6Li-7Li冷原子中绝对跃迁频率和精细结构常数的精密测量

Precise measurements of both absolute frequencies and small frequency differences of atomic energy levels have played an important role in the development of physics. For example, high precision measurements of absolute frequencies of the 2S½ → 2P ½ transition (D1 line) of alkali atoms form an important link in the measurement of the fine structure constant, α. Similarly, precise interferometric measurements of the local gravitational acceleration (g) rely on the knowledge of the absolute frequencies of the 2S½ → 2P3/2 transition (D2line) in alkali atoms. A number of recent experiments have measured the fine and hyper structure splittings as well as the isotope shifts for D1 and D2 lines at optical frequencies for stable 6Li and 7Li. These data offer an important comparison with current most accurate calculations including to quantum electrodyn amics, relativistic corrections and finite nuclear size in three electron atoms. Most importantly, they can be used to determine the nuclear size and measure fine structure constant at high level and test the fundamental theory. However, they are considerable discrepancies in experimental tests of the theories. The precision measurement of Lithium atoms has not yet reached an accuracy matching the of the structure calculations. And the previous measurements are in disagreement. In particular, there is a significant discrepancy in measurements of the isotope shifts of the D1 transitions of Li. Here we propose to perform the precise measurement of the absolute frequencies, isotope shift and fine structure constant based on the stabilized optical comb in lithium atoms. In contrast to the previous measurements in hot atomic beams, we use the ultracold atoms of 6Li and 7Li. We develop the two photon coherent spectroscopy with ultrahigh resolution and will greatly improve the accuracy of the measurement. We aim to get the measurements that are more than two orders of magnitude more accurate than the all previous measurements. The measurement of absolute frequency will be reached with an uncertainty of about 0.001ppb and the measurement of the fine structure constant is at the uncertainty of 0.2ppb. The relative nuclear charge radius will de determined with an uncertainty approaching 10^(-18) meter.

近来许多实验测量了6Li和 7Li中性原子的精细和超精细频率分裂和同位素频移。这些实验数据能验证当前最精确的包括量子电动力学、相对论修正和三个电子的有限原子核大小的理论计算。更重要的是他们能用来确定核半径和测量精细结构常数α，从而在更高的精度上测试基本的物理规律。 然而当前的实验数据和理论计算有着大的分歧，即使是这些测量也存在着许多不一致，锂原子的精密测量还没有达到和理论相一致的精度。 本项目将在冷的6Li和 7Li混合原子中使用更高精度的光学频率梳和发展更高灵敏度的量子相干光谱去精确的测量精细和超精细能级频率分裂和精细结构常数α，希望能解决当前对锂原子测量的分歧。目标是在当前的锂原子测量上提高1-2个量级：绝对频率测量在0.001ppb精度上；精细结构常数的不确定度在0.1ppb上；核电荷的半径的测量不确定度达到10^(-18)米。然后在如此高的精度和准确度上去测试基本的规律。

我们实现了费米6Li原子的全光冷却和俘获， 针对6Li原子冷却难的特点发展了D1线（灰MOT），冷却6Li原子的温度低于Doppler冷却极限；利用双光子光谱测量了D1线的精细分裂； 采用量子相干的双光子吸收（荧光）光学手段，极大的提高光谱的灵密度，采用相位调制和单光子延迟测量技术，压窄谱线线宽，在极弱探针光（万分之二个饱和光强）下得到了高精度的D1和D2线光谱；结合光学频率梳技术，精确的测量2S到2P的跃迁频率和精细分裂。 测量的结果和当前最精确的理论计算相一致