Research Progress on Rubidium Atomic Fountain Clock for Time-keeping

Even though the state-of-the art hydrogen masers exhibit high frequency stability, there are still long-term frequency drifts.To address this challenge, the National Institute of Metrology of China (NIM) has developed a rubidium fountain clock that shows a remarkable longterm stability and negligible frequency drift.By utilizing the rubidium atomic fountain clock to fast steer a hydrogen maser made by Beijing Institute of Radio Metrology and Measurement (BIRMM), a combined timekeeping system has been established.This system significantly mitigates the frequency drift of hydrogen masers, achieving improved long-term frequency stability over time intervals exceeding 4000 seconds, and improving the stability about 2.5 times in 1 day.Further research and optimization are still required to address the issue that the frequency stability of steered signal didn't fall as the -1/2 after 10,000 seconds.