Date of Award

Spring 2026

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical & Computer Engineering

Committee Director

Hani E. Elsayed-Ali

Committee Member

Linda Vahala

Committee Member

Grigory V. Eremeev

Abstract

Niobium (Nb) films play a central role in superconducting technologies used in particle accelerators and superconducting quantum circuits. Optimizing the physical properties of Nb films is therefore critical for improving both radiofrequency (RF) performance in superconducting radiofrequency (SRF) cavities and coherence in superconducting qubits. This thesis investigates the relationship between Nb film microstructure, impurity content, and electromagnetic response across these two application domains.

For particle accelerator applications, we studied Nb films deposited using high-power impulse magnetron sputtering (HiPIMS) with DC bias onto a 1.3 GHz elliptical SRF cavity. Nb film cavities exhibit a pronounced medium-field Q-slope, limiting their achievable accelerating gradients. To address this issue, a series of in-situ and vacuum furnace annealing treatments were performed to modify the film microstructure and impurity distribution. These treatments significantly improved RF performance, increasing the quench field from 10.0 MV/m to 17.5 MV/m and reducing the field dependence of the surface resistance (Rs). Material characterization and RF measurements indicate that these improvements arise from reduced impurity concentration, relaxation of lattice strain, and suppression of defect-related losses.

To investigate the intrinsic microwave dissipation of Nb films relevant for superconducting quantum circuits, we performed measurements using a three-dimensional SRF resonator coated with a DC biased HiPIMS Nb film and operated in the quantum regime at millikelvin temperatures. This configuration isolates the Nb film from other circuit elements and allows direct evaluation of its microwave losses. The measurements show that Nb films exhibit microwave dissipation comparable to that of bulk Nb cavities. These results indicate that the metallic Nb film itself is not the dominant limitation in present two-dimensional superconducting qubit architectures and highlight the significant role of dielectric interfaces, particularly native oxides, in limiting qubit coherence.

Finally, we investigate the impact of magnetic flux, which is generally believed to be detrimental for transmon qubits, on coherence times and their stability in Ta-capped Nb transmon qubits. Controlled magnetic field cooling experiments combined with wide-field nitrogen-vacancy (NV) diamond magnetometry reveal that moderate trapped magnetic fields (400-600 mG) suppress temporal fluctuations in the qubit energy relaxation time (T1) without significantly reducing the mean coherence time. In this regime, isolated trapped vortices do not significantly degrade T1. Instead, they can act as quasiparticle traps and suppress low-frequency noise through the polarization of paramagnetic defects and partial saturation of fluctuating magnetic two-level systems (TLS).

At larger magnetic fields, coherence degrades abruptly as vortices reorganize into spatially correlated vortex structures across the capacitor pads. Analysis of the vortex morphology shows that the onset of dissipation is governed by vortex correlation rather than vortex density, identifying the topology of trapped flux as the key factor controlling microwave loss in superconducting qubits. These results suggest that optimizing Nb film properties to prevent the formation of spatially correlated vortex structures while maintaining isolated vortices could enable higher trapped flux densities while preserving stable qubit coherence.

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ISBN

9798197809308

ORCID

0009-0007-3160-3575

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