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 clocks）通过将振荡器频率锁定于原子极稳定的量子化能级（quantized energy levels），是深空探测¹、全球导航卫星系统²等导航应用的核心技术，同时也是探索基础物理³⁻⁶问题的重要工具。这类卫星系统利用原子钟测定的信号传播时间（signal propagation times）的精确测量值，结合传播速度来计算位置。尽管低不稳定性（low instability）的空间原子钟（space atomic clocks）是全球导航的使能技术（enabling technology），但由于空间运行严苛环境（rigours of space operation）带来的性能限制⁷，它们尚未应用于深空导航，在天基基础物理研究中的应用也十分有限。电磁俘获与冷却离子（electromagnetically trapping and cooling ions）的方法已彻底革新了原子钟性能⁸⁻¹³。工作于光域（optical domain）的地面俘获离子原子钟（terrestrial trapped-ion clocks）性能较前代产品实现了数量级（orders-of-magnitude）提升，成为国家计量实验室（national metrology laboratory）研究项目的核心组件¹³，但将这一新技术推向太空的过程仍充满挑战。本文展示了一款在轨运行的俘获离子原子钟（trapped-ion atomic clock）的性能结果。在地面测试中，美国国家航空航天局（NASA）的深空原子钟（Deep Space Atomic Clock）展现出1.5×10⁻¹³/τ¹/²的短期分数频率稳定性（fractional frequency stability）（其中τ为平均时间）¹⁴。该钟于2019年发射，已在太空运行超过12个月，其在轨长期稳定性（long-term stability）在23天时达到3×10⁻¹⁵（未去除漂移），每日漂移（drift）估计值为3.0(0.7)×10⁻¹⁶。上述每项性能均较现有空间钟提升了一个数量级¹⁵⁻¹⁷。深空原子钟因对辐射、温度及磁场变化的低敏感性，尤其适应太空环境。这种级别的空间钟性能将实现基于原位（in situ）信号延迟测量的单向导航（one-way navigation），使深空探测器的近实时导航（near-real-time navigation）成为可能。