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Farrokh Vakili coauthored a modern stellar intensity interferometry experiment using two one-metre optical telescopes separated by fifteen metres. The team correlated photon-counting signals from bright stars, reviving a technique historically associated with Robert Hanbury Brown.

The measured quantity was the zero-delay correlation between detected light intensities. Its contrast contains information about angular source structure. The team measured reliable visibilities for two marginally resolved stars and found reduced contrast for a binary star, consistent with its more extended apparent structure.^[ref]

Intensity interferometry infers source coherence from correlations between measured intensities, rather than directly combining electric-field amplitudes. For stationary chaotic Gaussian light, the normalized correlations satisfy $g^{(2)}(\tau )=1+|g^{(1)}(\tau ){|}^2$, under the relevant statistical assumptions.

Separated detectors can therefore measure the magnitude of spatial coherence, which is related to the source's angular structure. Optical carrier phase need not be stabilized as in amplitude interferometry, but fast timing and sufficient photon statistics are crucial. The squared magnitude does not by itself retain every phase needed for unrestricted image reconstruction.

Jean-Pierre Rivet coauthored measurements of stellar spatial coherence using intensity interferometry. Two separated optical telescopes recorded photon arrivals, and fast electronics measured the correlation of their intensity fluctuations.

Unlike combining optical field amplitudes directly, the experiment estimated a statistical correlation from many detected events. Observations of three bright targets tested how the contrast changed with angular source structure. The work demonstrated that modern photon-counting detectors could recover useful stellar visibilities with metre-class telescopes.^[ref]

Robert Hanbury Brown and Richard Quentin Twiss developed intensity interferometry, using correlated fluctuations in detected light rather than direct combination of optical field amplitudes. Their experiments established how the correlation changes with the separation between detectors.

Applied to stars, the method connects spatial coherence to apparent angular size. It measures a statistical property of the arriving photons, allowing stellar diameters to be inferred even when atmospheric fluctuations make direct optical phase measurements difficult.^[ref][ref]