Controlling the charge dispersion of a nearly-open superconducting island*

Isolation from the environment determines the extent to which charge is confined on an island, which manifests experimentally through Coulomb oscillations such as charge dispersion. In superconducting circuits, the link to the environment has typically been formed from tunnel junctions. If instead a transparent ballistic junction forms the link between the superconducting island and the environment, Coulomb oscillations are predicted to suppress far more rapidly than for tunnel junctions due to imaginary-time Landau-Zener tunneling. Here we investigate the charge dispersion of a nanowire transmon hosting a quantum dot in the junction. We observe rapid suppression of the charge dispersion consistent with the scaling law resulting from diabatic transitions between Andreev bound states. We also observe greatly improved qubit coherence times at the point of highest charge dispersion suppression. Our observations further our fundamental understanding of charging effects in superconductors and suggests novel approaches for building charge-insensitive qubits.