  Optical imaging with microarcsecond resolution will reveal details across and
outside stellar surfaces but requires kilometer-scale interferometers,
challenging to realize either on the ground or in space. Intensity
interferometry, electronically connecting independent telescopes, has a noise
budget that relates to the electronic time resolution, circumventing issues of
atmospheric turbulence. Extents up to a few km are becoming realistic with
arrays of optical air Cherenkov telescopes (primarily erected for gamma-ray
studies), enabling an optical equivalent of radio interferometer arrays.
Pioneered by Hanbury Brown and Twiss, digital versions of the technique have
now been demonstrated, reconstructing diffraction-limited images from
laboratory measurements over hundreds of optical baselines. This review
outlines the method from its beginnings, describes current experiments, and
sketches prospects for future observations.
