Observation of the exceptional point in cavity magnon-polaritons

Dengke Zhang,Tie-Fu Li,Xiao-Qing Luo,J Q You,Yi-Pu Wang

doi:10.1038/s41467-017-01634-w

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 intrinsically 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.

Highlights

Magnon–polaritons are hybrid light–matter quasiparticles originating from the strong coupling between magnons and photons
In the unbroken-symmetry regime, the total output spectrum has two coherent perfect absorption (CPA) frequencies, but no CPA occurs in the broken-symmetry regime. c Normalized microwave magnetic-field distributions of the cavity modes TE101 and TE102. d Variations of the microwave magnetic fields of the TE101 and TE102 modes along the moving path of the yttrium iron garnet (YIG) sphere marked in c
The gray area corresponds to our experimental region, where the magnetic-field intensity of the TE102 mode shows an approximately linear relation with the displacement of the YIG sphere and the varying slope is estimated to be 1.3 MHz/mm. e The maximal coupling of the TE102 mode to magnon is reached at the displacement jxj $ 11 mm of the YIG sphere, with the fitted coupling strength given as 9.2 MHz and achieved in our experiment

Summary

Introduction

Magnon–polaritons are hybrid light–matter quasiparticles originating from the strong coupling between magnons and photons. 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. Recent work on magnon–photon interaction has explored the strong and even ultra-strong couplings of microwave cavity photons to the ferromagnetic magnons in yttrium iron garnet (YIG)[5,6,7,8, 21, 22]. By tuning the magnon–photon coupling, we observe the polaritonic coherent perfect absorption (CPA) and demonstrate the phase transition at the exceptional point. It paves the way to Hermitian Hamiltonian (Supplementary Note 2), explore the non-Hermitian physics of the cavity magnon– polaritons

Methods

Results

Conclusion