| The precise spectrum of atomic and molecules plays a central role in the process of human exploration of nature,especially since the birth of laser,the ac-curacy of measurement of atomic molecular frequency transition has been greatly improved.Each increase in spectral frequency resolution has led to a new rev-olution in human awareness,leading to a series of major discoveries:such as fine and hyperfine splitting,Stark effects,Zeeman splitting,and temporal vari-ations of fine constants.In this series of important scientific and technological advances,high-precision spectral line measurement of simple atoms with energy level structure plays a vital role in the fields of basic constant calibration,exotic nuclear structure measurement and quantum electrodynamics correctionRecent high-precision laser spectra of lithium atoms have attracted great attention in both theoretical and experimental fields.The energy level structure of the lithium atom is relatively simple,and there are only three electrons in the outer layer and many properties of lithium atoms can be obtained by the first principle.For example,the Hylleraas variational algorithm modified by quantum electrodynamics can accurately estimate the transition frequency,isotope shift and fine structure interval.The existing theoretical models can be tested ex-perimentally by measuring the precise transition spectra of lithium atoms.The energy levels of Li atoms have been modified by m?7 order QED theory,but the existing two sets of theoretical systems have small differences in the calculation results,and there are differences between the theoretical calculation and the ex-perimental results.Therefore,a set of high-precision measurement system is also needed to modify the theoretical calculation in order to verify the correctness and universality of the QED theoryMany experiments have measured 6,7Li neutral atoms fine and hyperfine frequency splitting and isotope shift,but the current experimental data and theoretical calculations have great differences,and there are many inconsistencies in these measurements,especially reflected in the lithium atoms D1 and D2 line isotope measurements.Almost all previous experiments were carried out in hot atoms or atomic beams,and the resulting frequency shift and other uncertainties limited the accuracy of the measurements.Although the use of cold atoms can improve the accuracy of the measurements,lithium atoms are the lightest metal that can be cooled by laser and have a high recoil momentum,which makes cooling difficult.This paper mainly studies the precise measurement of D line transition fre-quency in ~6Li cold atoms.We use the sub-Doppler cooling mechanism of D1 line in the experiment to make the atomic temperature lower than the Doppler cooling limit.The heterodyne detection method for the noise limit of photon scattering is developed.Precisely control the fluctuations of the cold atoms number and opti-cal density;Combined with optical frequency comb and hydrogen clock,spectral transition frequency in ~6Li cold atom was accurately measured,and the accuracy was nearly one order of magnitude higher than that of the most accurate NIST measurement reported so far.Specific research content has the following points:1.The laser cooling and trapping of lithium atoms is realized by the self-setup cold lithium atomic platform;the temperature of atomic is reduced to 50?K by gray molasses cooling technology which is far below the Doppler cooling limit,and the cooling efficiency of this process is more than 50%which retains a large number of atoms.2.The background magnetic field in the cold atomic region is measured by the same direction two-photon Raman spectrum technology,and the background magnetic field is suppressed to 0.8?T by adjusting the current of the compen-sation coil,which provides a good magnetic field environment for the precise measurement of the transition spectral line.3.The sub-natural linewidth spectral line is introduced into the existing cold atom system,and it is first obtained in the cold atom by the method of delay coherent measurement.Below MHz linewidth spectral line signal can be obtained theoretically by adjusting the delay time,but the D1 line transition spectral line signal with linewidth of 3.5 MHz is obtained under the condition of maintaining the observable signal-to-noise ratio,which is limited by the acquisition efficiency,the other broadening factors and the decrease of the spectral line amplitude By applying this technique to D2 line transitions,a partially distinguishable D2 line transition line is obtained.By improving the acquisition efficiency,further suppressing the remaining broadening,and improving the signal-to-noise ratio of the spectral line signals,it is expected that the non-resolved D2 line transition spectral lines interval can be measured experimentally4.The absolute frequency of ~6Li D line transitions was measured in cold atoms.Combined with optical frequency comb,hydrogen clock and GPS,a fre-quency chain system linking international units of seconds is constructed,and the frequency stability of the ultra-stable cavity in the optical comb is transferred to the detection laser.In the experiment,the method of heterodyne detection is developed,the weak detection light is amplified by local laser,and the signal-to-noise ratio of the absorption spectrum line is improved,and the whole measure-ment process is completed near the photon scattering noise limit.The method of combining probe laser with positive and negative scanning is used to reduce the error caused by the measurement in the spectral line measurement process At the same time,the measurement of absolute frequency tends to zero optical power density is obtained by using the method of optical power extrapolation The absolute frequency measurement of the transition lines ~6Li atoms and D2 lines is finally realized in the cold atomic system.This experiment improves the measurement accuracy by an order of magnitude compared to the previous ex-periments with the best accuracy,in which the absolute frequency uncertainty of the D1 line transition reaches 1 kHzAt present,the measurement of ~6Li and 7Li high-precision isotope shift ex-periments can verify the basic laws of physics with higher accuracy,and it is expected to correct QED and the theory of relativity in a higher order.It is even possible to find new physics beyond the standard model. |
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