Evaporative cooling is the final cooling step in our experiment. We remove the hottest atoms of the ensemble using rf transitions between atomic states to leave behind a sample that is, on average, cooler. A very neat interactive explanation of this process is here.
We use our chip trap to achieve very efficient evaporative and sympathetic cooling. We see efficiencies of about 3 orders of magnitude improvement in phase space density for each order of magnitude loss in atom number.
Absorption images of ultracold atom clouds after 9 ms time-of-flight, demonstrating momentum distributions.
Top Row: 87Rb atoms at three stages in the evaporative cooling process. On the far left, we see "thermal" atoms, which have expanded isotropically. In the centre image, we see a higher-density region of atoms, which have coalesced in the ground state and their anisotropic expansion tells us they are a BEC. On the right, we see atoms which have completely condensed.
Bottom Row: 40Rb atoms at two stages in the sympathetic cooling process. On the left, we see atoms at about 0.8 TF, where TF is the Fermi temperature. As the temperature is reduced, the fermions, which do not condense, fill momentum states up to the Fermi momentum, which is indicated by the white circle in the right image. In this right image, the atoms are at ~ 0.3 TF, and as they cool, will not get much smaller in momentum space than seen here.