On-Chip Superconducting Resonators for Gd³⁺ Spin Detection: Geometry Optimization and Comparison of Dry Etching-Based Fabrication Methods

Rare-earth ion spins diluted in non-magnetic crystals are promising candidates for physical qubits, offering long coherence times and rich energy-level structures from their large spin multiplicity. Achieving coherent control and detection of these spins requires strong coupling between the spin ensemble and the electromagnetic mode of a superconducting resonator. Building on previous work [1] demonstrating strong coupling between diluted Gd³⁺ (S = 7/2) spins and an on-chip superconducting resonator, we report ongoing efforts to optimize resonator geometry for implementing an active cooling scheme enabling spin initialization.

Temperature-dependent microwave spectroscopy is used to characterize the new cavity resonator designs and their coupling to the spin ensemble. We also examine the effects of different dry etching methods on resonator performance: while Ar ion-beam etching yields lower internal quality factors than reactive ion etching, a brief reactive ion etch fully restores the degraded quality, consistent with recent reports [2]. These results identify both geometric and fabrication-dependent strategies for advancing rare-earth spin-based hybrid quantum devices.

[1] G. Franco-Rivera et al., Phys. Rev. Appl. 19, 024067 (2023).

[2] J. Van Damme et al., Phys. Rev. Appl. 20, 014034 (2023).