Coherent spectroscopy with a single antiproton spin

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.