  Long-range active imaging has a variety of applications in remote sensing and
target recognition. Single-photon LiDAR (light detection and ranging) offers
single-photon sensitivity and picosecond timing resolution, which is desirable
for high-precision three-dimensional (3D) imaging over long distances. Despite
important progress, further extending the imaging range presents enormous
challenges because only weak echo photons return and are mixed with strong
noise. Herein, we tackled these challenges by constructing a high-efficiency,
low-noise confocal single-photon LiDAR system, and developing a
long-range-tailored computational algorithm that provides high photon
efficiency and super-resolution in the transverse domain. Using this technique,
we experimentally demonstrated active single-photon 3D-imaging at a distance of
up to 45 km in an urban environment, with a low return-signal level of $\sim$1
photon per pixel. Our system is feasible for imaging at a few hundreds of
kilometers by refining the setup, and thus represents a significant milestone
towards rapid, low-power, and high-resolution LiDAR over extra-long ranges.

Image and Video Processing;Optics