Stellar intensity interferometery: from Narrabri to CTA

In the early 1960's a new stellar interferometer began to emerge in dusty outback NSW. The highly unorthodox design was the brainchild of Robert Hanbury Brown and Richard Twiss and was led from the University of Sydney. The technology represents the culmination of a pioneering series of experiments which were accepted by radio engineers, but which courted controversy when performed in the optical, generating skepticism (and worse) from some of the most eminent physicists of the day. These critics were eventually silenced by the overwhelming success of the Narrabri Stellar Intensity Interferometer which made outstanding contributions to fundamental stellar physics. The diameter and effective temperature scale for hot stars was based on data only matched more than a 60 years later. However NSII's most profound legacy is as the foundational experiment in the field now known as Quantum Optics. Recent years have seen a resurgence of interest with several air ˇCherenkov arrays with large collectors, including the emerging CTA, building an intensity interferometer back-end. This review will draw lessons from the past and help inform prospects for the future from this enduring technique.