史庭云++高克林++嚴(yán)宗朝++鐘振祥++張佩佩

摘 要：精細(xì)結(jié)構(gòu)常數(shù)是無量綱的基本物理常數(shù)，它表征電磁相互作用強(qiáng)度，其高精度確定對基本物理理論的驗(yàn)證及新物理預(yù)言至關(guān)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(tài)精細(xì)結(jié)構(gòu)劈裂強(qiáng)烈依賴于精細(xì)結(jié)構(gòu)常數(shù)。開展對氦原子及類氦離子2S～2P態(tài)精密光譜測量可以檢驗(yàn)現(xiàn)有的兩電子量子電動(dòng)力學(xué)（QED）理論計(jì)算，同時(shí)也提供了高精度確定精細(xì)結(jié)構(gòu)常數(shù)的獨(dú)立方法。目前，氦原子的2P三重態(tài)精細(xì)結(jié)構(gòu)光譜測量實(shí)驗(yàn)達(dá)到ppb精度，已觸及到高階QED效應(yīng)，而與之有相似能譜結(jié)構(gòu)的Li+離子測量精度要低2～3個(gè)量級。由于QED高階效應(yīng)對電荷數(shù)Z極其敏感，提高Li+的光譜測量精度，可對QED理論計(jì)算進(jìn)行更為嚴(yán)格的檢驗(yàn)。該研究擬采用激光冷卻的Ca+對由Li+亞穩(wěn)態(tài)進(jìn)行協(xié)同冷卻，實(shí)現(xiàn)對Li+內(nèi)外自由度的精確控制，獲得幾十mK量級處于亞穩(wěn)態(tài)的Li+離子晶體，進(jìn)而對2S～2P三重態(tài)548.5 nm躍遷進(jìn)行光譜測量，預(yù)期相對精度達(dá)到10～8，較目前Li+同類實(shí)驗(yàn)精度提高兩個(gè)量級；同時(shí)，開展對現(xiàn)有QED理論進(jìn)行獨(dú)立檢驗(yàn)，提高理論預(yù)言的可靠性；通過實(shí)驗(yàn)光譜與理論預(yù)言的對比在10～8相對精度上獨(dú)立確定精細(xì)結(jié)構(gòu)常數(shù)。

關(guān)鍵詞：Li+離子 精密光譜 量子電動(dòng)力學(xué) 精細(xì)結(jié)構(gòu)常數(shù) 離子阱 協(xié)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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