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.

Caesium atomic fountain clock is a primary frequency standard, which realizes the duration of second. Its performance is mostly dominated by the frequency accuracy, and the C-field induced second-order Zeeman frequency shift is the major effect, which limits the accuracy improvement. By applying a high-precision current supply and high-performance magnetic shieldings, the C-field stability has been improved significantly. In order to achieve a uniform C-field, this paper proposes a doubly wound C-field solenoid, which compensates the radial magnetic field along the atomic flight region generated by the lead-out single wire and improves the accuracy evaluation of second-order Zeeman frequency shift. Based on the stable and uniform C-field, we launch the selected atoms to different heights and record the magnetically sensitive Ramsey transition |F = 3, m_F = –1〉 → |F = 4, m_F = –1〉 central frequency, obtaining this frequency shift as 131.03 × 10⁻¹⁵ and constructing the C-field profile (σ = 0.15 nT). Meanwhile, during normal operation, we lock NTSC-F2 to the central frequency of the magnetically sensitive Ramsey transition |F = 3, m_F = –1〉 → |F = 4, m_F = –1〉 fringe for ten consecutive days and record this frequency fluctuation in time domain. The first evaluation of second-order Zeeman frequency shift uncertainty is 0.10 × 10⁻¹⁵. The total deviation of the frequency fluctuation on the clock transition induced by the C-field instability is less than 2.6 × 10⁻¹⁷. Compared with NTSC-F1, NTSC-F2, there appears a significant improvement.

The Gaussian radius and temperature of cold atomic cloud are important parameters in describing the state of cold atoms. The precise measuring of these two parameters is of great significance for studying the cold atoms. In this paper, we propose a new method named knife-edge to measure the Gaussian radius and temperature of the cold atomic cloud. A near-resonant and supersaturated laser beam, whose size is controlled by a knife-edge aperture, is used to push away the cold atoms in the free falling process of cold atomic cloud. By detecting the intensity of fluorescence signal, the numbers of residual atoms under different-sized near-resonant beams can be obtained. According to the characteristic of cold atoms′ distribution, we construct a theoretical model to derive the Gaussian radius of cold atomic cloud from the recorded residual atom number and near-resonant beam size. Since the Gaussian radius and temperature of cold atomic cloud are associated with each other, we can finally obtain the temperature of cold atomic cloud through the recorded residual atom number and beam size. By using this method, we successfully measure the Gaussian radii of cold atomic cloud at the heights of 10 mm and 160 mm below the center of 3D-MOT (three dimensional magneto-optical trap) to be (1.54 ± 0.05) mm and (3.29 ± 0.08) mm, respectively. The corresponding temperature of cold atomic cloud is calculated to be (7.50 ± 0.49) μK, which is well consistent with the experimental result obtained by using the time-of-flight method under the same condition. This experiment is conducted on the platform of Cesium atomic fountain clock of National Time Service Center, China.

The caesium atomic fountain clock is a primary frequency standard. During its operation, a Majorana transition frequency shift will occur once a magnetic field at some special locations along the atomic trajectory is singular. In this study, by developing a physical model, we analyzed the magnetic field requirements for atomic adiabatic transition and calculated the influence of the Majorana atomic transition on the atomic state via a quantum method. Based on the simulation results for the magnetic field in the fountain clock, we applied the Monte Carlo method to simulate the relationship between the Majorana transition frequency shift and the magnetic field at the entrance of the magnetic shielding, as well as the initial atomic population. Measurement of the Majorana transition frequency shift was realized by state-selecting asymmetrically populated atoms. The relationship between the Majorana transition frequency shift and the axial magnetic field at the entrance of the magnetic shielding was obtained. The measured results were essentially consistent with the calculated results. Thus, the magnetic field at the entrance of the magnetic shielding was configured, and the Majorana transition frequency shift of the fountain clock was calculated to be 4.57 × 10⁻¹⁸.

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.

A cryogenic cesium atomic fountain clock is a novel clock with the microwave cavity and atomic free flight region placed in liquid nitrogen. On the one hand, the blackbody radiation shift is reduced at cryogenic temperature. On the other hand, the vacuum in the atomic free flight region is optimized, and the background gas collision shift reduced. The microwave resonant cavity is the most important unit in a cryogenic cesium atomic fountain clock. Through theoretical and simulative investigation, this study designs the configuration and dimensions for an optimized microwave cavity. Concurrently, experiments reveal the effects of temperature, pressure, humidity, and other factors on the resonant frequency of the microwave cavity. Combining the theoretical and experimental study, we obtain the resonant frequency difference between the microwave cavity in a cryogenic vacuum and at room temperature and ambient pressure. By subtracting this frequency difference, we adjust the microwave cavity for room temperature and ambient pressure, then vacuumize and immerse it in liquid nitrogen for verification and fine tuning. Finally, we determine that the microwave cavity resonant frequency deviation from the clock transition frequency is 10 kHz with an unloaded quality factor of 25 000, which meets the application requirements of the cryogenic cesium atomic fountain clock.