  Magnon-polaritons are hybrid light-matter quasiparticles originating from the
strong coupling between magnons and photons. They have emerged as a potential
candidate for implementing quantum transducers and memories. Owing to the
dampings of both photons and magnons, the polaritons have limited lifetimes.
However, stationary magnon-polariton states can be reached by a dynamical
balance between pumping and losses, so the intrinsical nonequilibrium system
may be described by a non-Hermitian Hamiltonian. Here we design a tunable
cavity quantum electrodynamics system with a small ferromagnetic sphere in a
microwave cavity and engineer the dissipations of photons and magnons to create
cavity magnon-polaritons which have non-Hermitian spectral degeneracies. By
tuning the magnon-photon coupling strength, we observe the polaritonic coherent
perfect absorption and demonstrate the phase transition at the exceptional
point. Our experiment offers a novel macroscopic quantum platform to explore
the non-Hermitian physics of the cavity magnon-polaritons.
