Bose-Einstein condensation of ultracold metastable helium atoms

In the cold atoms section of LCVU fundamental research is performed on a dilute helium gas (in the metastable 2 3S1 state: He*) close to absolute zero temperature. Applying laser cooling techniques 108 - 109 helium atoms (3He, 4He as well as mixtures) are cooled and trapped in a magneto-optical trap (MOT). The experimental setup allows detection of the cold cloud by absorption imaging on a CCD camera but also allows detection on two microchannel plate (MCP) detectors. One MCP detector measures ions, produced by collisions inside the cold cloud, whereas a second MCP, mounted 17 cm below the cloud, detects He* atoms directly.
In case of the bosonic isotope (4He) the cloud from the MOT, which has a temperature of 1 mK, is transfered to a magnetic trap and subsequently cooled by evaporative cooling to a temperature of 1 μK, where Bose-Einstein condensation (BEC) is observed. At a pressure of 10-11 mbar in the UHV chamber the cloud in the magnetic trap has a lifetime of ~3 minutes. Preliminary measurements show that a condensate contains about 106 atoms.
Future research aims at understanding the physics of this fascinating macroscopic quantum state. Parallel research aims at cooling 3He* fermions (via sympathetic cooling with 4He* atoms) to similar temperatures and densities allowing studies of a 3He* Fermi-degenerate gas as well as Bose-Fermi mixtures.

Contact: Wim Vassen, e-mail: w.vassen@few.vu.nl
Atomic, Molecular and Laser Physics

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Time-of-flight spectra of a cloud of ultracold He* atoms after 30 s evaporative cooling from
an initial radiofrequency of 90 MHz to several end frequencies corresponding to colder
and denser clouds. The changing shape in the < 9.9 MHz data reflects the appearance
of a Bose-Einstein condensate.