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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.A Penning trap confines a charged particle, in ultrahigh vacuum, using a uniform magnetic field, which causes the particle to undergo a circular cyclotron motion; and an electrostatic potential, which causes the particle to oscillate along the axis of the trap, parallel to the magnetic field (Fig.1).The magnetic moment of the particle, expressed as the dimensionless quantity known as the factor, can be measured very precisely in this setup because it is the ratio of two frequencies: the Larmor spin-precession frequency, which is the frequency of the (weak) oscillating magnetic field needed to flip the proton’s spin direction with respect to the magnetic field in the trap, and the frequency of the particle’s cyclotron motion.Writing in Physical Review Letters, they report the observation of spin flips of a single proton confined to a Penning trap [2].Using a Penning trap, the antiproton magnetic moment could be measured to better than one part in , a millionfold improvement over existing values [3].This work culminated in a precise measurement of the magnetic moment of the electron, which provided the most precise test of quantum electrodynamics theory and the most precise value for the fine structure constant .oscillates axially along the magnetic field lines due to the electric potentials applied to the ring electrodes (here, labeled + and − according to their polarity).The theorem is an essentially inviolable tenet of quantum field theory, and one that follows from the assumption that physics is the same in all inertial frames.A superconducting solenoid (not shown) that surrounds the trap produces a uniform magnetic field.The CPT theorem has been tested with baryons—composite particles made up of quarks bound by the strong interaction—by using a Penning trap to compare the masses of the proton and the antiproton to a precision better than one part in [1].

An important further test would be an accurate comparison of their intrinsic (spin) magnetic moments, which should also be identical, except for sign.