基于多粒子自旋压缩与量子纠缠的量子度量

According to the scientific goals of the program "physics of precision measurement" from the Major Research Plan of the NSF of China, we start from the basic principle of quantum metrology, and aim at developing new measurement theories, methods and techniques to beat standard quantum limit. Our main research topics include that (1) we generalize previous work of binary-outcome measurement into multi-outcome case. A linear combination of phase estimators that obtained by inverting experimentally observable signals is constructed to saturate Cramer-Rao lower bound over the entire phase interval. To confirm it, we plan to perform numerical simulations for the multi-outcome measurement. (2) For specific probe systems, we investigate the role of noises in quantum interferometry, and suppress the noises using dynamical decoupling and quantum error correction method. A new quantum interference device is proposed to eliminate noise-induced negative contribution in quantum Fisher information, which enables us to improve accuracy of phase estimation. (3) We investigate quantum-enhanced magnetometer based on spin squeezing and multi-particle entanglement. By optimizing the measurement scheme and data processing, together with multi-pulse sequence, we show that the sensitivity of ultracold atomic magnetometer can be further improved by 1 to 2 orders of magnitude. Based on our preliminary works and methods, the present research are sound and feasible, and hopefully make a breakthrough in quantum-enhanced precision measurement and its potential applications.

根据基金委《精密测量物理》重大研究计划的科学目标，本项目从量子度量学基本原理出发，研究突破标准量子极限的测量原理、方法与技术。具体研究内容包括： (1)将前期发展的双输出测量理论推广到多输出测量，通过线性组合实验上可测的反函数估计量，实现相位涨落全局饱和Cramer-Rao下限，并对多输出测量进行数值仿真；(2) 针对具体探测系统研究量子干涉仪的噪声机制，利用动力学退耦合及量子纠错等方法抑制噪声。设计新型量子干涉仪，消除噪声诱导的量子Fisher信息负值贡献，提高相位测量精度；(3)研究基于多粒子自旋压缩或纠缠的新型磁力计，通过优化测量方案和数据处理, 以及之前发展的多脉冲序列方法将单轴扭曲自旋压缩转化为双轴扭曲自旋压缩，力争将现有的超冷原子磁力计灵敏度提升1~2个数量级。本项目立足于前期工作积累和研究方法，研究内容和目标具有较强的可行性，有望在量子精密测量物理与潜在应用上取得突破。

量子度量(Quantum metrology)是对量子系统的物理参数进行高分辨和高灵敏度测量的学科。1981年，Caves提出压缩光干涉仪的相位灵敏度可优于散粒噪声极限。2008年，意大利的Pezze和Smerzi提出符合光子计数是压缩光干涉仪的最优测量方案，相位测量精度可达海森堡极限(Heisenberg limit)，这一结果源自干涉仪中路径对称纠缠态的出现。2010年，以色列Silberberg研究组成功地制备了5光子纠缠态，为多光子量子显微成像提供光源。本课题从量子度量的一般原理出发，具体取得如下原创性研究成果。1）基于双数态干涉仪，研究光量子显微成像的成像品质，提出两种消除图像散斑的方法；2）解析证明光强差测量是双数态干涉仪的最优测量方案，以6光子双数态为例，唯象地计及实验不完美数值模拟实验；3）针对压缩光干涉仪的符合计数探测，解析和数值地研究探测器的数分辨上限对最终相位测量精度的影响，发现探测器的数分辨上限与入射光光子数目可比拟时，相位测量精度依然遵丛海森堡标度律；4）针对任意多输出测量，发展出一个渐近无偏的、且能饱和Cramer-Rao下限的新相位估计量，以压缩光干涉仪的零拍探测为例，数值演示新的数据处理方法，为后续研究量子度量和量子感知提供理论依据。