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

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 laboratory prototype of mercury ion microwave clock aiming to spaceborne applications with the long-term stability below 1 × 10^- 15. 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 with the averaging time τ over 10⁵ s and achieves the long-term stability of 6.3 × 10^- 16 for averaging times of 200,000 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.