Gate-dependent offset charge shifts and anharmonicity in gatemon qubits in the weak tunneling regime

TL;DR

This paper investigates how gate-dependent charge offsets and anharmonicity affect gatemon qubits, providing theoretical predictions and proposing experimental detection methods for these effects in weak tunneling regimes.

Contribution

It extends previous many-body models to quantify observable impacts of charge offsets and anharmonicity on gatemon qubit spectra and proposes experimental detection protocols.

Findings

Charge offsets depend on gate voltage and junction asymmetry.

Anharmonicity varies with dot-gate voltages and junction transparency.

Theoretical predictions relate to simplified gatemon models.

Abstract

Gatemon qubits are based on a superconductor-quantum dot-superconductor (S-QD-S) junction which enables in situ electrostatic tuning via a gate electrode. For a single-channel QD this structure gives rise to two subgap Andreev bound states (ABSs), and generally leads to a richer quantum phase dynamics as compared to conventional transmons. In a recent work [Phys. Rev. B 111, 214503 (2025)] we derived the quantum phase dynamics from a many-body treatment which leads to an effective gate voltage-dependent Hamiltonian that self-consistently incorporates the phase quantization. It predicts (i) a renormalization of the junction's effective capacitance and (ii) the presence of gate voltage and occupation-dependent charge offsets in junctions with tunneling asymmetry. Here, we quantify the observable impact of these effects on the qubit's energy spectrum and anharmonicity, by studying the interplay of the two Andreev branches as a function of dot-gate voltages and junction transparencies. We show the relation of these predictions to simplified gatemon models and propose a protocol to experimentally detect the predicted charge offsets.