Precision spectroscopy on the 2 3S-2 3P transition of atomic helium

Precision measurement in few-electron atomic systems played an important role in testing fundamental physics and determination of the fundamental physical constants throughout the past few decades.Atomic helium,as the simplest multi-electron system,its energy levels can be calculated with a very high precision by means of ab-initio calculations, and can be accurately determined using precision spectroscopy.Test of quantum theories can be achieved by comparing theoretical predictions with experimental results.In case of any disagreement,it might imply that there are some undiscovered systematic effects,or might signal physics beyond the standard model.Particularly,the 2 3PJ energy level in atomic helium is considered as one of the best atomic systems for determining the fine-structure constant α.High precision helium spectroscopy can also be used for setting constraints on exotic spin-dependent interactions,and may provide an accurate determination of the helium nuclear charge radius.Comparison of results from electronic and muonic helium may provide a sensitive test of universality in electromagnetic interactions of leptons,and may help solve the socalled “proton size puzzle”.In this paper,we summarize our recent progress on precision spectroscopy of atomic helium. By using transverse cooling and deflection,we are able to prepare a low-noise bright source of atoms in the metastable state 2 3S1.The initial state preparation is completed by optical pumping,followed by laser spectroscopy in the 2 3S-2 3P transition.The 2 3P0-2 3P2 and 2 3P1-2 3P2 fine-structure intervals are determined to be (31908130.98 ±0.13) kHz and (2291177.56 ±0.19) kHz,respectively.Compared with calculations including terms up to α7m,the deviation for the α-sensitive interval 2 3P0-2 3P2 is only 0.22 kHz,which paths way for further improvement of theoretical predictions and independent determination of α with a 2-ppb precision.The 2 3S-2 3P transition frequency is determined with an accuracy of 1.4 kHz by utilizing comb-linked spectroscopy and first-order Doppler cancellation technique.Our result is not only more accurate but also differs by as much as 50 kHz (20 σ) from the previously reported result.This discrepancy remains unsolved and indicates the need for further independent measurements.In combination with ongoing theoretical calculations,this new result may provide the most accurate determination of helium nuclear charge radius.Prospects for future improvements in relevant precision measurements,including simple molecules,are also discussed.