Fig. 1. Density distribution of photon response time series corresponds to light intensity

Fig. 2. Schematic diagram of free-running counting

Fig. 3. Schematic diagram of two-channel coincidence counting

Fig. 4. Schematic diagram of two-channel coincidence counting photon heterodyne velocity measurement system

Fig. 5. Influence of coincidence counting time window size on signal-to-noise ratio (SNR) of velocity measurement

Fig. 6. Intermediate frequency (IF) signal power spectra. (a) Single channel free-running mode; (b) first order filtering of photon cumulative histogram curve; (c) two-channel coincidence counting mode

Fig. 7. Variation of SNR of IF signal with signal photon number under two counting modes

Fig. 8. Influence of local oscillator intensity on SNR of IF signal. (a) Single channel free-running mode; (b) two-channel coincidence counting mode

Fig. 9. Influence of background noise on SNR of IF signal. (a) Single channel free-running mode; (b) two-channel coincidence counting mode

Fig. 10. Influence of IF on SNR of IF signal. (a) Single channel free-running mode; (b) two-channel coincidence counting mode

Fig. 11. Influence of detection duration on SNR of IF signal. (a) Single channel free-running mode; (b) two-channel coincidence counting mode