Ultrastrong Coupling and Coherent Dynamics in a Gate-Tunable Transmon Qubit

I. Casal Iglesias; F. J. Matute-Cañadas; G. O. Steffensen; A. Ibabe; L. Splitthoff; T. Kanne; J. Nygard; V. Rollano; D. Granados; A. Gomez; R. Aguado; A. Levy Yeyati; E. J. H. Lee

arXiv · 2603.19438

Ultrastrong Coupling and Coherent Dynamics in a Gate-Tunable Transmon Qubit

I. Casal Iglesias·F. J. Matute-Cañadas·G. O. Steffensen·A. Ibabe·L. Splitthoff·T. Kanne·J. Nygard·V. Rollano·D. Granados·A. Gomez·R. Aguado·A. Levy Yeyati·E. J. H. Lee

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Abstract

Ultrastrong light-matter coupling (USC) gives access to exotic quantum phenomena and promises faster quantum gates, yet coherent time-domain control in this regime remains largely unexplored. Here, we realize USC in a hybrid system consisting of an InAs nanowire-based gatemon qubit coupled to a superconducting resonator. Spectroscopy reveals an avoided crossing that cannot be captured by the Jaynes-Cummings (JC) model, as well as photon-number-dependent transitions whose energies deviate markedly from the JC ladder expected in the strong coupling regime. Beyond demonstrating USC, we achieve time-resolved coherent control of the qubit and measure coherence times comparable to gatemons operating outside the USC regime. These results establish that hybrid semiconductor-superconductor qubits can retain coherent control in USC and provide a platform for exploring quantum dynamics and device concepts in this regime.

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I. Casal Iglesias, F. J. Matute-Cañadas, G. O. Steffensen, A. Ibabe, L. Splitthoff, T. Kanne, J. Nygard, V. Rollano, D. Granados, A. Gomez, R. Aguado, A. Levy Yeyati, E. J. H. Lee. 2026-03-19. Ultrastrong Coupling and Coherent Dynamics in a Gate-Tunable Transmon Qubit. https://arxiv.org/abs/2603.19438

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