AQP Seminar: Progress in Josephson Junctions for Classical and Quantum Applications

Progress in Josephson Junctions for Classical and Quantum Applications

Martin Weides (University of Glasgow)

Abstract Single-wall carbon nanotubes (SWCNTs) are an attractive material for quantum dots simply because of their small diameter, so that the energy scales can be larger compared with those fabricated by top-down (lithography) technologies. They can be metallic or semiconducting, depending on how they are rolled up from a graphene sheet. Both electrical and optical applications are possible, but the fabrication and processing difficulties hinder reliable and reproducible realization of potential quantum devices.

We are intereAbstract: Superconducting Josephson junctions form the foundation of both classical superconducting electronics and leading quantum computing platforms. Over the past two decades, superconducting qubits have achieved remarkable progress in coherence, control, and scalability, enabling increasingly complex quantum processors. However, a fundamental limitation remains: the requirement for deep cryogenic operation, which constrains system complexity, cost, and practical scalability.

A promising route to overcoming these limitations lies in the use of high-gap superconductors and advanced multilayer device architectures. In particular, niobium-based technologies offer intrinsic advantages due to their larger superconducting gap, enabling operation at higher characteristic frequencies and providing greater resilience against thermal excitations. These properties open opportunities for faster qubit operation, improved control bandwidth, and relaxed constraints on cryogenic infrastructure. At the same time, multilayer structures introduce additional degrees of freedom for engineering device properties through controlled interfaces, proximity effects, and tailored electromagnetic environments.

Recent experimental work has highlighted the importance of multilayer superconducting systems in shaping Josephson junction behavior, particularly in nanoscale geometries where interface effects and material composition play a dominant role. However, the design and optimization of such devices remain challenging. Conventional modeling approaches typically rely on simplified geometries or limited material descriptions, restricting their ability to capture the full complexity of realistic multilayer architectures.

In this work, we address this challenge by combining a three-dimensional numerical modeling framework with experimental investigations of multilayer superconducting devices. This approach enables quantitative prediction of key junction properties while maintaining direct relevance to fabricated structures. Building on this capability, we demonstrate trilayer niobium-based superconducting qubits exhibiting coherent operation at elevated temperatures, highlighting the potential of multilayer, high-gap superconducting platforms to move beyond conventional operating regimes.

References:

• Classical interfaces for controlling cryogenic quantum computing technologies, Brennan et al., APL Quantum 2, 041501 (2025)
• From macroscopic quantum circuits to scalable quantum systems, Zhao et al., Europhysics News 57 (1), 12-15 (2026)
• Modeling realistic multilayer devices for superconducting quantum electronic circuits, Colletta et al., Appl. Phys. Lett. 126, 142601 (2025)
• Gap Engineered Superconducting Multilayer Nanobridge Josephson Junctions, Colletta et al.,arXiv:2603.20757 (2026)sted in excitons in the SWCNT quantum dot. Our quantum dot is a short nanotube in which both ends are terminated by molecules. We show that potential profile in the nanotube can be modified by different chemical boings between the nanotube and the molecule, and demonstrate formation of the single and double quantum dots.

The semiconducting SWCNT quantum dot shows exciton emissions. Of our particular interest is to control the exciton states in the double quantum dots for the exciton-based quantum gate application. The single exciton in a dot forms a quantum bit (qubit), and its quantum state can be coherently controlled by the laser pulse. In the double quantum dots formed in a short SWCNT, an interaction between excitons in each dot makes the system to work as a conditional quantum gate. In my talk, formation of quantum dots, basic properties of the exciton emission and the quantum gate (CNOT) operation will be shown.

References

• Akira Hida and Koji Ishibashi, Appl. Phys. Express, 8, 045101 (2015)
• Akira Hida and Koji Ishibashi, ACS Photonics, 9, 3398-3403 (2022)