| With the rapid development of optical atomic clocks,their frequency instability and uncertainty have been better than the best microwave clocks at present,which make it possible to redefine the "second" by optical clocks instead of microwave frequency standard.So far,several optical frequency transitions have been selected as secondary definitions of the second,and the transition 6s2 1S0-6s6p 3P0 at 578 nm has been selected as one of them.Therefore,it is important to measure the absolute frequency of this transition.The absolute frequency measurement is to trace the measured value of the atomic transition frequency to the International System of Units(SI)"second".The content of this thesis will focus on the absolute frequency measurement of 171Yb optical clock,including frequency measurement system,time-frequency transfer link and the systematic evaluation of the optical clock,etc.As a bridge between microwave and optical frequency,the optical frequency comb is a very important tool in the process of tracing to the microwave frequency standard and measuring the absolute frequency of the optical clock transition.Therefore,the frequency stability of the optical comb is of paramount importance.To this end,an ultra-stable narrow linewidth 578 nm clock laser system was built and optimized.Firstly,the 1156 nm fundamental frequency laser is referenced to the resonance frequency of the FP cavity(Fabry-Perot cavity)through PDH(Pound-Drever-Hall)technique,thus achieving the stability of the 578 nm laser frequency to be 1×10-15@1 s.Afterwards,the locking noise introduced by various optical paths,circuits and environmental factors during the locking process and its influence on the frequency stabilisation were analyzed and optimised to obtain a narrow linewidth laser of Hz magnitude.The repetition frequency of the optical comb is locked by locking the beat note between the 578 nm narrow-linewidth laser and a certain tooth of the comb,so that each comb tooth has considerable instability compared to the 578 nm laser.Then the locked optical comb can be used to measure the clock transition frequency of 171Yb optical clock.Coordinated Universal Time(UTC),as the global time and frequency standard,undertakes the mission of maintaining time synchronisation.National institutes of metrology in each country trace the local time reference to a globally unified time frequency standard by making regular corrections for differences between the local UTC(k)and the UTC that published monthly by the International Bureau of Weights and Measures(BIPM).In collaboration with National Institute of Metrology,China(NIM),East China Normal University(ECNU)has set up a time frequency transmission link,in which the local hydrogen clock work as a flywheel clock trace to UTC(NIM)and then to SI "second" via UTC(NIM).Successful measurement of the absolute frequency of the 171Yb optical clock by this way.The lattice stark shift,clock stark shift,zeeman effect,collisional shift,blackbody radiation shift,servo error and minor shifts of the optical clock system were evaluated by synchronising two sets of 171Yb clocks(Ybl&Yb2)in the same environment.The total systematic frequency shift is-1.15Hz with a fractional uncertainty of 1.27×10-16.During the 15-day measurement period,384196 data points were obtained,in which the normal operation time of the optical clock is more than 31%of the total time.Combining the locally measured transition frequency,the systematic frequency shift of the optical clock,the gravitational redshift,and the frequency calibration of the hydrogen clock via the traceability link,the absolute frequency measurement of the cold ytterbium atomic optical clock at ECNU was calculated to be 518 295 836 590 863.30±0.38 Hz,which is in good agreement with the 2017 International Commission on Weights and Measures recommendation for this transition f2017CIPM171Yb=518 295 836 590 863.60±0.26 Hz.There are many system frequency shift terms that need to be evaluated during the absolute frequency measurement of an optical clock,each of which requires changing the relevant physical conditions and measuring the corresponding frequency changes.A complete evaluation is time consuming and tedious.In this thesis,by building a Bayesian hierarchical model and using the measured clock transition frequency values,the link traceability values of hydrogen clock and various systematic and statistical uncertainties,the total system frequency shifts of the optical clock was simulated in R to be-1.595 Hz with a relative uncertainty of 6.71 × 10-16,which is close to the actual measurement results.In the absence of experimental measurements,the frequency shifts and the uncertainty of the optical clock system can be quickly evaluated by simulation and used as a reference.In addition to the work on the absolute frequency measurement,the theoretical analysis and discussion of Floquet Rabi spectrum in optical lattice are also carried out in this thesis.At the same time,the initial detection of the modulated atomic spectra is also implemented in experiment.In the experiments,periodic sawtooth wave signals with different modulation amplitudes and modulation frequencies were added to the slow feedback PZT of a 759 nm laser and the Floquet sideband spectrum was observed.This provides the basis for the next step of multi-mode and multi-parameter Floquet modulation experiments.In addition,Floquet modulation can suppress the tunneling effect of atoms in shallow lattices,which creates the conditions for interrogating atomic spectral in shallow lattices and further suppression of lattice stark shift and collisional shifts. |
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