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The spin-magnetic moment of the proton $\mu_p$ is a fundamental property ofthis particle. So far $\mu_p$ has only been measured indirectly, analysing thespectrum of an atomic hydrogen maser in a magnetic field. Here, we report thedirect high-precision measurement of the magnetic moment of a single protonusing the double Penning-trap technique. We drive proton-spin quantum jumps bya magnetic radio-frequency field in a Penning trap with a homogeneous magneticfield. The induced spin-transitions are detected in a second trap with a strongsuperimposed magnetic inhomogeneity. This enables the measurement of thespin-flip probability as a function of the drive frequency. In each measurementthe proton's cyclotron frequency is used to determine the magnetic field of thetrap. From the normalized resonance curve, we extract the particle's magneticmoment in units of the nuclear magneton $\mu_p=2.792847350(9)\mu_N$. Thismeasurement outperforms previous Penning trap measurements in terms ofprecision by a factor of about 760. It improves the precision of the forty yearold indirect measurement, in which significant theoretical bound statecorrections were required to obtain $\mu_p$, by a factor of 3. By applicationof this method to the antiproton magnetic moment $\mu_{\bar{p}}$ the fractionalprecision of the recently reported value can be improved by a factor of atleast 1000. Combined with the present result, this will provide a stringenttest of matter/antimatter symmetry with baryons.

Direct high-precision measurement of the magnetic moment of the proton