All's well that ends well
In 1998, David Guéry-Odelin and Jean Dalibard asked the provocative
question "Is Bose-Einstein condensation of atomic cesium possible?"
(Europhys.Lett. 44, 25 (1998)). Rudi Grimm and his reasearch team at the
University of Innsbruck have now, finally, found an answer - and that answer
is "Yes!".
Although cesium had been used in countless laser cooling experiments long
before the first observation of Bose-Einstein condensation in the alkali
atoms lithium, sodium and rubidium in 1995, it did not seem to want to give
in to the efforts of a growing number of researchers who tried to coax it
into the Bose-condensed state. Although cesium had set the benchmark
low-temperature records in laser cooling thanks to its large mass, it
steadfastly resisted attempts at condensing it by evaporative cooling.
In order for evaporative cooling to be efficient, the "good" collisions
which allow a gas of atoms to rethermalize after the hottest atoms have
escaped have to dominate over the "bad" collisions which cause them to
escape from the trap. In magnetic traps, spin-flips induced by two-body
collisions can cause the atoms to end up in an untrapped magnetic state and
hence be lost. Cesium turned out to have the bad habit of suffering more and
more "bad" collisions the lower the temperature got.
Over the years, research groups at Oxford, Stanford and Paris, amongst
others, have developed ever more ingenuous methods for circumventing some of
the detrimental properties of the cesium atom - but so far to no avail.
Whenever the goal of Bose-Einstein condensation was in sight, cesium showed
another (previously unknwon) "bad habit". But now Rudi Grimm and his
co-workers have managed to combine all of these tricks into a successful
experiment. Following a piece of good advice already contained in the paper
by Guéry-Odelin, they used a dipole trap based on a C0_2 laser (as recently
employed by Michael Chapman at Georgia Technical University), a powerful
cooling technique developed by Vuletic and Chu at Stanford and exploited the
Feshbach resonances of cesium which allow its collisional properties to be
controlled. Finally, an additional laser was used to create a "dimple" in
which the atoms could be compressed and evaporatively cooled down to the
critical temperature for Bose-Einstein condensation.
The achievement by Rudi Grimms team is a technical feat in its own right,
but that is not all. Cesium, after all, is at the heart of many
precision-measurement applications such as atomic clocks. Bose-condensing it
opens up new avenues for pushing the frontiers of precision even further.
Details can be found at
http://exphys.uibk.ac.at/ultracold/CsBEC.html