We report the observation of the hyperfine spectrum of antihydrogen. By exposing trapped antihydrogen to microwave radiation and scanning the microwave frequency over two distinct transitions, we are able to extract the ground state hyperfine splitting. From a series of measurements involving a total of 194 detected atoms, we determine a splitting of 1,420.4 ± 0.5 megahertz, which agrees with the more precisely known value in ordinary hydrogen.

M. Ahmadi, B.X.R. Alves, C.J. Baker, W. Bertsche, E. Butler, A. Capra, C. Carruth, C.L. Cesar, M. Charlton, S. Cohen, R. Collister, S. Eriksson, A. Evans, N. Evetts, J. Fajans, T. Friesen, M.C. Fujiwara, D.R. Gill, A. Gutierrez, J.S. Hangst, W.N. Hardy, M.E. Hayden, C.A. Isaac, A. Ishida, M.A. Johnson, S.A. Jones, S. Jonsell, L. Kurchaninov, N. Madsen, M. Mathers, D. Maxwell, J.T.K. McKenna, S.Menary, J.M. Michan, T. Momose J.J. Munich, P. Nolan, K. Olchanski, A. Olin, P. Pusa, C.Ø. Rasmussen, F. Robicheaux, R.L. Sacramento, M. Sameed, E. Sarid, D.M. Silveira, G. Stutter, C. So, T.D. Tharp, J.E. Thompson, R.I. Thompson, D.P. van der Werf, J.S. Wurtele