Temporal intensity interferometry: photon bunching in three bright stars

We report the first intensity correlationmeasured with starlight since the historical experiments of Hanbury Brown and Twiss. The photon bunching g((2))(tau, r = 0), obtained in the photon-counting regime, was measured for three bright stars: a Boo, alpha CMi and beta Gem. The light was collected at the focal plane of a 1-m optical telescope, transported by a multi-mode optical fibre, split into two avalanche photodiodes and correlated digitally in real time. For total exposure times of a few hours, we obtained contrast values around 2 x 10(-3), in agreement with the expectation for chaotic sources, given the optical and electronic bandwidths of our set-up. Comparing our results with the measurement of Hanbury Brown et al. for alpha CMi, we argue for the timely opportunity to extend our experiments to measuring the spatial correlation function over existing and/or foreseen arrays of optical telescopes diluted over several kilometres. This would enable microarcsec long-baseline interferometry in the optical, especially in the visible wavelengths, with a limiting magnitude of 10.