Getting the elastic scattering length by observing inelastic collisions in ultracold metastable helium atoms.

We report an experiment measuring simultaneously the temperature and the flux of ions produced by a cloud of triplet metastable helium atoms at the Bose-Einstein critical temperature. The onset of condensation is revealed by a sharp increase of the ion flux during evaporative cooling. Combining our measurements with previous measurements of ionization in a pure Bose-Einstein condensate, we extract an improved value of the scattering length a=11.3(+2.5)(-1.0) nm. The analysis includes corrections that take into account the effect of atomic interactions on the critical temperature, and thus an independent measurement of the scattering length would allow a new test of these calculations.

Ionization rates in a bose-einstein condensate of metastable helium.

We have studied ionizing collisions in a BEC of metastable He. Measurements of the ion production rate combined with measurements of the density and number of atoms for the same sample allow us to estimate both the two- and three-body contributions to this rate. A comparison with the decay of the atom number indicates that ionizing collisions are largely or wholly responsible for the loss. Quantum depletion makes a substantial correction to the three-body rate constant.

A Bose-Einstein condensate of metastable atoms.

We report the realization of a Bose-Einstein condensate of metastable atoms (helium in the lowest triplet state). The excitation energy of each atom with respect to the ground state is 20 electron volts, but inelastic processes that would destroy the sample are suppressed strongly enough in a spin-polarized sample to allow condensation. Our detection scheme takes advantage of the metastability to achieve detection of individual atoms as well as of the decay products of inelastic processes. This detection opens the way toward new studies in mesoscopic quantum statistical physics, as well as in atomic quantum optics.