  Long-range active imaging has widespread applications in remote sensing and
target recognition. Single-photon light detection and ranging (lidar) has been
shown to have high sensitivity and temporal resolution. On the application
front, however, the operating range of practical single-photon lidar systems is
limited to about tens of kilometers over the Earth's atmosphere, mainly due to
the weak echo signal mixed with high background noise. Here, we present a
compact coaxial single-photon lidar system capable of realizing 3D imaging at
up to 201.5 km. It is achieved by using high-efficiency optical devices for
collection and detection, and what we believe is a new noise-suppression
technique that is efficient for long-range applications. We show that
photon-efficient computational algorithms enable accurate 3D imaging over
hundreds of kilometers with as few as 0.44 signal photons per pixel. The
results represent a significant step toward practical, low-power lidar over
extra-long ranges.
