Coherent Spectroscopy with a Single Antiproton Spin

Coherent quantum transition spectroscopy is a powerful tool in quantum-sensing and metrology (1), quantum information processing (2), accurate magnetometry (3), high-precision tests of the fundamental laws of nature, and searches for physics beyond the Standard Model (4). In atomic physics measurements, it was applied with great success in proton and deuteron magnetic moment measurements (5), which culminated for example in MASER spectroscopy with sub-parts per trillion resolution (6), in record-constraints on the neutron electric dipole moment (7), and many other experiments at the forefront of physics. All these experiments were performed on macroscopic ensembles of particles, while the coherent spectroscopy of a “quasifree” single nuclear spin has never been reported before. In this manuscript, we demonstrate the first non-destructive coherent quantum transition spectroscopy of the spin of a single trapped antiproton, stored in a cryogenic Penning-trap system. We apply a multi-trap technique (8), detect the antiproton spin-state using the continuous Stern-Gerlach-effect (9), and transport the particle to the homogeneous and stabilized magnetic field of a precision trap (PT). Here, we induce the coherent dynamics, and analyze the result by a quantum-projection measurement in the analysis trap (10). In our measurements, we observe for the first time Rabi-oscillations of a single nuclear magnetic moment, and achieve in time series measurements spin inversion probabilities above 80% at spin coherence times of ≈ 50s. Scans of single-particle spin resonance spectra show inversions > 70%, at transition line-widths 16 times narrower than in our previous measurements (8), limited by cyclotron frequency measurement decoherence. This achievement marks a major step towards at least 10-fold improved measurements of the proton and antiproton magnetic moments, and thus, substantially improved tests of matter/antimatter symmetry in the baryon-sector.