Fig. 1. Principle of atomic frequency standards and two interrogation modes. (a) Principle; (b) two interrogation modes

Fig. 2. Fountain frequency standard. (a) Setup of the physical unit; (b) photo of the fountain frequency standard device of ⁸⁷Rb of Shanghai Institute of Optics and Fine Mechanics (SIOM)

Fig. 3. ⁸⁷Rb atomic energy level diagram

Fig. 4. Typical Ramsey fringe obtained on the fountain frequency standard of ⁸⁷Rb of SIOM, the linewidth of the fringe is 1 Hz and its signal-noise-ratio (SNR) is about 500

Fig. 5. Typical curves of the stability of the fountain frequency standard (obtained by comparing experiments of the outputs of the fountain frequency standard of ⁸⁷Rb of SIOM and an H-maser)

Fig. 6. Composition diagram of photonic microwave generation system

Fig. 7.

Phase noise and stability curves of SIOMs photonic microwave, where figures (a) and (c) correspond to 6.834 GHz microwave signal for the ⁸⁷Rb fountain clock, and figures (b) and (d) correspond to 9.193 GHz microwave signal for the ¹³³Cs fountain clock. (a)(b) Phase noise curves; (c)(d) stability curves

Fig. 8. Structural diagram of the space cold atomic clock in Tiangong 2 Laboratory

Fig. 9. Appearance of the space cold atomic clock in Tiangong 2 Laboratory

Fig. 10. Representative experimental results of a space cold atomic clock. (a) Resonance microwave power spectrum curve; (b) Ramsey interference fringes

Fig. 11. Principle (left) and structural diagram (right) of a cold atom microwave clock based on in-situ measurement scheme for the space station

Fig. 12. Primary frequency standards and secondary frequency standards contributed to the generation of International Atomic Time since 2003, as well as their operating rates as recorded on the website of the Bureau International des Poides et Measures (BIPM)^[99]