Shape optimization of superconducting transmon qubits for low surface dielectric loss

Surface dielectric loss from superconducting transmon qubits is believed to be one of the dominant sources of decoherence. Reducing surface dielectric loss is crucial for achieving a high quality factor and a long relaxation time ( T 1 ), which can be engineered by carefully changing the geometry of a transmon. In this paper, we present a shape-optimization approach to reduce surface dielectric loss in a transmon qubit. The capacitor pads and junction wires of transmons are shaped as spline curves and optimized through the finite-element method and a global optimization algorithm. We find that the participation ratio of capacitor pads and junction wires can be reduced by 15 % − 18 % and 22 % − 26 % compared to previous designs, while the overall footprint and anharmonicity are maintained at acceptable values. As a result, the two-level system-limited quality factor and the corresponding T 1 were increased by 21.6%.