Observation of individual spin quantum transitions of a single antiproton

We report on the detection of individual spin quantum transitions of a single trapped antiproton in a Penning trap . The spin-state determination, which is based on the unambiguous detection of axial frequency shifts in presence of a strong magnetic bottle, reaches a fidelity of 92.1 % . Spin-state initialization with > 99.9 % fidelity and an average initialization time of 24 min are demonstrated. This is a major step towards an antiproton magnetic moment measurement with a relative uncertainty on the part-per-billion level.