  We present the first study of the statistics of GHz photons in quantum
circuits, using Hanbury-Brown and Twiss correlations. The superpoissonian and
poissonian photon statistics of thermal and coherent sources respectively made
of a resistor and a radiofrequency generator are measured down to the quantum
regime at milliKelvin temperatures. As photon correlations are linked to the
second and fourth moments of current fluctuations, this experiment, which is
based on current cryogenic electronics, may become a standard for probing
electron/photon statistics in quantum conductors
