铷原子D1线真空压缩光场的产生及态重构

Generation and quantum state reconstruction of a squeezed vacuum light field resonant on the rubidium D1 line

碱金属原子是光量子存储的良好介质,与碱金属原子共振的非经典光场是量子信息处理的重要资源.本文采用周期极化磷酸氧钛晶体作为非线性介质,利用参量振荡过程产生了795 nm(铷原子D1线)的真空压缩光场.通过对平衡零拍探测系统的时域信号进行采集,得到压缩光场不同相位角下的噪声分布;利用极大似然估计法对压缩光场进行了态重构,得到了密度矩阵及相空间的Wigner函数.理论计算了真空压缩场的光子数分布和Wigner函数,并对理论计算结果和极大似然重构结果进行了分析和比较.

The squeezed light field is a kind of important continuous variable quantum resource. It has wide applications in precision measurement and quantum information processing. Quantum storage is the foundations of quantum repeater and long distance quantum communication, and alkali metal atoms are an ideal quantum storage medium due to long ground state coherent time. With the rapid development of quantum storage technology in atomic medium, the preparation of the squeezed light which resonates with alkali metal atoms has become one of the research hotspots in the field of quantum information. In this paper, we report the generation of squeezed vacuum at 795 nm （resonant on the rubidium D1 transition line） by using an optical parametric oscillation based on a periodically poled KTiOPO4 crystal. The generated squeezed light field is detected by a balanced homodyne detector, and the squeezing of -3 dB and anti-squeezing of 5.8 dB are observed at a pump power of 45 mW. By using a maximum likelihood estimation, the density matrix of the squeezed light field is reconstructed. The time-domain signals from the balanced homodyne detector are collected to acquire the noise distribution of the squeezed light under different phase angles. The likelihood function is established for the measured quadrature components. An identity matrix is chosen as an initial density matrix, and the density matrix of the squeezed field is obtained through an iterative algorithm. The diagonal elements of the density matrix denote the photon number distribution, which includes not only even photon number states but also odd photon number states. The occurrence of odd photon number states mainly comes from the system losses and the imperfect quantum efficiency of detector. The Wigner function in phase space is calculated through the density matrix, and the maximum value of the Wigner function is 0.309. The standard deviation of the squeezed component is 64.4% of that of the vacuum state, corresponding to the squeezing degree of -3.8 dB. The standard deviation of the anti-squeezing component is 1.64 times that of the vacuum state, corresponding to the anti-squeezing degree of 4.3 dB. We theoretically calculate the photon number distribution and the Wigner function of the vacuum squeezed field, and compare the results obtained by theoretical calculation with those obtained by maximum likelihood reconstruction. The probability of vacuum state [0） obtained by maximum likelihood reconstruction is greater, and the probability of photon number state In/ （n = 1, 2, ） is smaller than the corresponding theoretical calculation results. From the theoretical calculation, the maximum value of Wigner function is 0.231, and the short axis and long axis of noise range deduced from the contours of the Wigner function are larger than the results from the maximum likelihood reconstruction. The possible reasons for the discrepancy are as follows. 1） The phase scanning is nonuniform during the measurement of the quadrature components. 2） The low-frequency electronic noise is not completely filtered out in the datum acquisition process. 3） The datum points of measured quadrature components are not enough. In conclusion, we produce a vacuum squeezed field of Y95 nm, and obtain the photon number distribution and the Wigner function in phase space through maximum likelihood estimation and theoretical calculation, respectively. This work will provide an experimental basis for generating the Schrodinger cat state