Abstract
We report microwave spectroscopy of ¹⁶⁷Er³⁺ doped in CaWO₄ which reveals the hyperfine splitting of the erbium electronic ground state (Z1, Jeff.=15/2) induced by the I=7/2 nuclear spin. From spectra measured below∼50 mK in magnetic fields up to 200 mT, we extract spin Hamiltonian parameters including the electron g, hyperfine A, and nuclear electric quadrupolar Q tensors. Crucially, our analysis demonstrate unambiguously, that the previously unobserved nuclear electric quadrupolar moment is essential to reproduce the experimental data. With these refined parameters, we identify zero first-order Zeeman (ZEFOZ) transitions at zero magnetic field. Extending the analysis to finite fiields, we uncover that ZEFOZ points lie either along the c axis or within the a–b plane. These results establish CaWO₄ as a promising host for long lifetime quantum memories