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 +/- 1.1 x 10(15) cm(-3), corresponding to a condensate population of 10(9) atoms. The BEC transition occurs at about T = 50 mu K and n = 1.8 x 10(14) cm(-3). [S0031-9007(98)07617-0].