This PhD project aims to revolutionize QC by advancing the qubit fabrication, with a specific focus on Transmon qubits. The research centres on developing high-quality Nb/AlOx/Nb tunnel barriers using a novel trilayer process pioneered at UofG and based on work from Uof Chicago, Phys. Rev. Applied 21, 024047 (’24). It eliminates the need for dielectric spacers, resulting in significant improvements in qubit coherence and operational efficiency (e.g. operation at higher temperatures). It has both academic and industrial impact, culminating in the fabrication of a four-qubit Transmon qubit chip by the end of the project.

Key Research Objectives:

1. Junction Quality and Stability at Scale: Fabricate qubit junctions with exceptionally low defect densities. These junctions will be rigorously characterized for variability, reproducibility, and two-level system (TLS) densities. These trilayers junctions with reduced TLS are also essential for advanced classical superconducting electronics. Seamlessly integrated into coherent qubits for superior qubit stability and performance.
2. Scalable Fabrication: Scalable fabrication techniques suitable for 100mm wafers, potentially also 150mm wafers, giving access to increased design variations, managing process drifts, and minimizing yield losses, which are important for production.
3. Tunnel Barrier Optimization: Optimization of deposition conditions and patterning techniques to ensure the production of junctions with precisely controlled properties, critical for enhancing qubit performance and enabling large-scale manufacturing.

Characterization and Analysis:

1. Advanced Characterization Techniques: Detailed transport measurements will be conducted to analyse the structural and electrical properties. Identify the dynamics of oxide formation and assess its impact on junction performance.
2. Characterization via Qubits: Characterize operational qubits, assessment of coherence, gate fidelities, and other performance metrics. Provide direct feedback on materials influence on performance, refined understanding of junction quality and qubit behaviour.
3. Josephson Junction Resonator Spectroscopy: Assess the impact of tunnel oxide growth on resonator losses, with a focus on TLS densities and phase noise. Coherence evaluations links materials with fabrication, insights into factors that influence qubit stability and efficiency.
4. Transmission Electron Microscopy (TEM): Utilize the Kelvin Nanocharacterisation Centre (KNC) at UofG for high-resolution TEM imaging for interface and surface studies.

1Q, 2Q, and Randomized Benchmarking: The qubits will be characterized at the NQCC, focusing on single-qubit (1Q) and two-qubit (2Q) operations, as well as randomized benchmarking. Comparisons with other quantum chips, such as those from the NQCC’s quantum testbeds or NPL, to evaluate the relative performance.

Commercialization Strategy: The project will collaborate with a new startup, KNT and the key industrial partners to translate the technological advancements into commercial products. Focus on providing foundry services based on a superconducting Process Design Kit and offering bespoke chip designs, thereby accelerating the practical application of quantum technologies.

Impact: Make substantial contributions to both academic research and industrial applications. Academically, the advanced fabrication and characterization techniques will significantly deepen the understanding of qubit performance, leading to high-impact publications. On the industrial side, the scalability of the project and its focus on commercialization will drive the practical application of QC with superconducting circuits. Key industry partners, such as the NQCC, NPL, Oxford Instruments, SPTS/KLA, KNT, OQC and SeeQC are expected to benefit from these advancements.

By the end of the research period, we will fabricate and characterise a four-qubit Transmon qubit chip, advancing the next generation of scalable, stable QCs. More broadly, optimised trilayers are crucial for improving both classical and quantum superconducting electronics, highlighting the project’s broader significance in the technology landscape.

Research Goals: Optimize Nb/AlOx/Nb junctions by developing high-quality qubits with improved coherence times and reduced two-level system (TLS) densities. Scale the fabrication process to 100mm wafers, enabling the creation of 1Q, 2Q, and 4Q circuits and with high yield. The primary objective is to fabricate and optimize a fully functional four-qubit Transmon chip. Additionally, commercialize superconducting qubits by collaborating with KNT to develop a superconducting Process Design Kit (PDK) and translate the research into commercial products.

Key Milestones: In the first 12 months, focus on optimising trilayer process and fabricating single qubit circuits. Success will be measured by the successful fabrication and characterization. By the end of the 2nd year, scale fabrication to 100mm wafers and start working on two-qubit (2Q) designs. Success at this stage will be defined by the fabrication of 2Q qubits and the improvement of performance metrics such as coherence. In the 3rd year, shift to fabricating and benchmarking 1Q and 2Q systems, comparing their performance with other quantum testbeds. The success of this phase will be determined by achieving target (100+usec) coherence times and gate fidelities (99%) for the 1Q and 2Q systems. In the 4th year, demonstrate a four-qubit Transmon chip.

Success Measures: Meeting target coherence times and gate fidelities for 1Q, 2Q, and 4Q systems. Successful scaling to 100mm wafers with high yield and minimal defects. Commercial access through the collaboration with KNT and the development of a superconducting PDK, for the translation into marketable products. Further validation through high-impact publications and strong partnerships with NQCC and industry collaborators.