Experimental signatures of quantum phase dynamics in gatemons

Unlike conventional transmons employing superconductor–insulator–superconductor junctions, gatemons enable in situ electrostatic tuning of junction properties via a gate electrode. Previous models, which attribute the Josephson energy solely to Andreev bound states, predict a limited tunability of the qubit anharmonicity. However, in recent experiments (see e.g. Ref.[1]), anharmonicity variations that exceed these theoretical bounds have been observed.

It has recently been proposed that accounting for the static effects of the continuum states can explain the discrepancy [2]. In this work, we extend this analysis to include the effects that originate from the quantum dynamics of the superconducting phase, based on the recent many-body treatment of gatemon [3], which predicts (i) a renormalization of the junction's effective capacitance and (ii) the presence of two additional gate voltage–dependent charge offsets in junctions with tunneling asymmetry. We quantify the impact of these effects on the energy spectrum and anharmonicity of the qubit, and propose a protocol to experimentally detect the predicted charge offsets by measuring the gate dependence of the qubit charge dispersion (amplitude and shifts) across different junction voltages.

[1] Z. Hao et al., Appl. Phys. Lett. 124, 254003 (2024)

[2]  V. Fatemi et al., SciPost Phys. 18, 091 (2025)

[3] U. Güngördü et al., Phys. Rev. B 111, 214503 (2025)