Macroscopic Quantum Coherence In Rotating Whispering Gallery Optomechanical Systems

Quantum coherence which originates from the superposition principle of quantum states is the core of interference phenomena and the correlation functions for light fields were proposed by Glauber to describe it at first.Recently quantum coherence has been well defined with the proposal of coherent resource theory,and many schemes have been presented to measure quantum coherence in some specific quantum systems.Moreover quantum coherence is widely accepted as a physical resource which has potential applications in quantum information processing,quantum computation,quantum metrology and so on.Very recently,the research on macroscopic quantum coherence has gradually become a hot topic in quantum optics.The purpose of this thesis is to investigate the macroscopic quantum coherence in a spinning Whispering-gallery-mode(WGM)optomechanical system which is driven by external laser fields via a tapered fiber.The cavity field in the WGM resonator consists of two optical modes:clockwise(CW)mode and counterclockwise(CCW)mode,coupling with each other by back scattering.Meanwhile both optical modes couple with the mechanical vibration mode of the WGM resonator through the radiation pressure.The influence of the various physical quantities including the power of external driving laser field_,the coupling constant between the optical modes,the decaying rates for the optical modes_and the mechanical mode_and the spinning angular velocity?of the resonator on the macroscopic quantum coherences have been studied by numerical simulation.This thesis mainly includes the following two parts:In the first part,without consideration of the centrifugal force,the quantum coherences of the single mode,two modes and three modes are studied numerically when the external laser light only drives the CW mode and the influence of the various physical quantities mentioned above is investigated.It is found that the changing trend of the single mode coherence has a great influence on the two-mode coherence and the three-mode coherence,indicating that the single mode quantum coherence still plays a major role in the quantum coherence of the multiple modes.It is also found that the quantum coherences related to the CW mode increase monotonically with the increase of?,while the quantum coherences related to the CCW mode decrease monotonically,which indicates the Sagnac effect caused by resonator spinning results in the nonreciprocity in the quantum coherence,and the greater the spinning angular velocity?,the more obvious the nonreciprocity of the quantum coherence.In the second part we investigate the effect of the centrifugal force on the quantum coherences in the same spinning WGM resonator system as in the first part.By adopting the similar numerical simulation method,we find in such a case that a dip emerges in the coherence curve with the increase of spinning angular velocity?and the maximum nonreciprocity occurs at the dip position.The presence of the dip can be attributed to a centrifugal-force-dependent term in the Hamiltonian of the system,making the Hamiltonian depend on the spinning velocity?more obviously and more complex.Moreover our numerical results also show that single-mode quantum coherence still plays an important role in two-mode coherence and three-mode coherence.