  An accurate measurement of the bunching of photons in the fluorescent
emission from an ultracold ensemble of thermal 87Rb atoms in a steady-state
magneto-optical trap is presented. Time-delayed-intensity-interferometry (TDII)
performed with a 5-nanosecond time resolution yielded a second-order intensity
correlation function that has the ideal value of 2 at zero delay, and that
shows coherent Rabi oscillations of upto 5 full periods - much longer than the
spontaneous emission lifetime of the excited state of Rb. The oscillations are
damped out by ~150ns, and thereafter, as expected from a thermal source, an
exponential decay is observed, enabling the determination of the temperature of
the atomic ensemble. Values so obtained compare well with those determined by
standard techniques. TDII thus enables a quantitative study of the coherent and
incoherent dynamics, even of a large thermal ensemble of atomic emitters.
