Bose-Einstein Condensation of Atomic Hydrogen

We report observation of Bose-Einstein condensation (BEC) of a trapped, dilute gas of atomic hydrogen. The condensate and normal gas are studied by two-photon spectroscopy of the $1S$- $2S$ transition. Interactions among the atoms produce a shift of the resonance frequency proportional to density. The condensate is clearly distinguished by its large frequency shift. The peak condensate density is $4.8\pm 1.1\times {10}^{15}{\mathrm{cm}}^{-3\mathrm{}}$, corresponding to a condensate population of ${10}^9$ atoms. The BEC transition occurs at about $T=50\mathrm{}\mu \mathrm{K}$ and $n=1.8\mathrm{}\times {10}^{14}{\mathrm{cm}}^{-3\mathrm{}}$.