We report a phase sensitive two-photon interference effect when the incoming thermal fields are mixed with weak coherent reference fields. The complex second-order coherence function of the input thermal fields can be extracted.
In 1956, Hanbury Brown and Twiss performed their famous intensity-correlation experiment with two 'coherent' beams of light from a thermal source. They argued that such intensity-correlation must entail an excess of photon arrival-time coincidences over chance ones for the two beams, and went on (with Little) to demonstrate this in an experiment (Twiss, Little and Hanbury Brown 1957). About the same time, Brannen and Ferguson (1956) performed a photon-coincidence experiment with coherent thermal light, and obtained a null result (due to lack of sensitivity).