Cesium Bose-Einstein condensate at Innsbruck

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