基于相干反馈操控的纠缠源的分析

Analysis of entanglement source based on coherent feedback control

基于非测量的量子相干反馈控制系统不会引入额外的噪声,可以用于稳定、操控和改善多种量子系统的性能.利用相干反馈的方法可以操控非简并光学参量放大器,在一定条件下能够增强其输出Einstein-Podolsky-Rosen (EPR)纠缠态光场的纠缠度.相干反馈控制系统中的核心光学元件是控制耦合镜,其透射率的选取直接影响反馈控制的效果.本文针对控制耦合镜对偏振相互垂直的种子光场透射率不同的情况,从理论上分析了该情况对相干反馈控制效果的影响,得出相干反馈的正作用达到最佳时对控制镜透射率的要求,理论分析与实验结果相吻合.同时分析了相干反馈控制效果随其他物理参量的变化关系,得出系统进一步优化的实验条件.为今后相干反馈控制系统中物理参量的选择提供依据,也为利用相干反馈操控更多的量子系统提供参考.

Entangled state of light with quantum correlations between amplitude and phase quadratures is a necessary quantum resource in optical continuous variable(CV) quantum information systems. The CV EinsteinPodolsky-Rosen(EPR) entangled optical field is one of the most basic quantum resources, which can be generated by a non-degenerate optical parametric amplifier(NOPA) operated below the threshold pump power.Manipulating the EPR entangled state of light effectively can break through the limitation of the imperfect performance of optical components in optical cavities and then further improve the entanglement level under certain conditions. So it is necessary to find out an effective optical scheme of manipulating quantum state of light. The non-measurement based coherent feedback control(CFC) system without introducing any extra noise into the controlled system, can be used to stabilize, control and improve the performance of various quantum systems. Only by selecting the right experimental parameters can the CFC system play its positive role in reaching a maximum efficacy. The key optical component, i.e. optical controller in CFC system, greatly affects the final manipulation effects. In 2015, using the method of CFC, our research team experimentally realized the enhancement of entanglement to different levels by changing the optical controller with different transmissivity values for seed optical beams. At the same time, the threshold pump power of the NOPA is reduced to different levels also. Due to the technical reasons, the transmissivity of the optical controller selected in the experiment is almost the same for the signal optical field and idle optical field. In this paper, we emphasize the condition that the transmissivity of the optical controller for the signal optical field is different from that for idle optical field.Firstly, we theoretically study the final effects of manipulating entanglement source by using the coherent feedback optical cavity under the above conditions. It is concluded that if the transmittance of control beam splitter(CBS) is low, the feedback control optical cavity works best when the optical controller has different transmissivity for signal optical beam and idle optical beam, and that if the transmittance of CBS is high, the transmittance of the optical controller for signal optical beam almost equals that for idle optical beam to make the feedback control optical cavity work best. Then we theoretically investigate the dependence of the quantum correlation noise of the quadrature amplitude and quadrature phase of the output optical fields from CFCNOPA system on other physical parameters. Combining with the actual experimental conditions, we can find the optimal transmissivity of the optical controller and appropriate range of frequency to optimize the effects of CFC, which provides the basis for correctly selecting the actual experimental parameters in CFC systems.Theoretical analysis results also show that with the higher input and output coupling efficiency and higher nonlinear conversion efficiency of NOPA, the entangled state of light with higher entanglement degree can be obtained experimentally. This provides the reference for obtaining better quantum resources needed for studying the CV quantum information.