Fig. 1. Cutaway and picture of the transportable fountain clock assembly. The size and weight of the clock are about 96cm×76cm×180cm and 350 kg, respectively.

Fig. 2. Model diagrams of the optical benches. (a) The laser source bench with ECDL and TA, which is used to produce and amplify the cooling laser and the repumping laser. ①, ②: ECDLs; ③, ④: the saturation absorption modules; ⑤: TA; ⑥: the repumping laser output; ⑦: cooling laser output. (b) The laser-regulating bench divides and combines the cooling laser or repumping laser. ①: cooling laser input; ②: repumping laser input; ③: upward cooling laser output; ④: downward cooling laser output; ⑤: detection laser output; ⑥ repumping laser output.

Fig. 3. Ramsey pattern obtained by scanning the microwave frequency of the Ramsey cavity using 0.1 Hz steps. The inset curve is the transition probability versus detuning of the microwave signal by several Hz; the linewidth of the central fringe is approximately 1.02 Hz.

Fig. 4. Short- and medium-term stabilities of the fountain clock with MOT on. The square dots are the Allan deviations obtained with the clock operating in our laboratory where the temperature fluctuation is about ±1.5°C. The triangle dots are the Allan deviations obtained with temperature control of the optical bench. The solid line is a fitted curve corresponding to a white frequency noise of 2.3×10−13τ−1/2.

Fig. 5. Fluctuation in the number of atoms, laser power, and temperature of the optical bench before temperature controlling of the optical bench.

Fig. 6. Fluctuation in the number of atoms, temperature of the microwave source, and the optical benches when the fountain clock was operated in the thermostat.

Fig. 7. Allan deviation of the fountain clock frequency with MOT off when it was put into the thermostat. The solid line is the fitting curve of short-term stability. After the averaging time exceeds 100,000 s, the long-term stability reaches the order of 10−16 and continues going down.