Hybrid quantum systems with high-T$$_c$$ superconducting resonators

Superconducting microwave resonators are crucial elements of microwave circuits, offering a wide range of potential applications in modern science and technology. While conventional low-T$$_c$$ c superconductors are mainly employed, high-T$$_c$$ c cuprates could offer enhanced temperature and magnetic field operating ranges. Here, we report the realization of $$\textrm{YBa}_2\textrm{Cu}_3\textrm{O}_{7-\delta }$$ YBa 2 Cu 3 O 7 - δ superconducting coplanar waveguide resonators, and demonstrate a continuous evolution from a lossy undercoupled regime, to a lossless overcoupled regime by adjusting the device geometry, in good agreement with circuit model theory. A high-quality factor resonator was then used to perform electron spin resonance measurements on a molecular spin ensemble across a temperature range spanning two decades. We observe spin-cavity hybridization indicating coherent coupling between the microwave field and the spins in a highly cooperative regime. The temperature dependence of the Rabi splitting and the spin relaxation time point toward an antiferromagnetic coupling of the spins below 2 K. Our findings indicate that high-Tc superconducting resonators hold great promise for the development of functional circuits. Additionally, they suggest novel approaches for achieving hybrid quantum systems based on high-T$$_c$$ c superconductors and for conducting electron spin resonance measurements over a wide range of magnetic fields and temperatures.