Fig. 1. (a) Schematic setup of the POP clock with dispersive detection. PD, photodetector. (b) Timing sequence for the POP clock. Top red, optical pulses for state preparation and detection; middle blue, two microwave pulses for Ramsey interrogation; bottom yellow, trigger pulse for signal acquisition. These pulses correspond to the switches in different colors in (a). (c) Detailed energy level structure of Rb87 involved in the POP clock. The π transition between |52S1/2,F=1,mF=0〉 and |52S1/2,F=2,mF=0〉 is chosen as the clock transition with a frequency of about 6.8346 GHz.

Fig. 2. (a) Schematic of the new physics package. The cell filled with Rb87 and buffer gases has a size of ϕ20×20 mm fixed in a TE011 microwave cavity resonant at 6.8 GHz. The size of the physics package is about ϕ130×170 mm with a volume of about 2.2 L. (b) Microwave–light double resonance signal. The three peaks correspond to the π transitions (ΔmF=0) between hyperfine levels of Rb87. (c) Ramsey oscillating signal. We fixed the microwave frequency on the clock transition frequency and measured the Ramsey signal by changing the microwave power. (d) Ramsey fringe measured with the new physics package. The fringe has a contrast of about 97%, an SNR of about 2400, and a line width of about 140 Hz.

Fig. 3. (a) Fractional frequency under different temperatures. Black square, experimental data; red curve, fit line. (b) Temperature stability in terms of the Allan deviation and the Hadamard deviation under different average times.

Fig. 4. Frequency stability of the POP clock compared with the H maser. A drift of about −4.5×10−13/day is removed from the data. A clock frequency stability of 3.53×10−13 at 1 s is obtained, and the medium-term fractional frequency stability of 4.91×10−15 is achieved at an averaging time of τ=2000  s.