Bose-Einstein Condensation of Atomic Hydrogen
  We have observed Bose-Einstein condensation (BEC) of trapped atomic hydrogen, and studied it by two-photon spectroscopy of the 1S-2S transition. In these lecture notes we briefly review the history of spin-polarized atomic hydrogen and describe the final steps to BEC. Laser spectroscopy, which probes the difference in mean field energy of the 1S and 2S states, is used to study the condensate, which has a peak density of 4.8e15 cm^-3 and population of 10^9. 
Daniel Kleppner, Thomas J. Greytak, Thomas C. Killian, Dale G. Fried,   Lorenz Willmann, David Landhuis, and Stephen C. Moss