José Alberto Nava Aquino

Topic

Theory of Charge and Flux Noise in Superconducting Wires

Department of Physics and Astronomy

Date & location

• Tuesday, August 20, 2024
• 1:00 P.M.
• Clearihue Building, Room B017

Examining Committee

Supervisory Committee

• Dr. Rogério de Sousa, Department of Physics and Astronomy, University of Victoria (Supervisor)
• Dr. Pavel Kovtun, Department of Physics and Astronomy, UVic (Member)
• Dr. Mohammad Amin, Chief Scientist, D-Wave Systems (Outside Member)

External Examiner

• Dr. Bill Coish, Department of Physics, McGill University

Chair of Oral Examination

• Dr. Michelle Wiebe, Department of Curriculum and Instruction, UVic

Abstract

Superconducting qubits are at the forefront of efforts to develop scalable quantum computers due to their potential to perform complex computations beyond the capabilities of classical systems. However, maintaining the quantum coherence of these qubits remains a significant challenge, primarily due to various noise sources such as flux noise, dielectric loss, and quasiparticle poisoning. This dissertation presents a detailed theoretical investigation into two noise mechanisms affecting superconducting qubits: flux noise from spin impurities and charge/flux noise from non-equilibrium superconducting quasiparticle distributions.

The first part of the research focuses on developing a general theoretical framework to calculate flux noise arising from spin impurities. This framework accounts for spin diffusion and spin-lattice relaxation, incorporating a discrete diffusion model to handle confinement effects and inhomogeneities. Analytical and numerical results show that the spin relaxation model aligns with experimental observations in aluminum devices, while the spin diffusion model better matches experiments in niobium devices.

The second part of the thesis proposes a theory addressing charge and flux noise due to non-equilibrium superconducting quasiparticle distributions within superconducting wires. This theory highlights the significant impact of ohmic loss generated by these quasiparticles, revealing their contribution to charge noise at intermediate frequencies and a nearly white flux noise background.

Comparative analysis with experimental data provides some validation for the theoretical models and gives insights into the temperature-dependent behavior of flux noise and the distinctive noise characteristics in aluminum and niobium devices. The findings highlight the necessity of addressing wire-resident quasiparticles and magnetic impurities to enhance the performance and scalability of superconducting qubits.