Observation of enhanced coherence in Josephson SQUID cavities using a hybrid fabrication approach

We study the coherence of flux-tunable Josephson junction resonators made with two different fabrication processes. In the first process, devices are made using a single step of evaporation in which the resonator and the junctions of the SQUID are made at the same time. In the second process, devices are made with an identical geometry, but in which the resonators are made from a MoRe superconding layer to which an the junctions are added later in a second step. To characterize the coherence of the two types of SQUID cavities, we observe and analyze the quality factor of their resonances as a function of flux and photon number. Despite a detailed cleaning process applied during fabrication, the single-step Al devices show significantly worse quality factor than the hybrid devices, and conclude that a the hybrid technique provides a much more reliable approach for fabricating high-Q flux-tunable resonators.