Spin-Orbit Qubits of Rare-Earth-Metal Ions in Axially Symmetric Crystal Fields

Contrary to the well-known spin qubits, rare-earth-metal 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 μs) and the Rabi frequency ΩR is anisotropic. Here, we present a study of the variations of ΩR(→H₀) with the magnitude and direction of the static magnetic field →H₀ for the odd ¹⁶⁷Er isotope in a single crystal CaWO₄:Er³⁺. The hyperfine interactions split the ΩR(→H₀) 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.