Optimization And Performance Evaluation Of ¹⁷¹Yb-1 Optical Lattice Clock

The time unit "second" is one of the seven basic units of the International System of Units,and it is also the physical quantity with the highest measurement accuracy among the basic units.The precise measurement of time has important applications in the fields of time-frequency reference,satellite navigation and basic physics research.At present,the definition of "second" is based on the unperturbed ground-state hyperfine transition frequency of cesium(133Cs)atoms.Along with technological development for years,its system uncertainty has reached the level of 10-16.However,the system uncertainty of atomic optical clocks based on neutral atoms or single ion has reached the level of 10-18 or even below,which is more than two orders of magnitude better than the current best cesium atomic fountain clock.Therefore,the optical clock will be discussed to replace the Cesium Fountain Reference Clock as the nextgeneration time-frequency standard around 2026.As one of the best optical clocks in the world,the 171 Yb atomic optical lattice clock has great value in basic physics research and engineering applications.At present,the frequency uncertainty of the best 171Yb atomic optical lattice clock in the world has reached 1.4 ×10-18,the frequency stability has reached 3.2 × 10-19@105 s,and the clock transition frequency has been accepted as the secondary representation of the "second"definition.We realized the closed-loop operation of our Yb lattice clock in 2017.However,factors,such as the design of the quadrupole magnetic field coil,its heating to the main chamber and higher atomic temperature,restrict the further improvement of performance of Yb lattice clock.Around the above problems,we have carried out several improvements and optimization and finally completed the performance evaluation.During my PhD,the main research contents are as follows:1.Optimization of the parameters of Zeeman slower,first and second stage magneto-optical trap(MOT)of Yb lattice clock.The relationship between the atomic temperature,number and density and the laser intensity,detuning and magnetic field gradient at 556 nm was obtained.The atomic temperature reaches 6μK after second stage cooling.After the atoms are loaded into the optical lattice,the lifetime is 6 s.The pure state preparation of Zeeman sublevel was achieved when polarization laser at 556 nm was applied,with a purity of 99.7%.2.The axial cooling of cold Yb atoms in optical lattice and the ground state population preparation of the axial motion state are realized with Raman sideband cooling technology.The ratio between the first-order red sideband and blue sideband was measured,so the longitudinal temperature of the cold atoms in the optical lattice was measured around 0.7 μK,and the corresponding average quantum number was 0.024.3.Realization of sub-hertz clock transition spectrum with Rabi interrogation and instability of the order of 10-18 with self-comparison method.The clock transition linewidth around 0.78 Hz is obtained with 1040 ms Rabi spectroscopy time.The closedloop operation of the Yb optical lattice clock is realized with a clock transition linewidth of 8 Hz.The system instability is estimated to be 6.5 × 10-16/√τ with self-comparison method,and it reaches 7.2 ×10-18 averaging 4500s.Independent comparison between the two Yb lattice clocks is elementarily realized,and the instability is 5×10-16/√τ.4.Observation of Ramsey-fringe clock spectra of 171Yb optical lattice clock.Time sequence with two π/2 clock laser pulses of 11 ms and free evolution time of 120 ms is applied,clock spectrum linewidth of 4.2 Hz with Ramsey interrogation is observed.5.Realization of the uncertainty evaluation of Yb-1 lattice clock,including contribution from BBR shift,probe light shift,second-order Zeeman shift and collision shift.Based on real-time measurement of 13-point chamber’s absolute temperature and the finite element analysis method,the effective temperature felt by the atoms in the vacuum chamber is accurately obtained.The uncertainty of the effective temperature is 160 mK,and the uncertainty due to the blackbody radiation shift is 5.3 × 10-18;The relationship between the frequency shift of the probe laser and the Rabi frequency of the clock excitation is 5.86(11)×10-6/Hz with cross-self-comparison method.For Rabi interrogation with a π pulse of duration 150 ms,the frequency shift caused by the probe light is 80(15)μHz,and the corresponding frequency uncertainty is 3×10-20.By measuring the frequency shift along with the variation of the polarization magnetic field,the second-order Zeeman coefficient is determined to be-0.0655(3)Hz/G2,and the second-order Zeeman frequency shift correction is-79(29)mHz,and the corresponding frequency uncertainty due to second-order Zeeman effect is 5.7×10-17.Similarly,the cross-self-comparison measurement is used to modulate the number of the atoms of the two clock loops,and the change of the frequency shift due to the difference of the atomic number is measured under different excitation ratios,and finally the collision frequency shift is determined about 7.5 × 10-18.