Spectral Narrowing due to Elastic Collisions

Abstract

As already explained, inhomogeneities in $\delta$ over the ensemble lead to dephasing of a stored coherence. Intriguingly, fluctuations in $\delta$ can prolong the coherence time—a phenomenon called motional narrowing. Historically, motional narrowing was first observed in liquid NMR, where a large reduction in the width of spectral lines was observed in comparison to solid NMR due to the thermal motion of the nuclei. Later the effect was also reported in other fields such as molecular physics, semiconductor microcavities and quantum dots. In this chapter we study motional narrowing due to elastic collisions in a dense atomic ensemble. In contrast to previous experiments, our apparatus enables precise and independent control over the thermodynamic parameters of the system. Owing to this, we are able to quantitatively analyze the dependance of the narrowed linewidth on the fluctuations rate and strength, and demonstrate an inverse linear dependence on the former and quadratic dependence on the latter. We also find that the narrowed linewidth exhibits universal scaling with the atomic phase space density.