Chew, Henry;
(2025)
Nonlinear Capacitors for Superconducting Quantum Device Applications.
Doctoral thesis (Ph.D), UCL (University College London).
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Abstract
The key components in superconducting quantum computing architectures are qubits, couplers, microwave elements and low-noise amplifiers among others. In a variety of these subsystems, nonlinear circuit elements are an essential ingredient which enables their functionality. In this thesis, we study two different superconducting elements used to implement nonlinear capacitors. First we simulated a novel Josephson-junction-based tuneable nonlinear capacitor to couple flux qubits. Such a device implements a YY interaction, which has the potential to enhance the performance and extend the capabilities of current generation quantum annealers. We show that the interaction strength of the YY coupling is on the order of MHz and tuneable across zero. We discuss experimental realisations of this device to verify its function. Next, we present our study of a low temperature parametric amplifier utilizing the nonlinear capacitance of quantum phase-slip nanowires. We have fabricated a single-nanowire and two-nanowire variants which shunt a quarter-wavelength superconducting coplanar waveguide resonator. We characterised the nonlinear properties of the first variant such as the power-dependent resonance frequency, idler generation and signal gain, while in the second variant we developed a lock-in detection method to investigate the gate voltage response and demonstrate that the device acts as a gate-voltage-tuneable resonator. We also report on the progress towards a fabrication process for sputtered Al overlap junctions in the LCN cleanroom. Back-sputter and in-situ oxide growth techniques are developed as an alternative to shadow evaporation. These techniques are better suited for large-scale integration as they do not rely on angular deposition, but they require better control over the interfaces due to patterning the two electrodes separately. We characterised the processes using resistance measurements and characterised their low temperature properties. We show that our junctions exhibit overdamped dynamics and the critical current is suppressed with RF radiation.
| Type: | Thesis (Doctoral) |
|---|---|
| Qualification: | Ph.D |
| Title: | Nonlinear Capacitors for Superconducting Quantum Device Applications |
| Open access status: | An open access version is available from UCL Discovery |
| Language: | English |
| Additional information: | Copyright © The Author 2025. Original content in this thesis is licensed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) Licence (https://creativecommons.org/licenses/by-nc/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request. |
| UCL classification: | UCL UCL > Provost and Vice Provost Offices > UCL BEAMS UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Electronic and Electrical Eng |
| URI: | https://discovery.ucl.ac.uk/id/eprint/10215670 |
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