The field of superconducting qubits is constantly evolving with new circuit designs. However, when it comes to qubit readout, the use of simple transverse linear coupling remains overwhelmingly prevalent. This standard readout scheme has significant drawbacks: in addition to the Purcell effect, it suffers from a limitation on the maximal number of photons in the readout mode, which restricts the signal-to-noise ratio (SNR) and the Quantum Non-Demolition (QND) nature of the readout. Here, we explore the high-power regime by engineering a nonlinear coupling between a transmon qubit and its readout mode. Our approach builds upon previous work by Dassonneville et al. [Physical Review X 10, 011045 (2020)], on qubit readout with a non-perturbative cross-Kerr coupling in a transmon molecule. We demonstrate a readout fidelity of 99.21% with 89 photons utilizing a parametric amplifier. At this elevated photon number, the QND nature remains high at 96.7%. Even with up to 300 photons, the QNDness is only reduced by a few percent. This is qualitatively explained by deriving a critical number of photons associated with the nonlinear coupling, yielding a theoretical value of nˉrcrit=377 photons for our sample's parameters. These results highlight the promising performance of the transmon molecule in the high-power regime, establishing it as a compelling platform for high-fidelity qubit readout.
@article{arxiv.2507.03642,
title = {High-power readout of a transmon qubit using a nonlinear coupling},
author = {Cyril Mori and Vladimir Milchakov and Francesca D'Esposito and Lucas Ruela and Shelender Kumar and Vishnu Narayanan Suresh and Waël Ardati and Dorian Nicolas and Giulio Cappelli and Arpit Ranadive and Gwenael Le Gal and Martina Esposito and Quentin Ficheux and Nicolas Roch and Tomás Ramos and Olivier Buisson},
journal= {arXiv preprint arXiv:2507.03642},
year = {2025}
}