Photon transport in cold atom clouds:

Perhaps the most intensively studied feature of photon transport in cold atom clouds is that of Radiation Trapping. This occurs  in an illuminated gas of atoms when the atoms, instead of acting as independent scatterers of the light, start exchanging photons with each other. Photons are therefore "trapped" inside the sample for a little while before exiting. It is important to develop sensitive indicators of radiation trapping to help overcome it's deleterious decohering effects during the preparation of coherent atomic samples for quantum computing applications.

We recently detected the presence of small amounts of radiation trapping in dilute atomic samples at densities as low as 6x10⁸/cm³ (optical depth ~ 0.4), an order of magnitude less than where effects of radiation trapping in cold atom clouds have been previously observed. We achieved this by measuring the normalized intensity correlations of light scattered from the cold atoms. These correlations are equal to unity for a coherent state. The sensitivity of the correlations to radiation trapping was predicted, then demonstrated, as shown below. A representative error bar for each data set is shown. 
  
Here is a picture of the experimental setup used in 2004 for the detection of radiation trapping in cold dilute atomic samples. And here's a picture of what our home-built external cavity tunable diode lasers look like.

In order to complete the above measurement, we needed to change the number density of the trapped atoms by an order of magnitude, without significantly affecting the temperature. Here's a trick to do this - change the relative intensity of the x-y-z trapping beams such that the total intensity stays constant. This was demonstrated in 2005. Our results are shown below.