Observation Of The Anisotropic Exceptional Point In A Cavity-magnonics System

In recent years,with the development of quantum science and technology,a variety of hybrid quantum systems were proposed,including the systems composed of different spin ensembles and a cavity.For instance,very recently,there has been considerable interest in the hybrid system consisting of the Kittel-mode magnons in yttrium iron garnet(YIG)coupled to microwave photons in a cavity.Owing to the good coherence and tunability of this cavity-magnonics system,some novel physical phenomena were observed in experiments,such as the information gradient storage,the spintronics,and the control of the microwave-transmission direction.In addition,other interesting physical systems can also be constructed by means of the couplings between the cavity-magnonics system and different counterparts,such as the superconducting qubit,phonons and optical photons.In the future,it is expected to build a quantum network platform by using magnons as the core to implement interaction with other physical systems.Exceptional points(EPs)occur in various non-Hermitian systems and there were lots of investigations on them in both optical and acoustic systems.However,in the cavity-magnonics system,there were very limited studies on the EP and even no study on the anisotropic EP.In this thesis,we focus on the non-Hermitian physics of the cavity-magnonics system and investigate the properties of the anisotropic EP,both theoretically and experimentally.The thesis consists of four chapters.In Chapter 1,we first give a brief introduction to the basic concepts and research background,the advantages and disadvantages of different spin ensembles in constructing hybrid quantum systems,as well as the advancements in cavity-magnonics systems.Chapter 2 shows how to achieve the non-Hermitian Hamiltonian in a cavity-magnonics system,the realization of the second-order EP,and the PT phase transition.Chapter 3 presents our theoretical and experimental results on the anisotropic EP in a cavity-magnonics system.In this study,we find that in the vicinity of the EP,the imaginary part of the eigenvalue exhibits very different behaviors(i.e.,either linear-or square-root-function behavior)along two orthogonal directions in the parameter space.Chapter 4 gives a summary of the thesis and an outlook of the research.