Cold atom clocks have made remarkable progresses in the last two decades and played critical roles in precision measurements. Primary Cs fountain frequency standards have achieved a total uncertainty of a few parts in 1016, and the best optical clock has reached a type B uncertainty below 10−18. Besides applications in the metrology, navigation, etc., ultra-stable and ultra-accurate atomic clocks have also become powerful tools in the basic scientific investigations. In this paper, we focus on the recent developments in the high-performance cold atomic clocks which can be used as frequency standards to calibrate atomic time scales. The basic principles, performances, and limitations of fountain clocks and optical clocks based on signal trapped ion or neutral atoms are summarized. Their applications in metrology and other areas are briefly introduced.

A transportable fountain clock with high reliability is important for high-precision time-frequency measurements. Because of its relatively small cold atoms’ collision frequency shift and ease of attaining high quantum state preparation efficiency, the rubidium atomic fountain clock has an indicated higher stability and reliability. This paper reports the design and operation of a transportable rubidium atomic fountain clock developed by the Shanghai Institute of Optical and Fine Mechanics, Chinese Academy of Science. After being transported more than 1000 km from Shanghai to the Changping Campus of the National Institute of Metrology, China, the optical platform and other hardware of the fountain clock did not need to be adjusted. The rubidium fountain clock maintained a stability of 4.0 × 10−13τ1/2, reaching 5.0 × 10−16 at 300 000 s. After transportation, the rubidium fountain clock and a cesium fountain clock (NIM5) were operated together against the reference frequency of a hydrogen maser. In three separate operating periods, over a total of nearly three months, the average frequency repeatability of the rubidium fountain was less than 3.8 × 10−15.

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The absolute frequency of 87Rb 5S1/2 (F = 2)→5D5/2 (F″ = 4) two-photon transition at 778nm is measured in an accuracy of 44 kHz. A home-made erbium-doped fiber laser frequency comb with frequency stability of 5.0 × 10−13@1 s is employed for the light source. By using a periodically poled lithium niobate, the femtosecond pulse operating in 1556nm is frequency-doubled to 778nm to obtain the direct two-photon transition spectroscopy of thermal rubidium vapor. Through sweeping the carrier envelope offset frequency (fceo), the 5S1/2 (F = 2)→5D5/2 (F″ = 4) two-photon transition line is clearly resolved and its absolute frequency is determined via the peak-finding of the fitting curve. After the frequency correction, the measured result agrees well with the previous experiment on this transition. The entire system configuration is compact and robust, providing a potential candidate of optical frequency standard for telecommunication applications.

We present an oscillator of optically generated microwaves (OGMs) based on an optical frequency comb (OFC) and an ultrastable laser (USL). The USL provides short-term stability, and the OFC transfers the short-term stability to radio frequencies. Frequency synthesis is used to transfer the stability of the OGM oscillator to an oven-controlled crystal oscillator (OCXO). To improve the long-term stability of the OCXO, an additional compensating loop was applied to lock the long term stability of OGM at a hydrogen maser by compensating the frequency drift of the USL. The phase noise of the OCXO is maintained sufficiently low through these locked loops. The OCXO’s phase noise at 1 Hz is about −116 dBc/Hz. The short-term stability (1–100 s) of the OCXO with different compensating periods is better than 4 × 10−15. By replacing the H-maser (BIPM code: 1404850) with the OGM as the local oscillator, the stability of the NIM6 Cs fountain clock is improved from (1.2 × 10−13)τ−1/2 to (5.1 × 10−14)τ−1/2. This work should help optimize fountain clocks for better type A uncertainty or faster frequency offset evaluations.