Precision Laser Spectroscopy Of Helium Atomic Beam With Selected Velocity

The precise spectrum of simple atomic system is a powerful tool for accurate mea-surement of basic physical laws and constants.Helium atom is the simplest multi-electron atom,and high-precision theoretical results can be obtained by full quantum ab initio calculation method,which makes helium become a platform to directly test the theory of quantum electrodynamics(QED).On the one hand,the comparison of high-precision experimental measurement results and theoretical calculation values can test the relevant theories.If there are significant differences between them,there may be unknown system effects,and even provide experimental basis for discovering new physics beyond the Standard Model(BSM).On the other hand,on the premise that the theory is correct,some basic physical constants can be determined by high-precision experimental measurements.Helium atomic spectroscopy can be used to determine the fine-structure constant a and the radius of helium nucleus.It can even test the universal-ity of lepton electromagnetic interaction in the Standard Model at low energy scale by comparing the size of a measured by spectroscopic method,electron scattering method and the experiment of ?He+.Firstly,this paper introduces the progress of physics related to helium atom,the test of QED theory,and the attention to the precision spectroscopy research of helium.For the 23S-23P transition of helium,the recent situation of experimental measurement and theoretical calculation accuracy of different research groups in the world,as well as the research situation of nuclear charge radius and isotope shift related to this transition are described.According to the characteristics of the precise spectrum experiment based on atomic beam,we have built a helium atomic beam device with high brightness,high stability and adjustable speed.The structure and design principle of the experimental system are described in detail,which is divided into two parts:atomic beam and optical system.Compared with our previous experimental device,the system has made consider-able improvement.The optimized structure of the source cavity equipped with a large turbo pump,which improves the vacuum background pressure of the source cavity by an order of magnitude and reduces the collision loss of metastable helium atoms.Zee-man slower was added to realize the continuous adjustment of the longitudinal velocity of atomic beam,and at the same time,it also dramatically increases the flux of atoms at the selected velocity.An additional 2D-MOT was installed at the outlet of the Zeeman slower,which further compresses the atomic beam and improves the beam intensity.Using the feedback control method,the beam stability is improved by a beam stabiliza-tion system.The system performance test results show that the longitudinal velocity of the atomic beam can be varied from 50 m/s to 450 m/s,the beam temperature is less than 10 mK,the intensity in a single momentum and quantum state reaches 1.8 × 1013 atoms/s/sr,with a fractional fluctuation of 0.02%in a period of 100 s.We use the method of laser cooling,focusing and deflecting metastable helium atom beam,and use Stern-Gerlach magnet to realize single quantum state detection,at the same time,the optical frequency comb calibrates the laser frequency.In this paper,the experimental scheme for studying the 23S-23P transition of 4He and the systematic error analysis of the preliminary experimental results are described in detail.For the first-order Doppler frequency shift which usually needs to be considered in atomic beam experiments,due to the longitudinal velocity of the beam system can be adjusted,the transition frequency can be obtained by velocity extrapolation,and the first-order Doppler effect and other system errors can be better evaluated.At present,we have measured the center frequency of 23S-23P transition of 4He on this beam device,and the preliminary experimental results are consistent with the previous measured values of our experimental group within the error range.It is predicted that the experimental accuracy can be improved by two times,and it will enter the sub-kHz level for the first time.In principle,it is enough to get the helium nucleus radius better than 1%accuracy by further confirming the calculated results of this energy level with theory.At the same time,the measurement of 4He and 3He 23S-23P transition frequency is to be completed on this beam device,and we hope to help find out the reason why the difference of the squared nuclear charge radii of these two isotopes has considerable deviation.