Li+離子精密光譜與精細結構常數確定

史庭云++高克林++嚴宗朝++鐘振祥++張佩佩

摘 要：精細結構常數是無量綱的基本物理常數，它表征電磁相互作用強度，其高精度確定對基本物理理論的驗證及新物理預言至關重要。氦原子及類氦離子是最簡單的多電子體系，其（1s2p）P三重態精細結構劈裂強烈依賴于精細結構常數。開展對氦原子及類氦離子2S～2P態精密光譜測量可以檢驗現有的兩電子量子電動力學（QED）理論計算，同時也提供了高精度確定精細結構常數的獨立方法。目前，氦原子的2P三重態精細結構光譜測量實驗達到ppb精度，已觸及到高階QED效應，而與之有相似能譜結構的Li+離子測量精度要低2～3個量級。由于QED高階效應對電荷數Z極其敏感，提高Li+的光譜測量精度，可對QED理論計算進行更為嚴格的檢驗。該研究擬采用激光冷卻的Ca+對由Li+亞穩態進行協同冷卻，實現對Li+內外自由度的精確控制，獲得幾十mK量級處于亞穩態的Li+離子晶體，進而對2S～2P三重態548.5 nm躍遷進行光譜測量，預期相對精度達到10～8，較目前Li+同類實驗精度提高兩個量級；同時，開展對現有QED理論進行獨立檢驗，提高理論預言的可靠性；通過實驗光譜與理論預言的對比在10～8相對精度上獨立確定精細結構常數。

關鍵詞：Li+離子 精密光譜 量子電動力學 精細結構常數 離子阱 協同冷卻

Li+ Ion Precision Spectroscopy and Determination of Fine Structure Constant

Shi Tingyun Gao Kelin Yan Zongchao Zhong Zhenxiang Zhang Peipei

（Wuhan institute of Physice and Mathenatics，Chinese Academy of Sciences）

Abstract：The fine structure constant is a dimensionless fundamental physical constant， charactering the strength of electromagnetic interaction. To determine this constant with high precision is of great importance both for the verifications of the basic physical theories and predictions of new physics go beyond particle-physics Standard Model. Helium and helium-like ions is the simplest many-electron systems， whose fine structure of triplet （1s2p）P state strongly depend on the fine structure constant. Precision spectroscopy measurement of Helium and helium-like ions provide stringent test of existent two-electron bound state QED theory and calculation， and an independent method for determination of fine structure constant. Up to now， high accuracy of the fine structure intervals have been achieved to a degree of ppb， which means high-order QED effects can be touched. Li+ ion has a similar energy structure to helium. However， experimental measurement for Li+ ion fine structure intervals with lower accuracy， 2～3 order of magnitude then helium. Since the high-order QED effects are extremely sensitive on charge number Z， the improvement of measurement accuracy for Li+ spectroscopy would provide more stringent test of bound state QED. In this project， metastable 1s2sS triplet state of Li+ ions will be sympathetically cooled by the laser-cooled Ca+ ions， to realize the accurate controlling of the internal and external degree of freedom. Then， metastable Li+ ions， with the transitional kinetic energy around several-tens mK can be obtained. The 548.5 nm transition frequency from triplet 2S state to the triplet 2P states will be measured by the precision spectroscopy method with the relative accuracy about 10～10， more accuracy with two order of magnitude higher than the results up to date. Meanwhile， an independent check for two-electron QED theory and calculation will be carried out to guarantee the correctness of QED theoretic prediction. Comparing the results from our experimental spectroscopy and theoretical prediction， the fine structure constant is expected to be determined with the relative accuracy about 10～8.

Key Words：Li+ ion；Precision spectroscopy；Quantum dynamic mechamics；Fine structure constant；Ion trap；Sympathetic cooling

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