Coherent quantum transition spectroscopy is a powerful tool in metrology1, quantum information processing2, magnetometry3 and precision tests of the standard model4. It was applied with great success in proton and deuteron magnetic moment measurements5, which culminated in maser spectroscopy with sub-parts-per-trillion resolution6 and many other experiments at the forefront of physics7. All of these experiments were performed on macroscopic ensembles of particles, whereas the coherent spectroscopy of a ‘free’ single nuclear spin has, to our knowledge, never been reported before. Here we demonstrate coherent quantum transition spectroscopy of the spin of a single antiproton stored in a cryogenic Penning-trap system. We apply a multi-trap technique8, detect the antiproton spin state using the continuous Stern–Gerlach effect9 and transport the particle to the homogeneous magnetic field of a precision trap (PT). Here we induce the coherent dynamics and analyse the result by quantum-projection measurements in the analysis trap (AT)10. We observe, for the first time, Rabi oscillations of an antiproton spin and achieve in time-series measurements spin-inversion probabilities greater than 80% at spin coherence times of about 50 s. Scans of single-particle spin resonances show inversions greater than 70%, at transition linewidths 16 times narrower than in previous measurements8, limited by cyclotron frequency measurement decoherence. This achievement marks a notable step towards at least tenfold improved tests of matter/antimatter symmetry using proton and antiproton magnetic moments.
