Analysis of squeezed light source in band of alkali atom transitions based on cascaded optical parametric amplifiers

Abstract

The squeezed light field at the band of alkali metal atoms’ transitions is an important quantum resource in the field of quantum information and precision measurement. The wavelengths of atomic transition lines (760-860 nm) are relative short. Limited by the gray-tracking effect of nonlinear crystals, the squeezing degree of the squeezed light at this band generated by the optical parametric amplifiers is low. For now, the squeezing is about 3~5 dB. Considering the problems in the experimental generation of the squeezed light at the wavelengths of atomic transitions, the variation law of quantum noise of the light field output from the single optical parametric amplifier with its physical parameters is theoretically studied, and the optimal physical parameters are obtained. To further improve the squeezing at the band of alkali metal atoms’ transitions, the cascaded optical parametric amplifiers are considered. Based on the basic theory of the optical parametric amplifiers, the theoretical model of the cascaded optical parametric amplifiers is constructed, in which the optical loss and phase noise of the cascaded optical loops are considered. Based on this, the quantum noise characteristics of the light field output from the cascaded system versus the optical loss and phase noise are analyzed at the frequencies of 2 MHz and 100 kHz, respectively. It is found that for the squeezing at 2 MHz, cascading 2 to 3 optical parametric amplifiers can significantly improve the squeezing under the premise of the low optical path loss and phase noise; for the squeezing at low-frequency band, the enhancement of the squeezing for the cascaded system is quite weak. Under the current experimental parameters, the squeezing at 2 MHz of the squeezed light on rubidium resonance can be improved from -5 dB to -7 dB by cascading another DOPA. For the squeezing at low frequency band, the cascaded is proved to be useless, and the efforts should be made in reducing the technique noise at low frequency band. Furthermore, the quantum limit and spectral characteristics of the squeezed light field output from the cascaded system are further explored. This study can provide reference and guidance for the improvement of the squeezing at the band of atomic transitions.

Research on intensity–difference squeezing enhancement of phase-sensitive amplifier based on coherent feedback

“…1. 引言 压缩态光场是量子光学的重要概念， 其不仅对于量子物理的基础研究具有重 要意义，还可作为量子通信、量子计算和量子精密测量领域的重要量子资源。到 目前为止，已经在多种实验系统中实现了压缩态光场的制备。基于原子四波混频 效应的光参量放大器是最早实验实现压缩态光场的装置 [1] 。此外，将非线性光学 晶体置于谐振腔内，利用参量下转换也可产生此非经典光场 [2] 。与基于非线性晶 体的压缩光制备方案相比，四波混频系统具有独特的优势，例如与原子跃迁线天 然匹配、无需复杂的光学谐振腔、强的非线性相互作用、多空间模压缩特性等。 目前，利用铷原子蒸汽四波混频系统产生的强度差压缩态光场，实验上演示了此 非经典光场在量子信息以及量子精密测量领域的应用优势，例如 EPR 纠缠态的 可调谐延迟 [3] 、磁力梯度仪灵敏度的提升 [4] 、全光纠缠交换 [5] 等。在上述应用中， 为了提升量子信息传输的保真度以及精密测量的灵敏度， 对量子态的关联特性有 较高的要求。因此，提高光参量放大器输出光场的强度差压缩具有重要的意义。 对于光参量放大器输出光场的强度差压缩特性的提升，目前有很多种方案。 一种很自然的方案是级联， 即将第一个光参量放大器输出光场作为输入光场注入 到下一步光参量放大器。目前，有很多研究组开展了基于级联方案的量子特性增 强的理论研究 [6][7][8] ，进一步得到了实验验证 [9][10][11] 。另一个可行的方案是相干反馈的 方案，即将光参量放大器输出的非经典光场通过分束器分出一部分，反馈到光参 量放大器的输入端，最终输出光场的非经典特性得到提升。2009 年，Gough 等 人最早提出了相干反馈实现量子压缩增强的方案 [12] 。相干反馈的方案相比级联 的方案，其系统简单，无需另一套光参量放大器。因此，国内外很多研究组开展 了相关的理论和实验研究，有代表性的是日本东京大学 Furusawa 研究组 [13] 和山 西大学的贾晓军教授研究组 [14] ，其分别在实验上演示了基于相干反馈的连续变 量正交压缩态光场和正交纠缠态光场的量子特性增强。2017 年，华东师范大学 的荆杰泰教授课题组在铷原子四波混频的非相敏放大器系统中， 实验证实了相干 反馈对强度差压缩的增强效应 [15] 。2020 年，该研究组理论讨论了相干反馈对级 联非相敏放大器输出的三光关联特性的增强 [16] 。基于原子四波混频的光参量放 大器，除了上述非相敏放大器，还有相敏放大器。相敏放大器由于量子干涉效应 的存在 [17,18] ，其产生的强度差压缩要优于非灵敏光参量放大器。然而，目前还 没有将相干反馈用于相敏放大器的理论和实验报道。此外，上述相干反馈的非相 敏放大器为单端反馈 [15,16] )…”