  Intensity interferometry exploits a quantum optical effect in order to
measure objects with extremely small angular scales. The first experiment to
use this technique was the Narrabri intensity interferometer, which was
successfully used in the 1970s to measure 32 stellar diameters at optical
wavelengths; some as small as 0.4 milli-arcseconds. The advantage of this
technique, in comparison with Michelson interferometers, is that it requires
only relatively crude, but large, light collectors equipped with fast
(nanosecond) photon detectors. Ground-based gamma-ray telescope arrays have
similar specifications, and a number of these observatories are now operating
worldwide, with more extensive installations planned for the future. These
future instruments (CTA, AGIS, completion 2015) with 30-90 telescopes will
provide 400-4000 different baselines that range in length between 50m and a
kilometre. Intensity interferometry with such arrays of telescopes attains $50
\mu$-arcsecond resolution for a limiting visual magnitude ~8.5. Phase
information can be extracted from the interferometric measurement with phase
closure, allowing image reconstruction. This technique opens the possibility of
a wide range of studies amongst others, probing the stellar surface activity
and the dynamic AU scale circumstellar environment of stars in various crucial
evolutionary stages. Here we focuse on the astrophysical potential of an
intensity interferometer utilising planned new gamma-ray instrumentation.
