Study On The Evaluation Of Cold Atom Collisional Frenquency Shift Of Cesium Fountain Clock

As the unit of time,"second" is defined by the quantum transition frequency between the hyperfine levels of the unperturbed ground state of the cesium-133 atom..The cesium atomic fountain clock,as a device that realizes the definition of seconds,is used to calibrate the frequency of time keeping clock in the time system,which make it an indispensable part of the standard time generation process.Frequency uncertainty is the main index of the performance of cesium atomic fountain clock.It is obtained by accurately measuring all frequency shifts that may cause deviation from the defined frequency of 9,192,631,770 Hz.The uncertainty of each frequency shift was evaluated and amounted to the total frequency uncertainty,which is used as the performance index of cesium atomic fountain clock.In the operation of cesium fountain clock,a cloud of cold atoms??K?is trapped and launched vertically and fall back freely,interact with the microwave field twice.As the temperature decreases,the wavelength of the atom's de Broglie wave becomes larger.The collision between cesium atoms is essentially different from the collision at room temperature.A significant quantum effect,the spin-exchange effect,causes the shift of transition frequency.Cold atom collisional frequency shift is one of the most important systematic frequency shift in the uncertainty evaluation of the cesium atomic fountain clock.The research work focuses on the frequency shift in this paper.The physical mechanism and the evaluation methods were investigated theoretically and experimentally,and accurate evaluation was implemented.The research contents are summarized as following:1.Research on the physical mechanism and measurements of cold atom collisional frequency shift.Based on the relationship between the cold collisional frequency shift and the atomic density,the statistical uncertainty and the systematic uncertainty of the differential method were analyzed.The univariate Rabbi method was used to prepare high and low density atoms for measuring the collisionalfrequency shift.The linear relationship between frequency shift and atomic density ratio,which is the crucial part of the measurement,is analyzed based on the data and theoretical calculations.Factors that may cause deviation from the linearity were studied.2.The theoretical and experimental study of Rapid Adiabatic Passage method.In order to evaluate the uncertainty of the measurement,the evolution of the atomic state during the adiabatic transition process and the accuracy deviation are analyzed,and an equation for the evolution of the error is derived.The equation enables more simply analysis of the source of error,and helps us to determine proper parameters,and to choose better pulse used in the method.Each important parameter,including pulse shape parameters,time parameters,and frequency parameters,is studied through theoretical analysis and experiment.The four main factors that affect the uncertainty of the method were analyzed one by one.Firstly,the evolution process can deviates from adiabatic process and causes error if the parameters of the pulse are not suitable.The effection of the amplitude of the microwave power and frequency modulation was carefully studied.The trajectory of atoms was precisely measured,which can be used to determine the time parameters.This result can also be used for the evaluation of the microwave leakage shift by a Mach-Zehnder interferometric switch.Secondly,the discrepancy between the center frequency of microwave pulse and the resonant frequency of atomic transition is another source of error.To reduce the discrepancy,the magnetic field around the selection cavity is investigated.The resonant frequency at the end of the selection pulse is measured by the transition of a short microwave pulse,which eliminated a main factor that affected the accuracy of the Rapid Adiabatic Passage method.The third factor is the sideband effect of microwave pulse,it can be reduced effectively by increasing the magnetic field.Finally,some background atomic number which is not selected by microwave can lead uncertainty to the atomic number ratio.The basic nature of this factor is studied,and the adiabatic transition method is proved in agreement with theoretical expectation up to 10^-3 whendeducting the background atoms from the total atomic number.3.Study on the double variable Rabi method.On the basis of the existing double variable Rabi methods,it was re-evaluated with different standard.Even the transition probability is not homogeneous,it was proved that this method still can be used to prepare atoms with homogeneous transition probability ratio between high and low density,which is enough for measuring collisional frequency shift.The way to find suitable parameters to adjust the microwave intensity and frequency was developed,which enable preparing atoms of arbitrary density ratio.Through numerical analysis and experiments,the homogeneity of the atomic ratio of the double variable Rabi method was studied,and the local homogeneity of the ratio can be evaluated through the change of the parameters.The uncertainty of measuring the collisional frequency shift by this method was evaluated.4.The comparison between the cesium fountain clock and the Coordinated Universal Time was implemented.The uncertainty of the comparison was 2.9×10^-15.