  Light is a precious tool to probe matter, as it captures microscopic and
macroscopic information on the system. We here report on the transition from a
thermal (classical) to a spontaneous emission (quantum) mechanism for the loss
of light coherence from a macroscopic atomic cloud. The coherence is probed by
intensity-intensity correlation measurements realized on the light scattered by
the atomic sample, and the transition is explored by tuning the balance between
thermal coherence loss and spontaneous emission via the pump strength. Our
results illustrate the potential of cold atom setups to investigate the
classical-to-quantum transition in macroscopic systems.
