  Long baseline diffraction-limited optical aperture synthesis technology by
interferometry plays an important role in scientific study and practical
application. In contrast to amplitude (phase) interferometry, intensity
interferometry -- which exploits the quantum nature of light to measure the
photon bunching effect in thermal light -- is robust against atmospheric
turbulence and optical defects. However, a thermal light source typically has a
significant divergence angle and a low average photon number per mode,
forestalling the applicability over long ranges. Here, we propose and
demonstrate active intensity interferometry for super-resolution imaging over
the kilometer range. Our scheme exploits phase-independent multiple laser
emitters to produce the thermal illumination and uses an elaborate
computational algorithm to reconstruct the image. In outdoor environments, we
image two-dimension millimeter-level targets over 1.36 kilometers at a
resolution of 14 times the diffraction limit of a single telescope.
High-resolution optical imaging and sensing are anticipated by applying
long-baseline active intensity interferometry in general branches of physics
and metrology.
