| Optical clocks based on neutral atoms or ions have developed very rapidly in the last decade or so,and the accuracy has far surpassed that of cesium fountain atomic clocks.Optical lattice atomic clocks can interrogate multiple atoms at the same time,and their quantum projection noise is much lower than that of ion optical clocks based on single ions,thus providing higher frequency stability.Due to the rapid development of optical clocks,the International Committee for Weights and Measures（CIPM）plans to revise the definition of the international unit second with optical frequency,replacing cesium fountain atomic clocks that currently support the definition of the second.High-precision atomic clocks are not only precision tools for time and frequency measurement,but also widely used in fields such as relativistic geodesy,testing of fundamental physical laws,detection of dark matter and gravitational waves.The fermionic isotope of the ytterbium atom 171Yb has the simplest hyperfine energy level structure（nuclear spin of 1/2）.The clock transition has no tensor light shift,and is insensitive to magnetic fields.The optical lattice optical clock based on 171Yb atoms is one of the most accurate optical clocks in the world.Our group has been working on ytterbium atomic optical clocks for years.During my Ph.D.,I have been focused on the develop-ent of ytterbium optical clocks with instability of order 10-18.The main achievements are summarized as follows:1.We have designed and built the second ytterbium atomic optical clock system,including the vacuum system,the optical system and the time sequence control system.Compared with our first ytterbium atomic optical clock system,we have made several improvements for the new optical clock system:1)a full titanium science cavity is used to reduce the residual magnetic field;2)a blackbody radiation（BBR）shielding cavity is placed in the vacuum chamber,which not only reduces the BBR frequency shift uncertainty,but also fully suppresses the dc Stark effect caused by residual electric charges;3)a vertical optical lattice design is adopted.This measure not only effectively suppresses the atomic tunneling between the adjacent lattice sites,but also enables us to reduce the atomic collision frequency shift by using a larger beam waist to reduce the atomic density.4)The frequency stability of the clock laser is improved by using a 30-cm-long super-stable optical cavity.2.The BBR shield inside the vacuum has been employed to improve the temperature uniformity of the thermal radiation environment,so that the BBR temperature can be accurately characterized.To further reduces the influence of external thermal radiations,the internal surfaces of the copper shield has been coated with a film which is both highly thermally emissive and electrically conductive.The uncertainty of the effective temperature felt by the atoms is 13 mK;the overall uncertainty of the BBR frequency shift is 9.5× 10-19,including the effect of the atomic response.3.We have realized closed-loop operation of the newly constructed optical clock,and made an anti-synchronous frequency comparison between this clock and our oldversion optical clock.Rabi interrogation of the clock transition was able to achieve subHertz linewidth spectral lines.The anti-synchronous frequency comparison between the two clocks shows that the single-clock frequency instability reaches 5.4× 10-18 at an average time of 4500 s.The analysis shows that the instability of the optical clocks is mainly limited by Dick noise.4.The first-and second-order Zeeman frequency shift coefficients of the clock transition have been measured by the self-comparison method,and determined to be 199.44（5）Hz/G and-6.09（3）Hz/mT2,respectively,which are in agreement with the measurement data of other international groups.Using our measured coefficients,we have evaluated the second-order Zeeman frequency shift caused by the bias magnetic field with an uncertainty of 1.8 × 10-18.5.We have evaluated the systematic uncertainties for the second optical clock.The collision frequency shift was measured by modulating the number of atoms in the optical lattice,and the uncertainty of the collision frequency shift was 1.2× 10-17.The optical frequency shift due to lattice laser was estimated from the measured data of optical lattice trapping frequency and trap depth,and it can be determined with an uncertainty of about 7× 10-18.Combiring Monte Carlo simulation and experimental parameters,we also estimated the servo error,which leads to an uncertainty of less than 1× 10-18.Combining all the systematic frequency shifts,the total systematic uncertainty of the second ytterbium atomic optical clock is 1.4× 10-17. |
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