Observation of population fluctuations in two-level atoms driven by a phase diffusing field

We have measured the variance in fluorescence intensity as a phase diffusing laser is scanned through resonance with a beam of two-level atoms. For laser intensities below saturation, our results are in quantitative agreement with recent theoretical predictions provided Doppler effects are treated. Above saturation, the agreement is somewhat less quantitative primarily due to the sensitivity of the predictions to the laser intensity profile and the experimental uncertainty in measuring it.