单量子态氦原子的制备和精密测量

Quantum electrodynamic theory(QED) of the simplest bound-three-body system is now stringently tested by precise spectroscopic frequency measurements in helium. Alternatively, comparison between Helium measurements and theory could be used for accurate determination of some fundamental quantities, as for example, the fine structure constant α, and the differenced of nuclear charge radii between two 3^He and 4^He. The obvious discrepancy between the available experimental and theoretical values of the helium triple-state fine structure splitting demands new precise measurements. However, the difficulty of the precise measurements includes both to obtain a high signal-to-noise ratio measurement, and to exclude the impact of the external environment and the systematic error. Compared with other Group's measurement, in order to minimizing the systematic error and improving the accuracy of the experiment measurements,the present project plans to use the method of laser cooling to obtain a high brightness, single quantum state of the helium atoms beam. Ultimately Sub-kHz precision spectroscopy measurement of helium atoms will be made, and the results will potentially lead to the inspection of the fundamental quantum electrodynamics theory, and the associated important physics quantities determination.

氦原子是最基本的多电子原子，其精密光谱一直是检验束缚态系统量子电动力学理论的重要手段，通过比对氦原子理论计算和实验测量的结果，可以得到精细结构常数α、以及4^He与其同位素3^He原子核半径等基本物理参数。目前对于三重态能级精细结构分裂的实验测量和理论计算结果间存在kHz水平的偏离，亟待新的独立高精密光谱测量进行检验。实验中的关键是获得足够高的测量信噪比、以及排除外界电磁环境等产生的系统偏差。本项目计划利用激光横向冷却和二维磁光囚禁的方法，获得高亮度、单量子态的氦原子束流，提高实验测量信噪比，通过精密的电磁屏蔽，减小系统偏差的影响，实现亚kHz水平的氦原子精密光谱测量，最终实现在该原子系统中对量子电动力学的检验，及精细结构常数和氦核结构参数等重要物理学量的测量。

氦原子是少电子束缚态的量子电动力学理论的经典研究对象，也是少数几个基于量子力学和基本物理常数、而不需要任何可调参数就可以计算达到极高精度的量子体系之一。因此，氦原子精密谱一直是一个十分理想的在原子分子体系中检验量子电动力学理论的平台。本项目按计划完成搭建了高亮度、单一量子态的氦原子束流系统，并利用高精度激光光谱的方法，测量得到了氦原子的能级结构。项目关于氦-4原子23PJ能级23P1-23P2和23P0-23P2精细结构分裂的测量，目前获得了国际上最优的测量精度，结果分别是31 908 130.98(13)kHz和2 291 177.56(19)kHz。其中和精细结构常数敏感的23P0-23P2间隔测量结果，在实验上把测量精细结构常数α的精度提高到了2ppb，同时通过和目前最精确的、包含高阶α5Ry量子电动力学修正项的计算结果相比较，其结果偏差仅在0.22kHz。首次确认了在该精度上的高阶量子电动力学理论修正，为未来该方向理论的改进发展铺平了道路。