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.
@article{arxiv.2603.19438,
title = {Ultrastrong Coupling and Coherent Dynamics in a Gate-Tunable Transmon Qubit},
author = {I. Casal Iglesias and F. J. Matute-Cañadas and G. O. Steffensen and A. Ibabe and L. Splitthoff and T. Kanne and J. Nygard and V. Rollano and D. Granados and A. Gomez and R. Aguado and A. Levy Yeyati and E. J. H. Lee},
journal= {arXiv preprint arXiv:2603.19438},
year = {2026}
}