crystal coupled to on-chip superconducting resonators.*

Hybrid quantum systems based on solid state spin ensembles presents as promising candidates for quantum memories due to their long coherence times¹ and possibility to perform multi-state qubit operations due to their rich energy levels². We present the coupling of coplanar stripline geometry superconducting resonator to the well-defined qubit states of Gd hosted in a CaWO₄ single crystal. A number of transitions between the ground and an excited state of the electronic multiplet ⁸S_7/2 of Gd³⁺ were studied by continuous-wave spectroscopy of the cavity resonance field dependence (B₀ ⊥ c). Numerical calculations allowed to finely tune the high-order zero-field splitting parameters for the spin Hamiltonian at T ≈ 0.5K.

¹ S. Bertaina, et al., arXiv:2001.02417 (2020).
² E.I. Baibekov, et al., Phys Rev B 95, 064427 (2017).