content_image/372753.jpg content_image/372760.jpg content_image/372761.jpg
<img src='content_image/372753.jpg'>

where ω$_{z}$ is the frequency of the harmonic trap along the axis of light propagation ( q = ˆ z q ), the momentum transferred is 

<img src='content_image/372760.jpg'>

where V is the strength of the potential induced by the lasers and τ is the pulse duration. 

As noted recently by Arahata and Nikuni [10], the density response function can also be measured by exciting a density pulse in a uniform gas. Within linear response theory, the density fluctuation δn ( r , t ) induced by the application of an external perturbing potential δV ( r , t ) is 

where δV ( q , ω ) is the Fourier transform of the perturbing potential. Similar to the sound propagation experiments in [21, 22], consider a localized potential applied for a short du- ration τ < t < 0, and turned off at t = 0. Assuming for simplicity that the perturbing 

<img src='content_image/372761.jpg'>

