Study On The Dynamics Of Magnons And Phonons In Cavity Magnomechanical System

Cavity magnomechanical system can easily realize the coupling among microwave photons,magnons and phonons.This system has the advantages of high adjustability,low loss and strong coupling of photon-magnon at room temperature.According to this,cavity magnomechanical system can not only be used as an excellent platform to study quantum information and quantum optics,but also be used to explore macroscopic quantum phenomena.In this thesis,we study the dynamics of magnons and phonons by the cavity magnomechanics,quantum dynamic equations and open quantum system theory.We also study the entanglement,ground-state cooling and phonon laser.This work is of great help to the application of cavity magnomechanical system in quantum information and quantum optics.This thesis consists of six chapters as follows:In Chapter 1,we introduce the theory and development of cavity magnomechanics,phonon laser and ground-state cooling,and illustrate the starting point and purpose of this work by comparison.In Chapter 2,we mainly introduce the basic concepts of our work,including the quantization of cavity magnomechanical system,basic theory of open quantum system,input-output relation of cavity field,PT-symmetry theory,quantum entanglement and steering.In Chapter 3.the ground state cooling of mechanical oscillator is studied in a cavity magnomechanical system.It consists of a microwave cavity and a small ferromagnetic sphere,in which phonon-magnon coupling and cavity photon-magnon coupling are achieved via magnetostrictive interaction and magnetic dipole interaction,respectively.Considering experimentally feasible parameters,we demonstrate that the extra magnetic damping can be utilized to achieve ground-state cooling of the magnomechanical resonator via an effective dark-mode interaction.It is found that the magnomechanical cooling mainly comes from the magnon-phonon interaction terms in Hamiltonian.And we show that the optimal cooling can be obtained by adjusting the external magnetic field.In Chapter 4,we study creation of entanglement and quantum steering in a PT-symmetric cavity magnomechanical system.There are three modes in the system:cavity mode,magnon mode and mechanical mode.By introducing a blue-detuned driving microwave field to the magnon mode,the bipartite entanglement of the PT-symmetry system is significantly enhanced versus the case of the conventional cavity magnomechanical systems(loss-loss systems).Moreover,the one-way quantum steering of the magnon-phonon mode and photon-phonon mode are obtained in the PT-symmetry regime.Finally,we investigate the boundary of stability and show that the steady-state solutions are more stable in the gain-loss systems.In Chapter 5,we theoretically study phonon laser in a cavity magnomechanical system,we find that phonon laser can appear by driving the magnon mode,and the intensity of driving field which can reach the threshold of phonon laser is given.Then we find that the phonon laser can be well controlled by an adjustable external magnetic field without changing other parameters,which provides an additional degree of freedom compared to the phonon laser in optomechanical systems.Finally,with the experimentally feasible parameters,threshold power of phonon laser in our system is close to the threshold power in optomechanical systems which have been well developed.In chapter 6,we give the summary and prospect.