© Mike Matthews, JILA.
The figure above shows three false color images of the velocity distribution within a cloud of 87Rb atoms in a magnetic trap. The fastest atoms are colored red, and as they slow down their colors change to yellow, green, blue, and finally white. Moving from left to right, the three images show successive stages of cooling. The leftmost peak corresponds to an atom cloud at ~10-6 K. At this temperature, the atoms behave as a classical gas with a relatively broad spread in velocity. Moving toward the right, the subsequent two peaks show the change in the velocity distribution as the trapped atoms are evaporatively cooled (see Unit 5). As the temperature is lowered, the classical gas gives way to a Bose-Einstein condensate with a sharply peaked velocity distribution. In the rightmost image, almost all of the bosonic 87Rb atoms are now in the lowest energy level of the trap. The residual non-condensed atoms actually indicate a temperature of ~10-8 K, a world record low at the time. (Unit: 6)