  The ground-state hyperfine splitting of antihydrogen promises one of the most
sensitive tests of CPT symmetry. The ASACUSA collaboration is pursuing a
measurement of this splitting in a Rabi-type experiment using a polarized beam
from a CUSP magnet at the Antiproton Decelerator of CERN. With the initial
intention of characterizing the Rabi apparatus, a polarized source of cold
hydrogen was built and the $\sigma_1$ transition of hydrogen was measured to a
few ppb precision. A measurement of the $\pi_1$ transition is being prepared.
The availability of this beam opens the possibility to perform first
measurements of some coefficients within the nonminimal Standard-Model
Extension.
