Magneto-optical study of Nb thin films for superconducting qubits

TL;DR

Problem: decoherence in Nb transmon qubits arises from spatial inhomogeneity of the superconducting state and possible magnetic-flux vortices, with the Nb/Si interfacial layer identified as a potential loss channel. Approach: the authors apply quantitative magneto-optical imaging to Nb films prepared under different sputtering conditions, extract the critical current density $j_c$, and correlate with microstructure and measured internal quality factors $Q_i$. Findings: flux penetration exhibits both Bean-like and thermo-magnetic dendritic regimes; higher $j_c$ and larger grains generally improve field screening, but dendritic activity reveals compromised heat transfer in some samples; the balanced Sample C shows the best quantum performance. Significance: the MO-imaging-based feedback can guide targeted optimization of Nb/Si interfaces, grain structure, and pinning to reduce decoherence in superconducting qubits.

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.

Figures (5)

• Figure 1: (Color online) Magneto-optical Faraday images of Nb thin films showing magnetic flux penetration in sample A-C. Color intensity is proportional to the magnetic induction on the film surface. The upper two rows are magnetic field applied in zero-field cooled state at 4 K and the bottom-most row is showing field cooled images with 320 Oe of applied field at 4 K. Sample B and Sample C exhibit the presence of thermo-magnetic dendritic avalanches even at an applied field of as low as 40 Oe.
• Figure 2: (Color online) Magneto-optical Faraday images of Nb thin films showing magnetic flux penetration in sample A-C. Color intensity is proportional to the magnetic induction on the film surface. Upper two rows are magnetic field applied in zero-field cooled state at 6 K. Sample B still exhibits the presence thermo-magnetic dendritic avalanches even in zero-field cooled state. This tells about higher flux pinning but poor thermal coupling in sample B as compared to other films as discussed in text.
• Figure 3: Magneto-optical Faraday images of Nb thin films showing trapped flux following Bean's model of flux trapping in field-cooled condition when 320 Oe of magnetic field was applied during the process. The films were field cooled to the temperature of 6 K as at higher temperatures the films are free of dendritic avalanches and exhibit trapped flux with sharp contrast. Bottom row contains fitting of Eq. with magnetic induction, calculated from line intensity profile across the film and later converted to magnetic induction, for the estimation of critical current density, $j_{c}$.
• Figure 4: The estimated critical current density, residual resistivity ratio, $RRR$, and the average grain size.
• Figure 5: Side-by-side comparison of estimated critical current densities with low and high power quality factor.