On August 18, 2004 we observed our first BEC
in Rubidium 87 atoms. A nice explanation of what BEC is and why it is
interesting can be found at the
physics 2000 website.
Our setup is different from most BEC experiments in that we use an
“all-optical” technique to reach the condensation conditions. What this
means is that we load our atoms into a trap which is formed by the overlap
of two focused laser beams. The atoms are trapped because they are
attracted to regions where the laser intensity is the highest. This happens
to be where the two laser beams intersect.
To make a BEC one needs to have a dense
sample of atoms at an extremely low temperature. For the density that we have in our experiments we must
reach a temperature that is only about 100 billionth of a degree
above absolute zero. This low temperature is achieved through evaporative
cooling, the process which will lead a cup of coffee to become cold if it
is left to stand for a few minutes. In our case, this evaporation is
performed by lowering the intensity of the laser beams the constitute the
optical trap to let the hottest atoms escape. The atoms which remain have a
lower average energy and hence a lower temperature. With this technique we can reach
the temperature necessary for a BEC to form, the only catch being that we
have thrown away some of the atoms. Typically we start with about 1 million
atoms and end up with about 10,000 in the Bose-Einstein condensate. The
whole evaporation process is carried out in about 1.5 seconds.
At the top right is a sequence of pictures
showing the formation of the BEC. These images were taken after the
condensate was allowed to freely expand for 7 milliseconds. You can see
that as the condensation temperature is crossed the peak goes up
dramatically. This is because the condensate is expanding at a much slower
rate than the non-condensate (also known as a thermal cloud).
The lower picture shows what happens when the
condensate is allowed to expand for different amounts of time. In this
situation a thermal cloud of atoms would expand uniformly in all
directions, so that after 12 milliseconds we should observe a spherical
atomic cloud. Instead the condensate turns into a cigar shaped object. This
effect is produced by the interactions between the atoms which are much
stronger in the vertical direction. This is because we confine the atoms
more tightly along this axis.