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Time-delayed intensity–interferometry (TDII) measurements of the fluorescent emission from an ultracold ensemble of thermal 87Rb atoms in a steady-state magneto-optical trap are presented, which reveal the underlying coherent and incoherent dynamics of the atoms. Measurements carried out with a 5 ns time resolution yielded a second-order intensity correlation function with the theoretically predicted value of 2 at zero delay. In addition coherent Rabi oscillations were seen for up to five full periods—much longer than the spontaneous emission lifetime of the excited state of Rb. The oscillations were damped out by ∼150 ns, and thereafter an exponential decay observed, from which the mean velocity of atoms and thus, the temperature of the ensemble was estimated. The values so obtained compare well with those determined by standard techniques. It is seen that TDII permits a quantitative study of the coherent and incoherent processes, even in a large ensemble of independent atomic emitters in random thermal motion. This measurement of second-order correlation powerful technique can reveal hidden periodicities such as coherent Rabi oscillations that are not directly seen in the emission from a large collection of atoms. In addition it can also reveal information about the mean velocity of the thermal ensemble of emitters, even though the Doppler broadening of emission due to the motion of atoms is smaller than the natural linewidth and is not directly measureable.
Shafi et al. (Wed,) studied this question.