Bose-Einstein Condensation of Metastable Helium

We have observed a Bose-Einstein condensate in a dilute gas of ${}^4\mathrm{He}$ in the ${}_{}{}^3{}_{}{}^{}2S_1^{}{}_{}{}^{}$ metastable state. We find a critical temperature of $(4.7\pm 0.5)\mu \mathrm{K}$ and a typical number of atoms at the threshold of $8\times {10}^6$. The maximum number of atoms in our condensate is about $5\times {10}^5$. An approximate value for the scattering length $a=(16\pm 8)\mathrm{nm}$ is measured. The mean elastic collision rate at threshold is then estimated to be about $2\times {10}^4{\mathrm{s}}^{-1\mathrm{}}$, indicating that we are deeply in the hydrodynamic regime. The typical decay time of the condensate is $2\mathrm{s}$, which places an upper bound on the rate constants for two-body and three-body inelastic collisions.