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Scientific Reports

Realization of a compact mercury ion microwave clock with frequency stability of 6.3 × 10− 16

Hao Liu, Yi-He Chen, Jian Wang, Shuhong Huang, Longbao Xiang, Shicheng Yu, Chengbin Li, Lijun Du, Lei She

Published Dec 15, 2025• 2 citations
Publisher

Abstract

Abstract The mercury ion microwave clock is considered to be one of the leading candidates for new generation satellite navigation atomic clocks due to its excellent long-term frequency stability, extremely low frequency drift rate, high reliability and good potential for miniaturization. In recent years, the performance and maturity of the space mercury ion microwave clock have improved rapidly, and the reported long-term stability results are mostly in low 10 − 15 level. In this article, we present a compact prototype of mercury ion microwave clock aiming to spaceborne applications with the long-term stability of 10 − 16 . By regulating the physical effects contributing to the clock transition frequency shifts, the clock maintains a white frequency noise Allan deviation of 2.8×10 − 13 / τ 1/2 even with the averaging time τ over 10 5 s and achieves the long-term stability of 6.3 × 10 − 16 for averaging times of 200000 s. The space mercury ion microwave clock with such level performance will benefit the Global Navigation Satellite Systems and a wide range of space science and missions.

DOI
10.1038/s41598-025-30416-4

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Abstract The mercury ion microwave clock is considered to be one of the leading candidates for new generation satellite navigation atomic clocks due to its excellent long-term frequency stability, extremely low frequency drift rate, high reliability and good potential for miniaturization. In recent years, the performance and maturity of the space mercury ion microwave clock have improved rapidly, and the reported long-term stability results are mostly in low 10 − 15 level. In this article, we present a compact prototype of mercury ion microwave clock aiming to spaceborne applications with the long-term stability of 10 − 16 . By regulating the physical effects contributing to the clock transition frequency shifts, the clock maintains a white frequency noise Allan deviation of 2.8×10 − 13 / τ 1/2 even with the averaging time τ over 10 5 s and achieves the long-term stability of 6.3 × 10 − 16 for averaging times of 200000 s. The space mercury ion microwave clock with such level performance will benefit the Global Navigation Satellite Systems and a wide range of space science and missions.
Abstract Optical clocks have well surpassed the frequency stability performance of microwave clocks. They have been mainly limited to tabletop setups and large rack-mount systems. Miniaturizing optical clocks will make the new technology and capability more ubiquitously applied to applications from navigation to fundamental science. Here we aim to develop a miniature space optical clock (mSOC) with the size of a compact microwave clock and capable of achieving a short-term stability of 10−14 τ −1/2 and noise floor of 10−16. In this paper, we will describe the mSOC concept, discuss the experimental setup, and present recent results. We have demonstrated trapping single ions and characterized the ion lifetime in a 16-cc trap tube. The trap tube is completely sealed off without any active pump. We also constructed a clock laser referenced to a small ultra-low expansion reference cavity at a low 10−14 level and observed the clock transition.
Abstract 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 the Global Positioning System (GPS)2 and as scientific tools for addressing questions in fundamental physics3,4,5,6. Atomic clocks that can be launched into space are an enabling technology for GPS, but to date have not been applied to deep space navigation and have seen only limited application to scientific questions due to performance constraints imposed by the rigors of space launch and operation7. The invention of methods to electromagnetically trap and cool ions has revolutionized atomic clock performance8,9,10,11,12,13. Terrestrial trapped ion clocks have achieved orders of magnitude improvements in performance over their predecessors and have become a key component in national metrology laboratories13. However, transporting this new technology into space has remained elusive. Here we show the results from the first-ever trapped ion atomic clock to operate in space. Launched in 2019, NASA’s Deep Space Atomic Clock (DSAC) has operated for more than 12 months, demonstrating a short-term fractional frequency stability of between 1 and 2 x 10-13 at 1 second of averaging time (measured on the ground), a long-term stability of 3 x 10-15 at 23 days, and an estimated drift of 3.0(0.7) x 10-16 per day. Each of these exceeds current space clock performance by as much as an order of magnitude14,15,16. We found the DSAC clock to be particularly amenable to the space environment, having low sensitivities to variations in radiation, temperature, and magnetic fields, and we were able to characterize these in detail. This level of space clock performance will enable new types of space navigation. In particular, the DSAC mission has demonstrated a process called one-way navigation whereby signal delay times are measured in-situ making near-real-time deep space probe navigation possible17.
In this paper, we report our recent effort on developing a micro mercury trapped ion atomic clock. In particular, we present preliminary results of a mercury-ion clock using a 15-cc vacuum trap tube. The mercury-ion clock can achieve a short-term fractional frequency instability of 1.5 ×10!12τ!1/2and reach a fractional frequency instability of 3 ×10!14after averaging for an hour. The clock can remain at the 10!14fractional frequency instability level up to several hours. Furthermore, the sealed 15-cc vacuum tube has demonstrated a very long ion lifetime of over two hundred days. We also investigated the mercury vapor density inside the vacuum trap tube by measuring the coherence time of the microwave clock transition.
Abstract The methods of trapping and cooling of atoms and ions have been transformative for atomic clocks due to the reduction, and in some cases elimination, of major systematic frequency shifts. Continuously operating atomic clocks based on trapped mercury ions have existed for decades but until recently have been restricted to terrestrial applications. The recently completed Deep Space Atomic Clock (DSAC) mission demonstrated the first trapped ion clock operation in space. Here we review DSAC as well as follow-on improvements towards the realization of high stability, long life Hg ion atomic clocks for foreseen ground and flight application.
In this work, we report a prototype of micro mercury trapped ion atomic clock. All the clock components have been packaged and demonstrated in a magnetic shield box of 4 liters. The prototype clock shows promising performance with a short-term fractional frequency instability below 10−11τ−12and can reach the 10−14-instability level after a day of averaging. The prototype clock is capable of reaching the10−15-fractional frequency instability range. Our demonstration of the integrated clock prototype paves the way for further SWaP reduction and maturation.
Trapped mercury ion is a promising candidate for portable and operational atomic clocks. It operates at the 40.5GHz ground state hyperfine transition of 199Hg ions. Mercury ions are confined in a linear RF trap, buffer-gas cooled, with a long ion storage lifetime in an ultra-high vacuum enclosure without an active pump. The clock transition is optically-pumped and detected with a Hg discharge lamp without need of a laser light source. This approach has proven to provide simplicity in design and implementation and robustness in operation. In this paper, we report our recently effort on developing a micro and low-power mercury trapped ion clock in the DARPA ACES program. In a 5 mm x 10 mm linear trap (0.25 cc), we have achieved the clock fractional frequency stability of .7 × 10-12τ-1/2and reached down to 7 × 10-14after 1000 seconds of averaging. We will discuss the clock performance under different conditions and implications toward a micro mercury trapped ion clock with low power and high stability.