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. 11803042, 11474282, 61775220), the National Key Research and Development Program of China (Grant No. 2016YFF0200201), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB21030100), the Key Research Project of Frontier Science of the Chinese Academy of Sciences (Grant No. QYZDB-SSW-JSC004), and the Youth Innovation Promotion Association the Chinese Academy of Sciences (Grant No. 2019400)
Received Date:26 July 2019
Accepted Date:17 September 2019
Available Online:27 November 2019
Published Online:05 December 2019
Abstract:In a one-dimensional Fermion optical lattice clock, the p-wave scattering can occur when collision energy is sufficient to overcome the centrifugal barrier of p-wave scattering. According to Pauli exclusion principle, the s-wave scattering is forbidden between two identical Fermions. However, the s-wave scattering may also exist due to inhomogeneous excitation which leads to some difference between two Fermions. In terms of the uncertainty evaluation of a neutral atomic optical lattice clock, the frequency correction and uncertainty caused by atomic interaction cannot be ignored, and it will affect the evaluation of AC stark frequency shift. So the uncertainty evaluation of the collision frequency shift should be as small as possible. Only in this way can a neutral atomic optical lattice clock have a state-of-the-art performance. The collision frequency shift originates from the interaction between atoms trapped in an identical lattice. In this study, the collision frequency shift of 87Sr optical lattice clock at the National Timing Service Center is measured experimentally. A horizontal one-dimensional optical lattice is constructed. The number of tapped atoms is about 104 at a temperature of 3.4 μK. A laser is used to pump the atoms to either of the Zeeman energy levels of mF = ± 9/2 in the ground state, and the clock transition spin polarization spectrum is obtained. In a spin polarized Fermions system, the collision frequency shift relating to atomic density is measured by the method of self-comparison. The method of self-comparison, which takes full advantage of the excellent short-term stability of the clock laser, can be used to measure the frequency difference caused by the variety of system parameters. Owing to the fact that the collision frequency shift is proportional to atomic density, the collision frequency shift can be measured by the method of self-comparison between high and low atomic density. In the experiment, the systematic state is changed between high and low atomic density by periodically changing the loading time of the first stage of cooling. In order to reduce the statistical uncertainty of the measurement, the collision frequency shift is separately measured 37 times. Finally, when the atomic density is 4 × 1010/cm3, the collision frequency shift is –0.13 Hz, and the statistical uncertainty of the measurement is 3.1 × 10–17. The Allan deviation of self-comparison between low and high atomic density reaches 4 × 10–17 after 8000 s averaging time, indicating that the accuracy of the measurement is reliable and on the order of 10–17. This work lays a foundation of the total uncertainty evaluation of 87Sr optical lattice clock. Keywords:collision frequency shift/ strontium optical lattice clock/ optical lattice/ spin-polarized spectrum
在只有单台光晶格钟的情况下, 通常用自比对的方法来衡量光晶格钟的性能, 自比对的原理图如图3所示. 锶原子光晶格钟系统在时域上被分为高密度和低密度两种工作状态如图3(a)所示; 高、低密度状态下获得的误差信号分别由PID1 (proportion integration differentiation)和PID2 计算频率纠正量, 如图3(b)所示. 这样相当于两台时间上独立工作的光晶格钟. 图 3 自比对方法 (a)自旋极化峰, fH和fL分别对应高密度和低密度状态下钟跃迁的中心频率, δvC为碰撞频移的值; (b) 锁定反馈原理, fH1 和fL1是初始设定的激光频率, $f'_{\rm H1} $和$f'_{\rm L1} $是修正激光频率, Err1和Err2是误差信号, Δf1和Δf2是频率修正量; (c) 时间序列; (d)交替改变原子密度获得的钟跃迁谱线; (e)高、低原子密度状态下原子的跃迁几率 Figure3. The method of self-comparison: (a) The spin-polarized peaks, fH and fL are the center frequency of locked clock transition, δvC is the value of collision frequency shift; (b) the feedback loop schematic, fH1 and fL1 are initial clock laser frequency of high-density and low-density respectively, $f'_{\rm H1} $ and $f'_{\rm L1} $ are the frequency of being corrected, Err1 and Err2 are error signals, Δf1 and Δf2 are revisionary frequency; (c) the time sequence; (d) the clock transition spectrum during alternately changing atomic density; (e) the excitation fraction at high and low atomic densities.
自比对测量碰撞频移的方法本质上是不断测量高低原子密度下钟跃迁的变化. 每一个钟跃迁反馈周期都能够获得一次碰撞频移测量值, 这样在较短的时间内便可获得大量的测量值. 利用自比对方法所得高低密度下碰撞频移数据的艾伦偏差如图4所示. 系统的不稳定度为4 × 10–15@1 s, 在积分时间为8000 s时达到了4 × 10–17, 表明我们碰撞频移不确定度评估结果在10–17量级是可靠的. 图 4 高低密度自比对艾伦偏差 Figure4. The Allan deviation obtained by the method of self-comparison between low and high atomic density.