For the first time a single trapped antiproton ( " p) is used to measure the " p magnetic moment " p . The moment " p ¼ " p S=ð@=2Þ is given in terms of its spin S and the nuclear magneton ( N ) by " p = N ¼ À2:792 845 AE 0:000 012. 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 " p = p ¼ À1:000 000 AE 0:000 005 to 5 ppm, for a proton moment p ¼ p S=ð@=2Þ, consistent with the prediction of the CPT theorem.

Previous measurements with a single trapped proton (p) or antiproton (p) detected spin resonance from the increased scatter of frequency measurements caused by many spin flips. Here a measured correlation confirms that individual spin transitions and states are rapidly detected instead. The 96% fidelity and an efficiency expected to approach unity suggests that it may be possible to use quantum jump spectroscopy to measure the p and p magnetic moments much more precisely.

Four billion positrons (e +) are accumulated in a Penning-Ioffe trap apparatus at 1.2 K and <6×10 −17 Torr. This is the largest number of positrons ever held in a Penning trap. The e + are cooled by collisions with trapped electrons (e −) in this first demonstration of using e − for efficient loading of e + into a Penning trap. The combined low temperature and vacuum pressure provide an environment suitable for antihydrogen (H) production, and long antimatter storage times, sufficient for high-precision tests of antimatter gravity and of CPT.

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