Fabien Boitier, L'École Polytechnique
Abstract
In 1956, Hanbury Brown & Twiss developed a new interferometer and simply used it to study a partially coherent source. They showed that, surprisingly enough at that time, photons originating from an incoherent source tend to hit a detector in bunch: photon correlations were born. More than fifty years after, photon correlation properties are harnessed in various experiments and applications at the classical level as well as the quantum level. In spite of this success, measurement techniques to characterize such correlations often address a particular kind of source – limited by the incoming power of the source, its spectrum width or the time response of the detector.
Here, we take advantage of the huge bandwidth offered by two photon absorption in a semiconductor to quantify the rate of photon coincidences of cw optical sources with output power down to 0.1 μW, bandwidth in the 1.1 to 1.8 μm range and time resolution in the femtosecond range. Experimentally, the system is similar to a Hanbury Brown–Twiss interferometer but, in our case, the two delayed sub-beams are recombined in a two-photon counting device. Thanks to the huge available bandwidth, broadband source correlations are characterized by measuring, with the same apparatus, the degree of second order coherence of a laser, a thermal source or a twin photon source.
For the first time, the bunching effect in a real blackbody is unambiguously shown down to the femtosecond level. Concerning parametric light, we demonstrate that our experimental set-up is able to determine the exact coincidence of twin photons as well as the accidental one between photons from different pairs by controlling chromatic dispersion phenomena with a prism pair set-up.