Contrary to the well known spin qubits, rare-earth qubits are characterized by a strong influence of crystal field due to large spin-orbit coupling. At low temperature and in the presence of resonance microwaves, it is the magnetic moment of the crystal-field ground-state which nutates (for several $mu$s) and the Rabi frequency $Omega_R$ is anisotropic. Here, we present a study of the variations of $Omega_R(vec{H}_{0})$ with the magnitude and direction of the static magnetic field $vec{H_{0}}$ for the odd $^{167}$Er isotope in a single crystal CaWO$_4$:Er$^{3+}$. The hyperfine interactions split the $Omega_R(vec{H}_{0})$ curve into eight different curves which are fitted numerically and described analytically. These spin-orbit qubits should allow detailed studies of decoherence mechanisms which become relevant at high temperature and open new ways for qubit addressing using properly oriented magnetic fields.
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