Fig. 1: Breit–Rabi diagram of ³He⁺.

The energies of the hyperfine states E₁, E₂, E₃ and E₄ are plotted as a function of the magnetic field according to equation (1). The arrows below m_j and m_I indicate the orientation with respect to the magnetic field of the total angular momentum of the electron \(j=1/2\) and the nuclear spin \(I=1/2\), which are antiparallel to the magnetic moments µ_e and µ_I, respectively. The four double-headed arrows indicate the hyperfine transitions measured in this work. The transition frequencies given on the right side refer to the magnetic field in the Penning trap \(B=5.7\) T, which is marked in the plot by the black dashed line.