Fig. 1. SEM images. (a) SEM image of SNSPD with 15 μm diameter photosensitive surface; (b) SEM image of nanowire after partial magnification

Fig. 2. Schematics of SNSPD operation principle based on hotspot model ^[12]

Fig. 3. SDE performance comparison of different SPDs^{[5,14,25,30-36]}

Fig. 4. TCSPC technology. (a) Schematic diagram of TCSPC technology ^[41]; (b) schematic diagram of TCSPC-based Lidar

Fig. 5. SNSPD ranging system. (a) Scanning imaging system based on SNSPD; (b) 1 km distance imaging results at different viewpoints of the scenes^[43]

Fig. 6. Laser ranging based on SNSPD. (a) Schematics of the LARES satellite ranging system 3000 km away based on SNSPD; (b) ranging result^[15]

Fig. 7. Few-photon scanning imaging with sub-millimeter depth resolution. (a) Reconstructed profile of a small key achieved by detecting and time-tagging reflected photons; (b) dwell time at each position of the scan was fixed to 20 ms meaning that the number of detected photons was different at each location; (c) distribution of the detected number of photons in the pixels of the scan^[28]

Fig. 8. Bioluminescence lifetime imaging. (a) Schematic diagram of confocal fluorescence lifetime imaging microscope based on SNSPD; (b) NIR-II confocal fluorescence lifetime imaging of rat glioma cells^[50]

Fig. 9. Experimental results. (a) Multi-pixel SNSPD schematic diagram based on double-ended readout technology and 590 pixel single-photon imaging demonstration experiment^[51]; (b) multi-pixel SNSPD camera and 16 pixel×16 pixel 3D single-photon image reconstruction process based on dual-end readout technology^[52]

Fig. 10. Experimental results. (a) Schematic of thousand-pixel array SNSPD read by row-column array; (b) optical micrograph of the array; (c) chip-scale layout of the array; (d)(e) log-scale count rate across the array under illumination with a focused laser beam; (f) persistence images when the laser spot sweeps across the array^[54]

Fig. 11. Schematic of ghost imaging^[59]

Fig. 12. Schematic diagram of single-pixel imaging ^[60]

Fig. 13. Experimental results. (a) Schematic diagram of compressed sensing imaging experiment based on SNSPD; (b) imaging results^[65]

Table 1. Typical parameters of commonly used SPDs

Table 2. Experimental results of laser ranging under different echo intensities^[71]

Table 3. Performance comparison of mid-infrared molecular spectrum detection system between SNSPD and InSb detector^[78]