Measuring photon bunching at ultrashort timescale by two-photon absorption in semiconductors

Photon beams exhibit temporal correlations that are characteristics of their emission mechanism. For instance, photons issued from incoherent sources tend to be detected in bunches. This striking 'bunching' behaviour has been observed in the seminal experiment by Hanbury-Brown and Twiss (HBT) in the fifties, who measured the time of arrival of partially coherent photons on two separate photon-counting modules 1 . Since then, HBT Interferometry has become a widespread technique to study photon correlations down to only the nanosecond range, because of the detector-limited bandwidth, preventing the observation of bunching for real thermal sources. It has been suggested later that two-photon absorption (TPA) could measure the photon temporal correlations at a much shorter timescale 2,3 , as it involves an almost simultaneous absorption of two photons, within a maximum delay given by the Heisenberg principle. Here, for the first time, this prediction is experimentally demonstrated using TPA in a GaAs photon-counting module. We have observed photon bunching in the femtosecond range for real blackbody sources (an enhancement of six orders of magnitude in the time resolution of present techniques), opening the way to monitor optical quantum statistics at the ultrashort timescale.