Shape optimization of superconducting transmon qubit for low surface dielectric loss

TL;DR

This paper introduces a shape optimization method for superconducting transmon qubits, reducing surface dielectric loss and enhancing qubit coherence by optimizing geometry with finite-element and global algorithms.

Contribution

The study presents a novel shape optimization approach for transmon qubits, significantly reducing dielectric loss and improving quality factors while maintaining device footprint and anharmonicity.

Findings

Surface participation ratio of capacitor pad reduced by 16%

Junction wire participation ratio reduced by 26%

Quality factor and relaxation time increased by approximately 21.6%

Abstract

Surface dielectric loss of superconducting transmon qubit is believed as one of the dominant sources of decoherence. Reducing surface dielectric loss of superconducting qubit is known to be a great challenge for achieving high quality factor and a long relaxation time ($T_{1}$). Changing the geometry of capacitor pads and junction wire of transmon qubit makes it possible to engineer the surface dielectric loss. In this paper, we present the shape optimization approach for reducing Surface dielectric loss in transmon qubit. The capacitor pad and junction wire of the transmon qubit are shaped as spline curves and optimized through the combination of the finite-element method and global optimization algorithm. Then, we compared the surface participation ratio, which represents the portion of electric energy stored in each dielectric layer and proportional to two-level system (TLS) loss, of optimized structure and existing geometries to show the effectiveness of our approach. The result suggests that the participation ratio of capacitor pad, and junction wire can be reduced by 16% and 26% compared to previous designs through shape optimization, while overall footprint and anharmonicity maintain acceptable value. As a result, the TLS-limited quality factor and corresponding $T_{1}$ were increased by approximately 21.6%.