Publication date: 15th December 2025
Superconducting quantum circuits are a versatile platform to study light-matter interaction and the Quantum Rabi model. The freedom in qubit and resonator designs and coupling engineering allows to study regimes of interaction beyond the bare frequencies of the system, entering the so-called ultrastrong coupling (USC) regime (0.1 < g/wr < 1) [1].
Non-perturbative ultrastrong couplings (0.3 < g/wr < 1) between a flux qubit and a resonator have been typically achieved using shared Josephson junctions. Recently, granular aluminum has been proposed as an alternative to reach ultrastrong couplings while keeping small persistent currents, and small qubit perimeters, which is advantageous for qubit coherence [2]. In this work, we present measurements of a flux qubit galvanically coupled to an LC oscillator by a shared wire of granular aluminum. The coupling is estimated to fall in the non-perturbative USC regime with g/wr ~0.3 and a persistent current below 40nA. This new approach opens the door to new designs with novel superinductors and the possibility to reach high coherence in ultrastrongly coupled flux qubit-resonator devices.
We would like to thank J. J. Garcia-Ripoll, L. Magazzu, and M. Grifoni for the fruitful discussions. We acknowledge funding from the Ministry of Economy and Competitiveness and Agencia Estatal de Investigacion (Nos. RYC2019-028482-I, PCI2019-111838-2, PID2021-122140NB-C31, and PCI2024-153468), the European Commission (FET-Open AVaQus GA 899561, QuantERA), and program“Doctorat Industrial” of the Agency for Management of University and Research Grants (2020 DI 41; 2024 DI 00004). IFAE is partially funded by the CERCA program of the Generalitat de Catalunya. This study was supported by MICIN with funding from European Union NextGenerationEU (No. PRTR-C17.I1) and by Generalitat de Catalunya. This work was (partially) supported by the Catalonia Quantum Academy, part of the Quantica- Mediterranean Valley of Quantum Science and Technologies initiative of the Generalitat of Catalunya.
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