The anharmonicity of a transmon qubit, defined as the difference in energy level spacing, is a key design parameter. In transmons built from hybrid superconductor-semiconductor Josephson elements, the anharmonicity is tunable with gate voltages that control both the Josephson energy and the weak link transparency. In Sn-InAs nanowire transmons, we use two-tone microwave spectroscopy to extract anharmonicity ranging in absolute value from the transmon charging energy Ec to values smaller than Ec/10. This behavior contrasts with the predictions of the multi-channel short-junction model, which sets a lower limit on anharmonicity at Ec/4. Coherent operation of the qubit is still possible at the point of the lowest anharmonicity. These findings demonstrate the potential of quantum circuits that benefit from widely electrically tunable anharmonicity.
@article{arxiv.2603.26895,
title = {Tunable anharmonicity in Sn-InAs nanowire transmons beyond the short junction limit},
author = {Amrita Purkayastha and Amritesh Sharma and Param J. Patel and An-Hsi Chen and Connor P. Dempsey and Shreyas Asodekar and Subhayan Sinha and Maxime Tomasian and Mihir Pendharkar and Christopher J. Palmstrøm and Moïra Hocevar and Kun Zuo and Michael Hatridge and Sergey M. Frolov},
journal= {arXiv preprint arXiv:2603.26895},
year = {2026}
}