Energetic Helium Yields Superatoms

Hank Hogan

First realized in the lab during the last decade, Bose-Einstein condensates have offered scientists a unique material with which to pursue research. Now a new breed of helium condensate has been created that could lead to an even better understanding of matter.

Since 1995, researchers have created a spate of Bose-Einstein condensates in trapped atomic gases. At temperatures just above absolute zero, normally distinct atoms condense into a single entity, or superatom. In such condensates, the energy of each atom is approximately 10^—10 eV.

The new condensates, in contrast, are formed from metastable helium atoms excited to approximately 20 eV above the ground state, explained Claude Cohen-Tannoudji, a professor at the Collège de France at École Normale Supérieure and co-leader of one of the research teams. "We are looking for new physical effects associated with this large internal energy," he said.

The team used 1083-nm laser diodes, an ytterbium-doped fiber amplifier and a Hamamatsu CCD camera in its experiments. The first two are part of a magneto-optical trap that captures and cools a 1000-m/s jet of helium-4 in the 2³S₁ metastable state. The camera measures the optical absorption of the resulting atomic cloud, thus verifying the existence and characteristics of the Bose-Einstein condensate.

(Independently, researchers at the Institut d'Optique in Orsay have used the same methods to produce a Bose-Einstein condensate from metastable helium-4. They employed a different detection technique, however, using a microchannel plate that identified the cascade of electrons arising from the helium atoms striking the plate.)

Research continues into the new condensate, Cohen-Tannoudji said, adding that he hopes current experiments will yield novel and interesting results. For instance, the density of atoms in the magnetic trap is sufficiently high that the elastic collision rate between the atoms is large compared with the oscillation frequencies in the trap. This puts the superatom into the hydrodynamic regime, a state that researchers have not yet fully investigated in trapped atomic gases.

The team reported its work in the April 16 issue of Physical Review Letters.