For the first time a single trapped antiproton (p-bar) is used to measure the magnetic moment μ_p-bar . The moment is given in terms of the nuclear magneton (μ_N) by μ_p-bar/μ_N = -2.792845 +- 0.000012. The 4.4 parts per million (ppm) uncertainty is 680 times smaller than previously realized. Comparing to the proton moment measured using the same method and trap electrodes gives μ_p-bar/μ_p = -1.000000 ± 0.000005 to 5 ppm, consistent with the prediction of the CPT theorem.

On the way to further increases in precision, individual spin flips of a single trapped proton have been observed. The 96% fidelity and an efficiency expected to approach unity, suggest that it may be possible to use quantum jump spectroscopy to measure the proton and antiproton magnetic moments much more precisely.

Argonne Physics Division Colloquium Schedule