Bose-Einstein Condensation of Atomic Hydrogen
  We report observation of Bose-Einstein condensation 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 \times 10^{15} cm^{-3}, corresponding to a condensate population of 10^9 atoms. The BEC transition occurs at about T=50 uK and n=1.8 \times 10^{14} cm^{-3}. 
Dale G. Fried, Thomas C. Killian, Lorenz Willmann, David Landhuis,   Stephen C. Moss, Daniel Kleppner, and Thomas J. Greytak