We present a theory of a flux-tunable superconducting qubit, the "Fraunhofer qubit," based on the Fraunhofer interference in a wide ballistic Josephson junction. As magnetic flux threads the junction, the Josephson potential is effectively averaged over a phase window proportional to flux. For perfectly transmitting junctions, as flux approaches one flux quantum h/2e, the flux averaging transforms the potential near its minimum from a quadratic to a triangular shape, resulting in significantly enhanced anharmonicity. This enhancement persists for junctions with lower transparency conducting channels. Microscopic tight-binding simulations that include inhomogeneous electrostatic potential and disorder confirm the enhancement of anharmonicity. These results establish a framework for flux control in hybrid superconducting circuits, providing an operating point where anharmonicity and charge-noise protection can be optimally balanced.
@article{arxiv.2603.06830,
title = {Anharmonicity and Charge-Noise Sensitivity of Fraunhofer Qubit},
author = {Longyu Ma and Tony Liu and Javad Shabani and Kasra Sardashti and Vladimir E. Manucharyan and Maxim G. Vavilov},
journal= {arXiv preprint arXiv:2603.06830},
year = {2026}
}