Detection and control of Gd³⁺ spins in a diluted spin ensemble via on-chip superconducting resonator

Rare-earth ion spins diluted in non-magnetic crystals have emerged as a promising candidate to realize physical qubits due to their long coherence times and rich energy level structure arising from their high spin degrees of freedom. The control and detection of these quantum spins require strong coupling of the spin ensemble with 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 present our ongoing research focused on further optimizing the resonator design to implement an active cooling scheme to achieve spin initialization. In this direction, we will present highly sensitive temperature-dependent cavity spectroscopy measurements to characterize and compare the new resonator designs and their coupling with the spin ensemble.

In addition, we will discuss our strategy for achieving coherence protection using a universal pulse protocol, previously demonstrated for cylindrical cavities [2]. We will outline our experimental framework, preliminary studies, and future plans to adapt this methodology for on-chip superconducting resonators.

[1] G. Franco-Rivera et al., Strong coupling of a Gd³⁺ multilevel spin system to an on-chip superconducting resonator, Phys. Rev. Appl. 19, 024067 (2023).

[2] S. Bertaina et al., Experimental protection of quantum coherence by using a phase-tunable image drive, Sci. Rep. 10, 21643 (2020).