Fig. 1. Schematic of vacuum setup

Fig. 2. Schematic of optical system

Fig. 3. Energy-levels diagram of a ⁸⁷Sr atom

Fig. 4. Collision-induced decoherence, broadening of the spectrum, and atomic loss. (a)(b) Rabi spectrum and the relationship between atom number and frequency detuning with 6000 atoms; (c)(d) Rabi spectrum and the relationship between atom number and frequency detuning with 2000 atoms

Fig. 5. Stability comparison. The blue triangles and black circles represent self-comparison stabilities of the clock at atomic numbers of 6000 and 2000, respectively, the error bars correspond to σ standard deviation, the solid lines are the linear fitting results with a fixed slope of -0.5, and the red dotted line is the self-comparison stability limited by Dick noise at τ_p=500 ms