Strong Coupling of a Gd3+ Multilevel Spin System to an On-Chip Superconducting Resonator

Overview & Synopsis

We report the realization of a strong coupling between a Gd 3+ spin ensemble hosted in a scheelite (CaWO 4) single crystal and the resonant mode of a coplanar-strip-line superconducting cavity leading to a large separation of spin-photon states of 146 MHz. The interaction is well described by the Dicke model and the crystal-field Hamiltonian of the multilevel spin system. We observe a change of the crystal-field parameters due to the presence of photons in the cavity, which generates a significant perturbation of the crystal ground state. Using finite-element calculations, we numerically estimate the cavity sensing volume as well as the average spin-photon coupling strength of g 0 /2π ≈ 620 Hz. Lastly, the dynamics of the spincavity states are explored via pulsed measurements by recording the cavity ring-down signal as a function of the pulse length and the amplitude. The results indicate a potential method to initialize this multilevel system in its ground state via an active cooling process.