Enhancement Of Quantum Effects With Coherent Feedback In Optomechanical Systems

Cavity optomechanics is an emerging field,which focuses on the interaction between optical(or microwave)cavity field and mechanical motion.In recent years,cavity optomechanics has attracted more and more research interest because of its potential in fundamental research and practical applications.Nowadays,more and more experimental platforms have successfully realized the cavity optomechanical system,and gradually become an ideal system model for studying quantum effects on the macro scale,such as ground state cooling,macroscopic entanglement,photon blockade effect,mechanical squeezing,ultra-high precision measurement and quantum-classical boundary.In addition,coherent feedback is a control method on quantum systems which is not based on measurement and posses some advatange over measured-based control method to manupicate quantum effects.Coherent feedback is introduced into the cavity optomechanical system to improve such as optical squeezing,which is a pure non-classical effect in quantum optics.The controller in the coherent feedback loop is a quantum system,which can modulate and feedback the output of the cavity and control the system in a coherent way.In this loop,no measurements are made.Therefore,no redundant measurement reaction noise is introduced into the system.Based on this characteristic,coherent feedback control plays a key role in control theory,to reduce the extral noise.In addition,the quantum ground state cooling of macroscopic mechanical resonators is of great significance to both fundamental physics and applications.In order to realize the effective quantum control of the macroscopic mechanical oscillator,in general,the mechanical oscillator needs to be cooled to the quantum ground state.According to previous investigation,cavity optomechanical interaction can be utilized to reduce the effect of thermal noise.In the typical cavity optomechanical cooling setup,the Stokes process will create phonons in mechanical motion to heat the system.Thus,how to suppress even eliminate the Stokes process is a key issue in optomechanical cooling.If squeezed optical field is injected to the cavity optomechanical system,the correlation generated by the squeezed field can be utilized to eliminate Stokes process,to realize the mechanical cooling beyond the resolved sideband limit.The specific research contents of this thesis are as follows:A coherent feedback scheme is used to enhance the degree of squeezing of the output field in a cavity optomechanical system.In the feedback loop,a beam splitter(BS)plays the roles of both a feedback controller and an input–output port.To realize effective enhancement,the output quadrature should take the same form as the input quadrature,and the system should operate at the deamplification situation in the meantime.This can be realized by choosing an appropriate frequencydependent phase angle for the generalized quadrature.Additionally,both the transmissivity of the BS and the phase factor induced by time delays in the loop affect optical squeezing.For the fixed frequency,the optimal values of transmissivity and phase factor can be used to achieve the enhanced optical squeezing.The effect of optical losses on squeezing is also discussed.Optical squeezing is degraded by the introduced vacuum noise owing to the inefficient transmission in the loop.We show that the enhancement of squeezing is achievable with the parameters of the current experiments.Moreover,we studied how to improve the optomechanical cooling by utilizing squeezed coherent feedback loop,in order to implement an effective cooling scheme for mechanical oscillators beyond the resolved sideband limit.We considered the cavity optomechanical system with the double-sided cavity mirrors.The output field from one mirror is injected in optical parametric amplifier,and the generated squeezed field is fed back to the system through the other mirror.The process consists the squeezed coherent feedback loop.Via calculating the optical force on the oscillator,it is found that the heating process,i.e.,the Stokes process,can be eliminated by the choosing the appropriate squeezing parameter including squeezing degree and phase factor.At the same time,the feedback loop can effectively decrease the optical dissipation rate,and the anti-Stokes cooling process can be enhanced,leading to the improvement on mechanical cooling.In addition,the optical loss in the feedback loop is taken into account.The Stokes process can be still eliminated,while an additional heating process caused by the introduced uncorrelated vacuum noise.However,the better cooling is still achievable via choosing the appropriate parameters.