级联四波混频相干反馈控制系统量子纠缠特性

本文在级联四波混频结构基础上,利用光学分束器作为反馈控制器理论构造了一种相干反馈控制系统.考虑相干反馈回路中光束传输损耗以及原子对光束吸收损耗,通过计算系统的协方差矩阵以及利用部分转置正定判据,分析了该系统在不同反馈强度、增益以及相位下的纠缠特性.结果表明,系统存在真正的三组份纠缠,但是反馈控制器过度反馈会破坏系统的三组份量子纠缠特性.另外,将相位设为180°,通过适当改变增益大小以及在0.1—0.4范围内调节分束器反射率的大小可以增强系统的量子纠缠程度.本文为实验上基于级联四波混频相干反馈控制系统制备多组份纠缠奠定理论基础,在量子通信领域有着潜在应用.

Quantum entanglement in coherent feedback system based on the cascaded four wave mixing processes

Based on the cascaded four wave mixing processes,a coherent-feedback control system is constructed by utilizing a linear beam splitter as the feedback controller.Considering the loss of optical propagation in the coherent feedback loop and the absorption effect of Rb vapor cells to beams,we theoretically investigate the entanglement properties of this system under different feedback ratio,gain and phase by calculating the covariance matrix of system and applying the positivity under partial transpose(PPT)criterion to all possible bipartitions.The result shows that the genuine tripartite entanglement exists in the coherent feedback control system,but the entanglement structure of system will be destroyed by the excessive feedback.In addition,when the phase isπ,we find that the tripartite entanglement can be enhanced by changing the gains and the reflectivity of the beam splitter in the range of 0.1 to 0.4.The results pave the way for manipulating multipartite entanglement by coherent feedback control and have potential application in quantum communication.