Not your usual NMR

The CPT theorem implies, except for the sign of various properties such as charge, a particle and its antiparticle should be absolutely identical. 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. 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 1010 [1]. An important further test would be an accurate comparison of their intrinsic (spin) magnetic moments, which should also be identical, except for sign. A collaboration between researchers at several institutions in Germany (Ulmer et al.) has now taken a major step toward this goal. Writing in Physical Review Letters, they report the observation of spin flips of a single proton confined to a Penning trap [2].