  The original intensity interferometers were instruments built in the 1950s
and 60s by Hanbury Brown and collaborators, achieving milli-arcsec resolutions
in visible light without optical-quality mirrors. They exploited a then-novel
physical effect, now known as HBT correlation after the experiments of Hanbury
Brown and Twiss, and nowadays considered fundamental in quantum optics. Now a
new generation of inten- sity interferometers is being designed, raising the
possibility of measuring intensity correlations with three or more detectors.
Quantum optics predicts some interesting features in higher-order HBT. One is
that HBT correlation increases combinatorially with the number of detectors.
Signal to noise considerations suggest, that many-detector HBT correlations
would be mea- surable for bright masers, but very difficult for thermal
sources. But the more modest three-detector HBT correlation seems measurable
for bright stars, and would provide image information (namely the bispectrum)
not present in standard HBT.
