Demonstration of a trapped-ion atomic clock in space

Atomic clocks, which lock the frequency of an oscillator to the extremely stable quantized energy levels of atoms, are essential for navigation applications such as deep space exploration1 and global navigation satellite systems2 , and are useful tools with which to address questions in fundamental physics3–6 . Such satellite systems use precise measurement of signal propagation times determined by atomic clocks, together with propagation speed, to calculate position. Although space atomic clocks with low instability are an enabling technology for global navigation, they have not yet been applied to deep space navigation and have seen only limited application to space-based fundamental physics, owing to performance constraints imposed by the rigours of space operation7 . Methods of electromagnetically trapping and cooling ions have revolutionized atomic clock performance8–13. Terrestrial trapped-ion clocks operating in the optical domain have achieved orders-ofmagnitude improvements in performance over their predecessors and have become a key component in national metrology laboratory research programmes13, but transporting this new technology into space has remained challenging. Here we show the results from a trapped-ion atomic clock operating in space. On the ground, NASA’s Deep Space Atomic Clock demonstrated a short-term fractional frequency stability of 1.5 × 10−13/τ1/2 (where τ is the averaging time)14. Launched in 2019, the clock has operated for more than 12 months in space and demonstrated there a long-term stability of 3 × 10−15 at 23 days (no drift removal), and an estimated drift of 3.0(0.7) × 10−16 per day. Each of these exceeds current space clock performance by up to an order of magnitude15–17. The Deep Space Atomic Clock is particularly amenable to the space environment because of its low sensitivity to variations in radiation, temperature and magnetic felds. This level of space clock performance will enable one-way navigation in which signal delay times are measured in situ, making near-real-time navigation of deep space probes possible.

原子钟（Atomic clock）是将振荡器（oscillator）的频率锁定至原子极其稳定的量子化能级的设备，其在深空探测¹、全球导航卫星系统（Global Navigation Satellite Systems）²等导航应用中不可或缺，同时也是用于解答基础物理相关问题的重要工具³–⁶。此类卫星系统通过精确测量由原子钟确定的信号传播时间，结合传播速度来计算位置。尽管低频率不稳定性空间原子钟是全球导航的支撑技术，但受太空严苛运行环境带来的性能限制⁷，其尚未应用于深空导航，且在天基基础物理研究中的应用也十分有限。电磁囚禁与冷却离子的方法革新了原子钟的性能⁸–¹³。工作在光频段的地面囚禁离子原子钟（trapped-ion atomic clock），其性能较前代设备提升了数个数量级，已成为国家计量实验室研究计划中的关键组件¹³，但将这项新技术应用至太空仍面临诸多挑战。本文展示了一款在轨运行的囚禁离子原子钟的实验结果。地面测试中，美国国家航空航天局（NASA）的深空原子钟（Deep Space Atomic Clock）实现了1.5 × 10⁻¹³/τ¹/²的短期分数频率稳定度（其中τ为平均时间）¹⁴。该钟于2019年发射升空，已在轨运行超过12个月，并在轨验证了其长期稳定度：在23天时长下可达3 × 10⁻¹⁵（未进行漂移校正），每日漂移估计为3.0(0.7) × 10⁻¹⁶。上述性能均较当前空间原子钟提升了至多一个数量级¹⁵–¹⁷。深空原子钟对辐射、温度与磁场的变化敏感度较低，因此尤其适配太空环境。这一水平的空间原子钟性能将支持原位测量信号延迟时间的单向导航技术，使得深空探测器的近实时导航成为可能。