Low-Temperature Multi-Mode Microwave Spectroscopy of Paramagnetic and Rare-Earth Ion Spin Impurities in Single Crystal Calcium Tungstate

We present experimental observations of dilute ion spin ensembles in an undoped low-loss single crystal cylindrical sample of CaWO_4 cooled to 30 mK in temperature. Crystal field perturbations were elucidated by constructing a dielectrically loaded microwave cavity resonator from the crystal. The resonator exhibited numerous whispering gallery modes with high Q-factors of up to 3× 10^7, equivalent to a low loss tangent of ∼ 3× 10^-8. The low-loss allowed precision multi-mode spectroscopy of numerous high Q-factor photonspin interactions. Measurements between 7 to 22 GHz revealed the presence of Gd^3+, Fe^3+, and another trace species, inferred to be rareearth, at concentrations on the order of parts per billion. These findings motivate further exploration of prospective uses of this low-loss dielectric material for applications regarding precision and quantum metrology, as well as tests for beyond standard model physics.