Tunable anharmonicity beyond short junction limit in hybrid Sn-InAs nanowire transmons

Superconducting quantum devices typically use Josephson junctions as nonlinear inductors to create anharmonic oscillators, with strong anharmonicity employed in qubits and weak anharmonicity in wave-mixing processes. For transmon qubits employing conventional superconducting tunnel junctions, the anharmonicity is set by the capacitive energy (Ec) of the designed circuit. However, with hybrid superconducting semiconducting Josephson junctions, the anharmonicity is tunable via electrostatic gate allowing it to deviate from the designed Ec value. In this work, we investigate anharmonicity of transmon qubits utilizing Sn-InAs nanowire Josephson junctions. Our experimental results demonstrate non-monotonic gate variation of anharmonicity. While capped by the designed E_c, we observe significant reduction in anharmonicity surpassing the E_c/4 limit of the short junction model. These findings suggest necessary corrections to the Josephson potential typically used to describe hybrid junctions in superconducting circuits considering the finite length of the junction.