Magneto-optical study of Nb thin films for superconducting qubits

TL;DR

This study uses magneto-optical imaging to analyze niobium thin films for superconducting qubits, revealing how film inhomogeneities and interfacial layers impact flux behavior and qubit coherence.

Contribution

It provides a detailed characterization of flux penetration and critical current density in Nb films, highlighting the influence of fabrication conditions and interlayers on qubit performance.

Findings

Flux penetration varies from ideal to dendritic regimes.

Nb/Si interlayer affects decoherence and must be optimized.

Critical current density correlates with film properties and qubit quality.

Abstract

Among the recognized sources of decoherence in superconducting qubits, the spatial inhomogeneity of the superconducting state and the possible presence of magnetic-flux vortices remain comparatively underexplored. Niobium is commonly used as a structural material in transmon qubits that host Josephson junctions, and excess dissipation anywhere in the transmon can become a bottleneck that limits overall quantum performance. The metal/substrate interfacial layer may simultaneously host pair-breaking loss channels (e.g., two-level systems, TLS) and control thermal transport, thereby affecting dissipation and temperature stability. Here, we use quantitative magneto-optical imaging of the magnetic-flux distribution to characterize the homogeneity of the superconducting state and the critical current density, $j_{c}$, in niobium films fabricated under different sputtering conditions. The imaging reveals distinct flux-penetration regimes, ranging from a nearly ideal Bean critical state to strongly nonuniform thermo-magnetic dendritic avalanches. By fitting the measured magnetic-induction profiles, we extract $j_{c}$ and correlate it with film physical properties and with measured qubit internal quality factors. Our results indicate that the Nb/Si interlayer can be a significant contributor to decoherence and should be considered an important factor that must be optimized.