We have observed spin quantum-jumps with a single trapped proton for the first time. The result is a pioneering step forward in the endeavor to directly measure the magnetic properties of the proton and the antiproton with high precision, and was just published in Physical Review Letters. The measuring principle is based on the observation of a single proton stored in a Penning trap.
The proton has an intrinsic angular momentum or spin, just like other particles. It is like a tiny bar magnet; in this analogy, a spin quantum jump would correspond to a (switch) flip of the magnetic poles. However, detecting the proton spin is a major challenge. While the magnetic moments of the electron and its anti-particle, the positron, were already being measured and compared in the 1980s with a fractional precision of 3 ppt, this has yet to be achieved in the case of the proton. The problem is that the magnetic moment of the proton is 660 times smaller than that of the electron, which means that it is considerably harder to detect.