Simplified geometry of the Penning trap used by Ulmer et al. The proton p oscillates axially along the magnetic field lines due to the electric potentials applied to the ring electrodes (here, labeled + and − according to their polarity). A superconducting solenoid (not shown) that surrounds the trap produces a uniform magnetic field. The (weak) radio-frequency magnetic field Brf causes the proton spin to flip. The interaction between the proton’s spin magnetic moment and the inhomogeneous magnetic field produced by the center electrode (made of ferromagnetic material), results in a tiny, but detectable, change in the proton’s axial oscillation frequency.