[论文解读] A liquid nitrogen-cooled Ca^+ optical clock with systematic uncertainty of 3*10^-18

Yao Huang, Baolin Zhang+7 more|arXiv (Cornell University)|Mar 16, 2021

Advanced Frequency and Time Standards被引用 7

一句话总结

本文提出一种液氮冷却的⁴⁰Ca⁺光频标，系统不确定度达到3×10⁻¹⁸，通过将黑体辐射（BBR）屏蔽冷却至82(5) K，显著降低了黑体辐射位移。通过采用低加热率离子阱、改进的冷却激光、超Ramsey方法以及高阶伺服算法，作者将运动相关与激光相关位移抑制至低于1×10⁻¹⁸和4×10⁻¹⁹，展示了液氦系统的一种实用且低成本的替代方案。

ABSTRACT

Here we present a liquid nitrogen-cooled Ca^+ optical clock with an overall systematic uncertainty of 3*10^-18. In contrast with the room-temperature Ca^+ optical clock that we have reported previously, the temperature of the blackbody radiation (BBR) shield in vacuum has been reduced to 82(5) K using liquid nitrogen. An ion trap with a lower heating rate and improved cooling lasers were also introduced. This allows cooling the ion temperature to the Doppler cooling limit during the clock operation, and the systematic uncertainty due to the ion's secular (thermal) motion is reduced to < 1*10^-18. The uncertainty due to the probe laser light shift and the servo error are also reduced to < 1*10^-19 and 4*10^-19 with the hyper-Ramsey method and the higher-order servo algorithm, respectively. By comparing the output frequency of the cryogenic clock to that of a room-temperature clock, the differential BBR shift between the two was measured with a fractional statistical uncertainty of 7*10^-18. The differential BBR shift was used to calculate the static differential polarizability, and it was found in excellent agreement with our previous measurement with a different method. This work suggests that the BBR shift of optical clocks can be well suppressed in a liquid nitrogen environment. This is advantageous because conventional liquid-helium cryogenic systems for optical clocks are more expensive and complicated. Moreover, the proposed system can be used to suppress the BBR shift significantly in other types of optical clocks such as Yb^+, Sr^+, Yb, Sr, etc.

研究动机与目标

• 将⁴⁰Ca⁺光频标的系统不确定度降低至10⁻¹⁸以下，以提升时间测量精度。
• 通过使用液氮而非液氦将BBR屏蔽冷却至82(5) K，以抑制黑体辐射位移。
• 通过改进的阱设计与先进的激光控制技术，最小化离子运动与激光光强位移的不确定度。
• 通过与室温光频标进行差分比较，验证BBR位移测量，并确定静态差分极化率。
• 证明液氮冷却在光频标中作为可扩展、低成本替代传统低温冷却系统的可行性。

提出的方法

• 在真空中使用液氮将BBR屏蔽冷却至82(5) K，以抑制黑体辐射位移。
• 采用加热率更低的离子阱，以减少⁴⁰Ca⁺离子的热运动。
• 使用改进的冷却激光，实现多普勒冷却极限，最小化因摆动运动引起的频率位移。
• 应用超Ramsey方法，将探测激光光强位移不确定度降低至1×10⁻¹⁹以下。
• 采用高阶伺服算法，将伺服误差降低至4×10⁻¹⁹。
• 以7×10⁻¹⁸的分数统计不确定度，测量低温与室温光频标之间的差分BBR位移。

实验结果

研究问题

• RQ1 液氮冷却能否有效抑制⁴⁰Ca⁺光频标中的黑体辐射位移？
• RQ2 通过改进的阱与激光系统，离子运动与激光控制的不确定度可降低到何种程度？
• RQ3 低温与室温光频标之间的差分BBR位移可测量到何种精度？
• RQ4 由差分BBR位移推导出的静态差分极化率是否与先前测量结果一致？
• RQ5 液氮冷却能否作为光频标中液氦系统的实用且低成本的替代方案？

主要发现

• 液氮冷却的⁴⁰Ca⁺光频标的整体系统不确定度为3×10⁻¹⁸。
• 通过将屏蔽冷却至82(5) K，BBR位移得到抑制，显著降低了热辐射效应。
• 通过多普勒冷却与低加热率阱设计，离子运动引起的频率位移降低至1×10⁻¹⁸以下。
• 通过超Ramsey方法，激光光强位移不确定度降低至1×10⁻¹⁹以下。
• 通过高阶伺服算法，伺服误差降低至4×10⁻¹⁹。
• 差分BBR位移以7×10⁻¹⁸的分数统计不确定度测得，可精确确定与先前结果一致的静态差分极化率。