Fig. 1. Sectional view of the ultra-stable cavity assembly

Fig. 2. Schematics of temperature stabilizer circuit. (a) Schematic of temperature collection and amplification circuit; (b) schematic of PID control circuit

Fig. 3. Resonant frequency of ultra-stable cavity at different temperatures

Fig. 4. Variation of measured resonant frequency or calculated resonant frequency with time

Fig. 5. (a) Temperature at different positions of active temperature stabilized layer; (b) environmental temperature of laboratory

Fig. 6. Temperature noise power spectrum density of three measuring points in the active temperature stabilized layer (solid line), temperature noise power spectrum density of three measuring points in the ultra-stable cavity (dotted line) calculated by transfer function, and the requirement of the temperature noise power spectrum density at 10-8 K-1and 10-9 K-1 to reach the thermal noise limit

Fig. 7. (a) Temperature fluctuation of the ultra-stable cavity which calculated from the temperature of temperature stabilized layer; (b) temperature stability of the three point (middle, top, bottom) in stabilized layer (solid points), temperature stability of the ultra-stable cavity (hollow points) calculated from the temperature of the stabilized layer, and the requirement of the temperature stability at 10-8 K-1 and 10-9 K-1(brown line and green line) to reach the thermal noise limit

Fig. 8. Experimental schematic of the clock transition detection of 199Hg cold atoms

Fig. 9. (a) Clock transition spectrum; (b) clock laser frequency drift