Localized quasiparticles in a fluxonium with quasi-two-dimensional amorphous kinetic inductors

Disordered superconducting materials with high kinetic inductance are an important resource to generate nonlinearity in quantum circuits and create high-impedance environments. In thin films fabricated from these materials, the combination of disorder and the low effective dimensionality leads to increased order parameter fluctuations and enhanced kinetic inductance values. Among the challenges of harnessing these compounds in coherent devices are their proximity to the superconductor-insulator phase transition, the presence of broken Cooper pairs, and the two-level systems located in the disordered structure. In this work, we fabricate tungsten silicide wires from quasi-two-dimensional films with one spatial dimension smaller than the superconducting coherence length and embed them into microwave resonators and fluxonium qubits, where the kinetic inductance provides the inductive part of the circuits. We study the dependence of loss on the frequency, disorder, and geometry of the device, and find that the loss increases with the level of disorder and is dominated by the localized quasiparticles trapped in the spatial variations of the superconducting gap.