The study is authored by Alba Torras, Luca Cozzolino, Ariadna Gómez-del-Pulgar, Elia Bertoldo, and Pol Forn-Díaz, and will be submitted for publication.

The paper presents the design and characterization of a flux qubit coupled to a superconducting resonator through a novel coupling element made of granular aluminium, a material distinct from the standard aluminium typically used in superconducting circuits. This architecture enables operation in the ultrastrong coupling regime and is implemented in a fully in-house fabricated chip, developed at IFAE in collaboration with CNM and ICN2.

This work represents the first publication entirely led and executed by the QCT group at IFAE using locally fabricated devices, and constitutes a key outcome of the European Quantera SiUCs project. It also forms the core of Alba Torras Coloma’s PhD thesis, scheduled for defense in November, and highlights IFAE’s capacity to carry out end-to-end fundamental research in superconducting quantum circuits with direct relevance for the development of emerging quantum technologies.