Building on technological developments over the last 35 years, intensity interferometry now appears a feasible option by which to achieve diffraction-limited imaging over a square-kilometer synthetic aperture. Upcoming Atmospheric Cherenkov Telescope projects will consist of up to 100 telescopes, each with ~100m2 of light gathering area, and distributed over ~1km2. These large facilities will offer thousands of baselines from 50m to more than 1km and an unprecedented (u,v) plane coverage. The revival of interest in Intensity Interferometry has recently led to the formation of a IAU working group. Here we report on various ongoing efforts towards implementing modern Stellar Intensity Interferometry.
Stellar amplitude interferometry is limited by the need to have optical distances known to a fraction of the wavelength. We suggest reviving intensity interferometry, which requires far less accurate hardware (~1cm mechanical precision) at the cost of more limited sensitivity. We present an algorithm that uses the very high redundancy of a uniform linear array to increase the sensitivity of the instrument by more than a hundredfold. An array of a hundred ~100m diameter elements can achieve a limiting magnitude of mb=14.4. Off-line processing of the data will enable such a ground-based facility to transform a two-dimensional field of point-like sources to a three-dimensional distribution of micro-arcsec resolved systems, each imaged in several optical bands. Each system will also have its high resolution residual timing, high quality (inside each band) spectra and light curve, emergent flux, effective temperature, polarization effects and perhaps some thermodynamic properties, all directly measured in a single observation run of such a dedicated facility. Coronagraphy, selectively suppressing large scale structures of the sources, can also be achieved by specific aperture shapes. We conclude that after three decades of abandonment optical intensity interferometry deserves another review, also as a ground-based alternative to the science goals of space interferometers.