  We report our experimental measurements and theoretical analysis of the
response function of cold atoms in a 3D optical molasses. The response function
of the atomic cloud is measured by applying a pulsed homogeneous magnetic field
which provides the perturbing force. We observe an interesting transition from
a damped oscillatory to an over-damped behaviour of the response function
stemming from a competition between the viscous drag provided by the 3D optical
molasses and the restoring force of the Magneto-Optical Trap (MOT). Our
observations are in good qualitative and quantitative agreement with the
predictions of our theoretical model based on the Quantum Langevin equation. We
also study the variation of the Diffusion coefficient of the cold atoms as the
atomic cloud is cooled to lower temperatures via sub-Doppler cooling in a 3D
optical molasses and compare our experimental data with the theoretical
calculations using the Stokes-Einstein-Smoluchowski relation.
