Primary frequency standards in the microwave range

Currently, the definition of the SI unit second is based on the cesium-133 hyperfine transition frequency in the microwave range. The primary frequency standards use cesium atomic clocks, which evolved from hot beam clocks to laser-cooled fountain clocks, and the total uncertainty reached 2×10^-16. The primary microwave clock plays an important role in time and frequency systems，it can be used not only for time keeping clock calibration, but also for steering the hydrogen maser as a time keeping clock with technical progress and reliability improvement.