非线性吸收损耗对压缩态光场量子噪声特性影响的实验与理论研究

The continuous-variable squeezed states are important resources for quantum information, gravitational wave detection, ultraprecise measurement, and so on. And the squeezed states in the wave bands of rubidium (Rb) and cesium (Cs) atoms absorption line (780nm-860nm) are the necessary resources for extending the quantum information storage time and constructing the quantum information networks. However, different from the experiments of the wave bands of communication wavelengths (1342nm and 1550nm), the wave bands of atoms absorbed wavelengths are too short, and the absorption of the signal and pumping lights are strong in the process of parametric down-conversion. When the two fields are injected into the optical parametric oscillator (OPO) at the same time, the blue light deduces the absorption of infrared light. It will increase the thermal effects of the nonlinear crystal, and form thermal lens and thermal aberrations effect, which will make the mode matching efficiency of the lasing to optical parametric oscillator decrease. The two effects increase the intracavity losses of the OPO, hence it is difficult to improve the squeezing level at the short wavelength. In this subject, by measuring and analysing the characteristics of the nonlinear absorption in the process of parametric conversion, the characteristics of nonlinear absorption losses relating to quantum noise will be investigated, meanwhile several methods will be taken to mitigate the absorption effect. It will provide a theoretical and experimental basis for the generation of high-quality short wavelength squeezed light.

连续变量压缩态光场是量子信息、引力波探测、精密测量等研究领域的一种重要光源。处于Rb、Cs原子吸收线附近的压缩态光场（780-860nm波段）是提高量子存储时间，构建量子网络的必需光源。然而，区别于光通信波段（1342、1550nm等）激光的实验，处于原子吸收线的激光波长较短，在参量转换的过程中，信号光与泵浦光的吸收较为严重；当两者同时注入参量振荡器中时，还会伴随着蓝光导致红外吸收效应，增加了非线性晶体的热效应，形成热透镜效应与热致像差，造成光学参量振荡器模式匹配变差，两种效应进一步加剧了内腔损耗，从而导致难于提高短波长压缩态光场的压缩度。本课题旨在通过采用一定的技术手段，测量并分析短波长激光在参量转换过程中的非线性吸收特性，研究非线性吸收损耗对量子噪声特性影响，同时采取一定的技术手段减轻非线性吸收损耗，为高质量短波长压缩光的产生提供实验与理论依据。

连续变量压缩态光场是量子信息、引力波探测、精密测量等研究领域的一种重要光源。处于Rb、Cs原子吸收线附近的压缩态光场（780-860nm波段）是提高量子存储时间，构建量子网络的必需光源。然而，区别于光通信波段（1342、1550nm等）激光的实验，处于原子吸收线的激光波长较短，在参量转换的过程中，非线性晶体对信号光与泵浦光的吸收较为严重；当两者同时注入参量振荡器中时，还会伴随着蓝光导致红外吸收效应；增加了非线性晶体的热效应，形成热透镜效应与热致像差，造成光学参量振荡器模式匹配变差，两种效应进一步加剧了内腔损耗，从而导致难于提高短波长压缩态光场的压缩度。本课题提出一种利用温度变化导致光学腔模式匹配发生改变测量吸收的方法，测量并分析了短波长激光在参量转换过程中的非线性吸收特性。研究表明，蓝光导致红外吸收与蓝光功率密度的二次方成正比，引入了严重的非线性损耗，从而导致泵浦功率高于阈值的30%后，压缩度急剧下降。我们提出采用四镜环形腔结构可有效降低泵浦光功率密度，提高压缩度，实验中采用四镜环形腔直接探测到4dB的压缩态光场，扣除量子效率的影响后压缩度可达5.3dB。在完成以上课题任务的基础上，我们研究了影响压缩度的两个因素：光学损耗与位相起伏。并采取系列措施降低了系统损耗，提高了位相锁定稳定性，在1064nm波段分别制备得到12.6dB和13.2dB的明亮振幅压缩态和压缩真空态光场，掌握了高压缩度光场制备的关键技术。后续，我们将高压缩度非经典光场应用于量子雷达、光谱测量、引力波探测中，通过降低经典背景噪声，实现突破量子噪声极限的测量，为解决国家重大研究课题提供技术支持。