Quasiparticle dynamics in microwave resonators from granular aluminum close to the superconductor to insulator transition

Superconducting high kinetic inductance elements constitute a valuable resource for quantum circuit design and millimeter-wave detection. Granular aluminum (GrAl) is a particularly interesting material since it has already shown a kinetic inductance in the range of nH/square and its deposition is compatible with conventional Al/AlOx/Al Josephson junction fabrication. We characterize microstrip resonators in the GHz range fabricated from GrAl thin films with resistivity and geometry dependent kinetic inductances between 0.3 and 1.7 nH/square. Our results suggest that non-equilibrium quasiparticles limit their internal qualitiy factors at a level of 10⁵. We extract quasiparticle relaxation times on the order of 1 s and we observe quasiparticle producing events approximately every 30 s. The current level of coherence of GrAl resonators makes them attractive for integration in quantum devices, while it also evidences the need to reduce the density of non-equilibrium quasiparticles.