Research On The Measurement Of The Cold Collision Shift In The Cesium Atomic Fountain Clock

The time unit,"second" is defined as the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom without any interference at the sea level.The cesium atomic fountain clock is a primary frequency standard that can realize the definition of the second?in SI?,and is used to calibrate the frequency values of other atomic clocks.The national time service center of the Chinese Academy of Sciences undertakes the task of producing and maintaining the national standard time,UTC?NTSC?.The developing cesium atomic fountain clock in NTSC can real-timely calibrate the local atomic time TA?NTSC?,which can avoid the dependence of the delaying one month data released by the International Bureau of Weights and Measures?BIPM?and can enhance the performance of the national standard time UTC?NTSC?.The working medium in the cesium atomic fountain clock is cold cesium atoms.The collision between the cold cesium atoms makes the spin state of the valence electrons exchange,which causes the shifts of the hyperfine magnetic energy levels and finally results in the variation of the transition frequency between the hyperfine magnetic energy levels.Based on the physics principle mentioned above,the cold atomic collision frequency shift is produced,which is one of the main systematic errors limiting the promotion of the frequency uncertainty of cesium atomic fountain clock.The research of this thesis focuses on the measurements of cold atomic collision frequency shift and on the evaluation of the frequency uncertainty of the cold atomic collision frequency shift in NTSC-F1.In the aspect of the theory,the physical mechanism of cold atom collision frequency shift is analyzed.A new scheme is proposed to reduce cold atomic collision frequency shift in the cesium atomic fountain clock.In the aspect of the experiments,the difference method used to measure the cold collision frequency shift is realized.The evaluated type B uncertainty of the cold atomic collision frequency shift of NTSC-F1 is 6.5×10^-16.The main research contents of this thesis are as follows:1.This thesis carried out some theoretical study on the measurement of cold atomic collision frequency shift.On the basis of quantum scatter theory,the relation between the collision frequency shift and the collision parameters has been analyzed.The usual differential method to measure the cold collision frequency shift is studied based on the research results mentioned above and the imperfections of the usual differential method are also studied.Thus,the preparation method of the cold atom clouds with the adiabatic transition technique is studied,which can realize the precise ratio between the high and low atomic density.In this chapter,the Feshbach resonance technique used to reduce the cold atomic frequency shift is also studied.2.Carry on the research on improving and enhancing the frequency stability of cesium atomic fountain clock in order to accurately measure cold atomic collision frequency shift.It is necessary to be under the condition of high frequency stability to precisely measure the cold atomic collision frequency shift in the cesium atomic fountain clock.In order to better study on the measurement of cold atom collision frequency shift,this thesis carries out two parts work to optimize the performance of NTSC-F1 frequency stability:The first part work is that the detection laser power should be stabilized in order to reduce the detection noise caused by the intensity fluctuation of detection light.This thesis proposes a new method to obtain the monitoring optical path by using uncoated quartz thin glass.This new method can avoid a type of laser power fluctuation caused by the random fluctuation of the polarization direction after the fiber transmitting.By using this method,the power of the two detection laser beams can be stabilized at the same time and the noise level of the two beam are both reduce from 10^-2 to 10^-5 order.The TOF signals obtained in experiments of the NTSC-F1 show that the constructed power stabilization system applied in the detection laser can effectively reduce the detection noise in NTSC-F1 and thus,the S/N ratio of cold atomic fluorescence signal collected by NTSC-F1 is improved.The second part work is to calibrate the optical fiber beam expanding collimation system used in trapping cold atoms.According to the decentered Gaussian beam theory,this thesis proposes a novel method to realize the detection and adjustment of the light spot intensity distribution uniformity.By using CCD image detection,the new method can realize the detection of concentricity between the position of Gauss intensity maximum?Gauss center?and the position of light spot center.The new method effectiveness experimental results demonstrate that the new method proposed in this thesis is effective for the detection of the intensity distribution non-uniformity caused by the incident tilt.The proposed CCD spot image detection system is constructed,and the corresponding calibration work is completed.From the comparison of the images of the captured cold atomic clouds before and after adjustment,the calibration of the beam expanding collimation system can obviously improve the shape and density distribution of the trapped cold atomic clouds.The density distribution of the trapped cold atomic clouds is approximated as a spherical Gauss distribution.It is improvable for the operation parameters of NTSC-F1 after the adjustments of the beam expanding collimation system.The temperature of the cold atomic clouds is reduced from 10?K to 3?K and the number of the captured atoms is increased from 107 to 108.3.Carry on the experimental study on the differential method to measure of the cold atom collision frequency shift.Based on the optimizations of the technology in the cesium atom fountain,NTSC-F1,the measurement experiments of the cold atomic collision frequency shift were carried out.First of all,the steps and processes of measurement are established by analyzing the theory of the difference method of measuring the cold atomic collision frequency shift.Then,the closed-loop locking process is studied for reason of the acquiring coarse error data in the closed-loop locking phase when measuring the cold atomic collision frequency shift.By the theoretical analysis,the condition to discriminating the coarse error existing in the data of the difference of the transition probability is found.Based on the condition to discriminating the coarse error,a new closed-loop locking algorithm is proposed.Based on the theoretical analysis mentioned above,the calculation of the frequency shift and its uncertainty after measurement of the cold atomic collision frequency are studied.Finally,according to the established measurement steps and processes,measuring in the state of closed loop lock for 5 days,the cold atomic collision frequency shift of NTSC-F1 is evaluated preliminarily.The evaluated type B uncertainty of the cold atomic collision frequency shift is 6.5×10^-16...