The ground state hyperfine splitting (GSHFS) is one of the best measured transitions in hydrogen and plays an important role in astrophysics (where it is known as the 21-cm line) and in tests of fundamental physics. In a magnetic trap, the ground state of antihydrogen (and hydrogen) is split into four hyperfine states (see Figure 1). Two of those states have a positron spin that is anti-aligned with the magnetic field and trappable by ALPHA’s magnetic minimum trap. The other two states have a positron spin that is aligned with the magnetic field and are untrapped.

Transitions between the two trappable states (c and d) correspond to a flip of the anti-proton spin and transitions from trappable states to untrappable states correspond to flip of the positron spin (d to a or c to b). The transition frequencies occur in the microwave range, from hundreds of MHz in the case of the antiproton spin flip transitions to roughly 30 GHz for the positron spin flip transition (at 1 T magnetic field).  To measure the ground state hyperfine splitting of antihydrogen, we sweep the frequency of a microwave source across the c-b and d-a transitions (shown in Figure 1). When resonant, the microwaves flip the spin of the positron, causing the antihydrogen to seek a high magnetic field and annihilate on the wall of the trap. The difference between these two frequencies is equal to the GSHFS. We can then compare our measurements to the measured GSHFS  in hydrogen for a precision test of CPT symmetry. Our most recent published measurement can be found here: https://doi.org/10.1038/nature23446