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

TL;DR

This paper demonstrates ultrastrong light-matter coupling in a hybrid semiconductor-superconductor qubit system, enabling coherent control and revealing quantum phenomena beyond traditional models, thus opening new avenues for quantum technology.

Contribution

It reports the first realization of USC in a gate-tunable transmon qubit with coherent control, surpassing the limitations of the Jaynes-Cummings model.

Findings

Observation of photon-number-dependent transitions deviating from JC model

Coherent control of qubit in USC regime with comparable coherence times

Spectroscopic evidence of avoided crossing indicating USC

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.