1.China Academy of Space Technology (Xi’an), Xi’an 710100, China 2.State Key Laboratory of Quantum Optics and Quantum Optics Devices, Institute of Opto-Electronics, Shanxi University, Taiyuan 030006, China 3.Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 11654002, 61575114, 61501368, 11505135), the Program for Sanjin Scholar of Shanxi Province, China, the Fund for Shanxi “1331 Project” Key Subjects Construction, China, and the Program for Outstanding Innovative Teams of Higher Learning Institutions of Shanxi, China.
Received Date:29 December 2018
Accepted Date:18 April 2019
Available Online:01 June 2019
Published Online:20 June 2019
Abstract:Squeezed states, which have fewer fluctuations in one quadrature than vacuum noise at the expense of increasing fluctuations in the other quadrature, can be used to enhance measurement accuracy, increase detection sensitivity, and improve fault tolerance performance for quantum information and quantum computation. In this paper, the influences of relative intensity noise (RIN) of all-solid-state single-frequency laser and single-frequency fiber laser on the squeezing factor of squeezed vacuum states are experimentally and theoretically studied. Here, an all-solid-state single-frequency laser and a single-frequency fiber laser each are used as a light source of the system generating squeezed vacuum states. The homodyne detection is used to compare the RIN of all-solid-state single-frequency laser and that of single-frequency fiber laser at the analysis frequency of 1 MHz. The results show that the RIN of the all-solid-state single-frequency laser and single-frequency fiber laser are higher than those of the shot noise limitation 2.3 dB and 30 dB at the analysis frequency of 1 MHz, respectively. The RIN of all-solid-state single-frequency laser is far less than that of the single-frequency fiber laser. As a result, squeezed vacuum state with maximum quantum noise reduction of (13.2 ± 0.2) dB and (10 ± 0.2) dB are directly detected. Theoretical calculation shows that the influence of the RIN on the measurement accuracy is the major factor of degrading the squeezing factor with the fiber laser as the pump source. The measurement error of squeezed vacuum state caused by the RIN of single-frequency fiber laser is about 2.6 dB. The discrepancy of the pump power between the two lasers is another factor of affecting the squeezing factor, corresponding to 0.6 dB quantum noise difference. The theoretical calculations are consistent with the experimental results, which provides some guidance for developing the practical squeezed states with highly squeezing level. Keywords:squeezed vacuum state/ relative intensity noise/ solid-state laser/ fiber laser
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--> --> --> 1.引 言量子精密测量是继激光精密测量之后兴起的一项颠覆性技术, 传统的激光精密测量技术利用激光的相干性实现目标相位信息的高精度采集, 实现对目标信息的高精度重构; 该技术具有非接触性和高灵敏度的特性, 已经在科学研究、生物医学、工业生产、空间技术和国防技术中得到了广泛应用[1—7]. 随着科技的进步和发展, 基于相干态光场的经典测量已不能满足人们对测量精度的要求. 为了突破散粒噪声基准(shot noise limit, SNL)进一步提升测量精度, 量子技术成为人们研究的焦点. 而压缩态光场作为一种潜在应用十分广泛的非经典光场, 可用于量子信息、量子成像和精密测量等诸多领域, 所以性能优良的压缩态光场是提升量子态保真度和测量灵敏度的必要条件. 而制备性能优良的压缩态光场的一个基本前提就是需要一款性能优良的激光光源. 近年随着技术和工艺不断进步, 半导体激光二极管(laser diode, LD)抽运的全固态激光系统体积更小、功率更高, 可长期稳定运转且相对强度噪声(relative intensity noise, RIN)可在1.5 MHz之后达到SNL[8—10]. 这期间LD抽运的单频光纤激光器同样也发展的更加成熟和稳定[11]. 激光光源的不断完善解决了基于激光器的各种光学系统的实用化问题, 如激光雷达、激光通信以及用于探测引力波的激光干涉仪等. 随着量子技术的发展, 激光器作为制备非经典光场的光源, 成为制备高品质非经典光场的关键, 其强度噪声特性与非经典光场的噪声水平直接相关[12—14]. 如在压缩真空态光场的测量中, 激光器的RIN会通过本底光耦合到平衡零拍探测过程进而影响测量精度. 本文从全固态单频固体激光器和单频光纤激光器的强度噪声特性出发, 对比研究了这两种单频激光器作为非经典光场实验产生系统的光源, 在制备得到压缩真空态光场后使用平衡零拍探测系统测量压缩真空态光场噪声水平时, 本底光的RIN对压缩度测量精度的影响, 进而为研制高压缩度压缩真空态光源和优化平衡零拍探测系统的性能提供了新的思路. 2.实验分析图1所示为制备压缩态光场的实验系统. 本实验系统的光源部分由一台1064 nm单频激光器构成. 分别采用山西大学研制的单频Nd:YVO4固体激光器和华南理工大学研制的掺Yb磷酸盐单频光纤激光器作为光源进行对比研究[4]. 本实验系统采用外腔倍频的方式获得532 nm激光, 用于抽运光学参量振荡器产生压缩真空态光场. 故该实验系统的核心部分为倍频腔和光学参量振荡腔. 然后是实验系统的探测部分由测量压缩真空态光场压缩度的平衡零拍探测装置和测量本底光RIN的自零拍探测装置组成, 用于探测压缩真空态光场的噪声水平和本底光的RIN. 此外还在实验系统各部分光路中插入了多个模式清洁器(mode clear, MC), 用于优化系统各处光束的空间模式分布和滤除激光携带的经典技术噪声. MC的详细参数如下: 腔长为430 mm, 两个平面镜45°入射透射率均为1%@1064 nm/532 nm, 凹面镜0°入射反射率为99.95%@1064 nm/532 nm, 曲率半径为1000 mm. 为了保证实验系统的稳定运转, 改进了光学谐振腔和移相器的机械结构以及电光相位调制器和探测器的电学性能[15, 16], 而且还根据锁定环路的传递函数针对性地优化了边带锁频环路的各项参数, 保证了谐振腔腔长和光场相对相位的稳定, 为获得高压缩度的压缩态光场奠定了基础. 图 1 本底光RIN测量装置和压缩态光场产生实验系统(SHG, 倍频; EOM, 电光调制器; PZT, 锆钛酸铅压电陶瓷; BHD, 平衡零拍探测器; DBS, 分束镜; OPA, 光参量放大器; LO beam, 本底光; SA, 频谱仪) Figure1. Schematic of the experimental setup for measuring the local oscillator intensity noise and generating the squeezed state (SHG, second-harmonic generation; EOM, electro-optic modulator; PZT, piezoelectric ceramic transducer; BHD, balanced homodyne detector; DBS, dichroic beam splitter; OPA, optical parametric amplifier; LO, local oscillator; SA, spectrum analyzer).
为了分析激光器的RIN对实验测量压缩真空态光场压缩度的影响, 需要测量单频固体激光器和单频光纤激光器作为实验系统光源时本底光的RIN(测量光功率1 mW), 图2和图3所示为对本底光RIN的测量结果. 图2为采用光纤激光器为实验系统光源时, 对应的本底光的RIN在分析频率1 MHz处高出SNL约30 dB, 此处恰好对应单频光纤激光器的弛豫振荡峰. 图3为采用单频固体激光器为实验系统光源时, 对应的本底光的RIN在分析频率1 MHz处高出SNL约2.3 dB. 以上测量所用仪器为罗德斯瓦茨FSW公司的Signal & Spectrum Analyzer·2 Hz to 13 GHz频谱仪. 对比以上两种本底光的RIN数据, 发现单频固体激光器输出的激光经MC滤除一部分噪声后本底光的RIN在分析频率1 MHz处远低于单频光纤激光器输出的激光经MC滤除噪声后本底光的RIN. 图 2 单频光纤激光器经MC滤除一部分RIN和相位噪声后对应的本底光RIN Figure2. RIN of local oscillator with single-frequency Yb3+-doped phosphate fiber laser after MC.
图 3 单频固体激光器经MC滤除一部分RIN和相位噪声后对应的本底光RIN Figure3. RIN of local oscillator with single-frequency Nd:YVO4 laser after MC.
图4是采用山西大学研制的单频Nd:YVO4固体激光器作为实验系统的光源时, 直接测量到的压缩真空态光场的最大压缩度, 图5是采用华南理工大学研制的掺Yb磷酸盐单频光纤激光器作为实验系统的光源时, 直接测量到的压缩真空态光场的最大压缩度. 图 4 单频固体激光器制备的压缩真空态光场的噪声谱, 分析频率1 MHz (分辨带宽RBW = 300 kHz, 视频带宽VBW = 200 Hz) Figure4. Balance homodyne measurements of the quadrature noise variances at a Fourier frequency of 1 MHz, with a resolution bandwidth RBW of 300 kHz and a video bandwidth VBW of 200 Hz.
图 5 单频光纤激光器制备的真空压缩态光场的噪声谱, 分析频率1 MHz (RBW = 300 kHz, VBW = 200 Hz) Figure5. Balance homodyne measurements of the quadrature noise variances at a Fourier frequency of 1 MHz, with a RBW of 300 kHz and a NBW of 200 Hz.
图 1 本底光RIN测量装置和压缩态光场产生实验系统(SHG, 倍频; EOM, 电光调制器; PZT, 锆钛酸铅压电陶瓷; BHD, 平衡零拍探测器; DBS, 分束镜; OPA, 光参量放大器; LO beam, 本底光; SA, 频谱仪)
图 2 单频光纤激光器经MC滤除一部分RIN和相位噪声后对应的本底光RIN
图 3 单频固体激光器经MC滤除一部分RIN和相位噪声后对应的本底光RIN
图 4 单频固体激光器制备的压缩真空态光场的噪声谱, 分析频率1 MHz (分辨带宽RBW = 300 kHz, 视频带宽VBW = 200 Hz)
图 5 单频光纤激光器制备的真空压缩态光场的噪声谱, 分析频率1 MHz (RBW = 300 kHz, VBW = 200 Hz)
