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.
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