Bose-Einstein Condensation of Metastable Helium

We have observed a Bose-Einstein condensate in a dilute gas of ⁴He in the ³2S₁ metastable state. We find a critical temperature of (4.7±0.5)μK and a typical number of atoms at the threshold of 8×10⁶. The maximum number of atoms in our condensate is about 5×10⁵. An approximate value for the scattering length a0ex0ex=0ex0ex(16±8)nm is measured. The mean elastic collision rate at threshold is then estimated to be about 2×10⁴s^-1, indicating that we are deeply in the hydrodynamic regime. The typical decay time of the condensate is 2s, which places an upper bound on the rate constants for two-body and three-body inelastic collisions.