A sensitive frequency-shift technique is employed to monitor the magnetic quantum state of a single electron stored in a compensated Penning trap. The electron sees a weak parabolic magnetic pseudopotential in addition to the electric well, which causes the axial oscillation frequency to have a slight magnetic quantum state dependence. Transitions at both the spin-cyclotron-beat (anomaly) frequency and the cyclotron frequency have been measured in the same environment to yield a magnetic anomaly $a_e=(1\mathrm{}159652410\pm 200)\times {10}^{-12\mathrm{}}$.

• Received 5 November 1976

DOI:https://doi.org/10.1103/PhysRevLett.38.310