An atomic clock with excellent long-term stability is important for maintaining the Local Atomic Time (TA(k)). The ⁸⁷ Rb atomic fountain clock has the potential to fulfill these characteristics. It is required to operate continuously, similar to commercial cesium beam clocks and hydrogen masers. This paper describes the design and preliminary operation results of an ⁸⁷ Rb atomic fountain continuous clock ( ⁸⁷ RFCC) developed at the National Time Service Center (NTSC) of the Chinese Academy of Sciences. We adopted some improved design aspects and suppressed Majorana effect and microwave leakage to enhance the clock’s stability. Its continuous operation has been improved owing to the advantages of the high-reliability optical system and servo high-resolution offset generator (HROG) algorithm. We presented an algorithm for the 5 MHz/1 PPS signal output of the ⁸⁷ RFCC through the HROG. The ⁸⁷ RFCC exhibited a frequency stability of 1.91×10 ^-13 τ ^-1/2 , reaching 4.7×10 ^-16 at 200000 s. We referenced the 1 PPS output signal to the UTC(NTSC) and obtained the relative phase fluctuation of the ⁸⁷ RFCC, which was within ±1.75 ns for approximately 90 days. We estimated the preliminary drift of the ⁸⁷ RFCC relative to the UTC(NTSC) for three different periods at 0.000019±0.00148 ns/d/d, 0.00119±0.00182 ns/d/d, and -0.00155±0.00060 ns/d/d and discussed the results. During the 90-day evaluation period, although the frequency of the ⁸⁷ RFCC relative to the UTC(NTSC) had changed, it still exhibited a low drift compared to cesium beam clocks and hydrogen masers, which highlights its potential for excellent long-term stability. The ⁸⁷ RFCC investigated here has the potential to improve the local time scale.

The cesium atomic fountain clock is the world’s most accurate microwave atomic clock. The uncertainty of blackbody radiation (BBR) shift accounts for an increasingly large percentage of the uncertainty associated with fountain clocks and has become a key factor in the performance of fountain clocks. The uncertainty of BBR shift can be reduced by improving the system environment temperature. This study examined the mechanism by which the BBR shift of the transition frequency between the two hyperfine energy levels of the ¹³³Cs ground state is generated and the calculation method for the BBR shift in the atomic fountain. Methods used to reduce the uncertainty of BBR shift were also examined. A fountain system structure with uniform temperature and good heat preservation was designed, and related technologies, such as that for measuring the temperature of the cesium fountain system, were studied. The results of 20 days of measurements, in combination with computer simulation results, showed that the temperature uncertainty of the atomic action zone is 0.12 °C and that the resulting uncertainty of BBR shift is 2.4 × 10⁻¹⁷.

Second-order Zeeman frequency shift is one of the major systematic factors affecting the frequency uncertainty performance of cesium atomic fountain clock. Second-order Zeeman frequency shift is calculated by experimentally measuring the central frequency of the (1,1) or (−1,−1) magnetically sensitive Ramsey transition. The low-frequency transition method can be used to measure the magnetic field strength and to predict the central fringe of (1,1) or (−1,−1) magnetically sensitive Ramsey transition. In this paper, we deduce the formula for magnetic field measurement using the low-frequency transition method and measured the magnetic field distribution of 4 cm inside the Ramsey cavity and 32 cm along the flight region experimentally. The result shows that the magnetic field fluctuation is less than 1 nT. The influence of low-frequency pulse signal duration on the accuracy of magnetic field measurement is studied and the optimal low-frequency pulse signal duration is determined. The central fringe of (−1,−1) magnetically sensitive Ramsey transition can be predicted by using a numerical integrating of the magnetic field “map”. Comparing the predicted central fringe with that identified by Ramsey method, the frequency difference between these two is, at most, a fringe width of 0.3. We apply the experimentally measured central frequency of the (−1,−1) Ramsey transition to the Breit-Rabi formula, and the second-order Zeeman frequency shift is calculated as 131.03 × 10⁻¹⁵, with the uncertainty of 0.10 × 10⁻¹⁵.