A gate tunable transmon in the ultrastrong coupling regime

The ultrastrong coupling regime of light–matter interaction, where the coupling rate becomes a sizable fraction of the resonator frequency, opens access to quantum electrodynamics beyond the rotating-wave approximation. While this regime has been studied in different superconducting circuits, its realization in hybrid semiconductor–superconductor systems remains largely unexplored. This work presents the experimental investigation of a gatemon qubit based on a hybrid InAs-Al nanowire in the ultrastrong coupling limit. From the measurements, we estimate a vacuum Rabi splitting of g₀/h = 648 MHz, corresponding to a ratio g₀/f_r > 0.16. Further evidence of the ultrastrong coupling is obtained by means of spectroscopic measurements that probe the qubit transition for different photon occupancies of the resonator, which reveal a unique spectral signature of this regime.