1.CAS Key Laboratory of Time and Frequency Primary Standards, National Time Service Center, Xi’an 710600, China 2.School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 100049, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 61775220, 11803042), the Key Research Project of Frontier Science of the Chinese Academy of Sciences (Grant No. QYZBD-SSW-JSC004), and the Youth Innovation Promotion Association CAS (Grant No. 2019400)
Received Date:27 July 2020
Accepted Date:07 September 2020
Available Online:22 January 2021
Published Online:05 February 2021
Abstract:Transportable optical clocks have broad applications in scientific research and engineering. Accurate evaluation of systematic uncertainty for the transportable 87Sr optical lattice clock is a prerequisite for the practical realization of the optical clock. Four main frequency shifts of the 87Sr optical lattice clock are measured, i.e. blackbody-radiation (BBR) shift, collision shift, lattice alternating current (AC) Stark shift, and second-order Zeeman shift. Firstly, by measuring the temperature distribution on the surface of the magneto-optical trap cavity and analyzing the influence of different heat sources on atomic cloud, the BBR shift correction is measured to be 50.4 × 10–16 Hz with an uncertainty of 5.1 × 10–17. Secondly, the time-interleaved self-comparison method is used under high and low atom density condition to evaluate the collision shift of the system. The correction of collision shift is 4.7 × 10–16 with an uncertainty of 5.6 × 10–17. Thirdly, the lattice AC Stark shift is evaluated by the time-interleaved self-comparison method. By measuring the dependence of the lattice AC Stark shift on the wavelength of the lattice light, the magic wavelength is measured to be 368554393(78) MHz. As a result, the lattice AC Stark shift correction is 3.0 × 10–16 with an uncertainty of 2.2 × 10–16. Finally, using the time-interleaved self-comparison technology, the second-order Zeeman frequency shift is evaluated by measuring the fluctuation of the difference in center frequency between the ${m_{\text{F}}} = + {9 / 2} \to {m_{\text{F}}} = + {9 / 2}$ polarization spectrum and ${m_{\text{F}}} = - {9 / 2} \to {m_{\text{F}}} = - {9 / 2}$ polarization spectrum. The correction of second-order Zeeman shift is calculated to be 0.7 × 10–16, and corresponding uncertainty is 0.2 × 10–17. Experimental results indicate that the frequency shift correction due to the blackbody radiation is the largest, while the uncertainty caused by the lattice AC Stark effect is the largest in the evaluated shifts. The systematic shift is 58.8 × 10–16, the total uncertainty is 2.3 × 10–16. In the next work, the magneto-optical trap cavity will be placed in a blackbody-radiation cavity to reduce the blackbody-radiation shift. The uncertainty of the collision shift will be reduced by increasing the beam waist of the lattice and reducing the potential well depth of the lattice, which will reduce the density of atoms. What is more, the light source for the optical lattice after spectral filtering will be measured by an optical frequency comb locked to the hydrogen clock signal to reduce the uncertainty of the lattice AC Stark frequency shift. The systematic uncertainty is expected to be on the order of 10–17. The evaluation of the systematic uncertainty for the transportable 87Sr optical lattice clock lays the foundation for the practical application. Keywords:transportable 87Sr optical lattice clock/ systematic shift/ uncertainty/ time-interleaved self-comparison method
采用分时自比对方法测得的高低原子密度下频率差的阿伦偏差如图5所示, 其中黑点为不同积分时间的频率不稳定度, 点上的实线是误差棒, 红色实线表示对阿伦偏差的线性拟合, 拟合斜率为1/τ1/2. 从图5可看出, 分时自比对频率不稳定度为4.5 × 10–15@1 s, 当积分时间为4000 s时进入10–17量级, 表明碰撞频移不确定度评估结果进入10–17量级是准确可靠的. 图 5 单次高低原子密度自比对的阿伦偏差 Figure5. The Allan deviation obtained by the time-interleaved self-comparison method between high and low atomic density