Observation of the exceptional point in cavity magnon-polaritons

TL;DR

This paper reports the experimental observation of an exceptional point in cavity magnon-polaritons, revealing non-Hermitian physics and phase transitions in a tunable quantum electrodynamics system.

Contribution

It demonstrates the creation and control of non-Hermitian spectral degeneracies in cavity magnon-polaritons through engineered dissipation and coupling tuning.

Findings

Observation of polaritonic coherent perfect absorption

Demonstration of phase transition at the exceptional point

Control of non-Hermitian degeneracies in a macroscopic quantum system

Abstract

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.