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在离子光钟实验系统中，离子的运动效应是衡量一套光钟性能的主要指标之一，是目前限制各类不同离子光钟具有更高不确定度的关键影响因素。在第一套液氮低温钙离子光钟的基础上[PhysRevApplied.17.034041(2022)]，我们研制了新一套液氮钙离子光钟的物理系统，并对其离子囚禁装置进行了较大改进，主要包括以下两方面:通过引入射频电压的主动稳定装置，将液氮低温钙离子光钟的径向宏运动频率的长期漂移抑制到了小于$1\,\mathrm{kHz}$水平；通过改进离子阱鞍点位置剩余电压的补偿方案，进一步将液氮低温钙离子光钟中附加微运动造成的频移抑制至$<1.0\times10^{-19}$。这些改进有助于提升离子的冷却效率与提高离子温度的评估精度。通过对宏运动红蓝边带的测量，我们精确评估了Doppler冷却后离子的振动平均声子数，对应的离子温度为0.78 mK，接近Doppler冷却极限。此外，稳定的宏运动频率为下一步在液氮低温钙离子光钟上实施三维边带冷却创造了良好条件，也为推动液氮低温钙离子光钟的系统不确定度进一步降低至$10^{-19}$量级打下了基础。In ion optical clock systems, the motional effects of trapped ions are critical factors in determining clock performance and currently represent key limitations in achieving lower uncertainty across different ion-based optical clocks. Building on the first liquid nitrogen-cooled Ca$^+$ ion optical clock [PhysRevApplied.17.034041], we have developed a new physical system for a second Ca$^+$ ion optical clock and made significant improvements to its ion trapping apparatus. These improvements primarily focus on two aspects: First, we designed and implemented an active stabilization system for the RF voltage, which stabilizes the induced RF signal on the compensation electrodes by adjusting the RF source amplitude in real time. This approach effectively suppressed long-term drifts in the radial secular motion frequencies to less than 1 kHz, achieving stabilized values of $\omega_x = 2\pi \times 3.522(2)\,\mathrm{MHz}$ and $\omega_y = 2\pi \times 3.386(2)\,\mathrm{MHz}$. The induced RF signal was stabilized at 59 121.43(12) µV, demonstrating the high precision of the stabilization system. Second, we optimized the application of compensation voltages by integrating the vertical compensation electrodes directly onto the ion trap structure. This refinement enabled us to suppress excess micromotion in all three mutually orthogonal directions to an even lower level. With the RF trapping frequency tuned close to the magic trapping condition of the clock transition, we further evaluated the excess micromotion-induced frequency shift in the optical clock to be $2(1) \times 10^{-19}$. To quantitatively assess the secular motion of the trapped ion, we performed sideband spectroscopy on the radial and axial motional modes, measuring both red and blue sidebands. From these measurements, we accurately determined the mean phonon number in the three motional modes after Doppler cooling, corresponding to an average ion temperature of $0.78(39)\,\mathrm{mK}$, which is close to the Doppler cooling limit. The corresponding second-order Doppler shift was evaluated to be $-(2.71 \pm 1.36) \times 10^{-18}$. The long-term stability of the radial secular motion frequency provides favorable conditions for implementing three-dimensional sideband cooling in future experiments, which will further reduce the second-order Doppler shift. These advancements not only enhance the overall stability of the optical clock but also lay the foundation for reducing its systematic uncertainty to the $10^{-19}$ level.
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Keywords:
- Ca+ion optical clock /
- cryogenic /
- secular motion /
- excess micromotion
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