Spin-photon interaction between a ruby crystal and a high-critical-temperature superconducting microwave cavity

High-spin systems in molecules and inorganic solids hold significant potential for advancing quantum information storage and processing technologies, owing to their multilevel structure. However, the dynamics of spin-photon interactions in these systems remain underexplored in practical device settings. Here we investigate the quantum properties of an S = 3/2 spin ensemble in a ruby crystal embedded in a high-Tc superconducting microwave cavity. Using electron spin resonance techniques, we measure the coupling strength between the quantized electromagnetic mode of the cavity and the spin ensemble across a broad temperature range and for various crystal axis orientations relative to the applied magnetic field. The coupling strength, governed by thermal polarization of the spins and quantum mechanical transition probabilities, shows quantitative agreement with theoretical predictions based on spin Hamiltonian diagonalization and electromagnetic simulations of the cavity mode. These findings establish impurity spins in solids as controllable multilevel quantum systems and provide critical insights into the factors influencing spin-cavity interactions. This work opens new avenues for optimizing quantum information processing and developing versatile quantum technologies based on high-spin systems. Impurity spins in solids are controllable multilevel quantum systems that can be harnessed for implementing quantum memories. In this work, the authors investigate the coupling strength between spins in a ruby crystal and a quantized electromagnetic mode using electron spin resonance technique.