Fraunhofer Qubit

We present a theory of flux-tunable superconducting qubit based on the Fraunhofer interference of a single short 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 modifies the potential near its minimum from a quadratic to a triangular shape, resulting in enhanced anharmonicity. This enhancement persists for lower transparencies, where the flux averaging effectively modifies the curvature at the bottom of the potential well. Microscopic simulations confirm the enhancement of anharmonicity while maintaining exponential suppression of charge dispersion. Our design offers flux control in superconducting circuits and provides an operating point at which anharmonicity and charge noise protection can be optimally balanced.