cavity for inputting microwave photons and the related decay rates, the system of cavity photons coupled to magnons can be described by a non-Hermitian Hamiltonian. The hallmark of a non-Hermitian system is the existence of a singularity in its eigenvalues and eigenfunctions at some particular points in the parameter space of the system. This singularity is called the exceptional point.

**Fig. 1.** Observation of the exceptional point in cavity magnon–polaritons. (a) Measured total output spectrum vs. the displacement *x* of the YIG sphere in the cavity. (b) Calculated total output spectrum corresponding to the measured results in (a). Corresponding coupling strength is indicated according to the relation *g*_{m}/2π=1.3|*x*|.

Very recently, Zhang *et al. *at the CSRC [6] have experimentally demonstrated that the non-Hermiticity dramatically modiﬁes the mode hybridization and spectral degeneracies in cavity magnon-polaritons. In their experiment, they engineered the dissipations of magnons and photons to produce an effective non-Hermitian PT-symmetric Hamiltonian. By tuning the magnon-photon coupling, they observed the polaritonic coherent perfect absorption and demonstrated the phase transition at the exceptional point. Thus, cavity magnon-polaritons with non-Hermitian nature are explored and achieved in this experiment. It paves the way to explore the non-Hermitian physics of the cavity magnon-polaritons.

**References: **

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[2] Y. Tabuchi, S. Ishino, T. Ishikawa, R. Yamazaki, K. Usami, and Y. Nakamura, Phys. Rev. Lett. **113**, 083603 (2014).

[3] X. Zhang, C. L. Zou, L. Jiang, and H. X. Tang, Phys. Rev. Lett. **113**, 156401 (2014).

[4] D. Zhang, X. M. Wang, T. F. Li, X. Q. Luo, W. Wu, F. Nori, and J. Q. You, NPJ Quantum Inf. **1**, 15014 (2015).

[5] L. Bai, M. Harder, Y. P. Chen, X. Fan, J. Q. Xiao, and C. M. Hu, Phys. Rev. Lett. **114**, 227201 (2015).

[6] D. Zhang, X. Q. Luo, Y. P. Wang, T. F. Li, and J. Q. You, Nature Commun. **8**, 1368 (2017).