  Highest resolution imaging in astronomy is achieved by interferometry,
connecting telescopes over increasingly longer distances, and at successively
shorter wavelengths. Here, we present the first diffraction-limited images in
visual light, produced by an array of independent optical telescopes, connected
electronically only, with no optical links between them. With an array of small
telescopes, second-order optical coherence of the sources is measured through
intensity interferometry over 180 baselines between pairs of telescopes, and
two-dimensional images reconstructed. The technique aims at diffraction-limited
optical aperture synthesis over kilometre-long baselines to reach resolutions
showing details on stellar surfaces and perhaps even the silhouettes of
transiting exoplanets. Intensity interferometry circumvents problems of
atmospheric turbulence that constrain ordinary interferometry. Since the
electronic signal can be copied, many baselines can be built up between
dispersed telescopes, and over long distances. Using arrays of air Cherenkov
telescopes, this should enable the optical equivalent of interferometric arrays
currently operating at radio wavelengths.
