An open quantum systems picture of measurement-induced state transitions in fluxonium qubits (Part 2)

A key bottleneck in the readout fidelity of superconducting qubits arises from measurement-induced state transitions (MISTs), where photon occupation in the readout resonator elicits deleterious effects on qubit excitations in the computational subspace. These effects limit fast, high-fidelity readout and motivate a deeper examination of the underlying dynamics. This problem is better characterized in transmon qubits, where both diagnostic and mitigative strategies have been recently developed to avoid this phenomenon when possible. However, these techniques are not easily generalizable to other artificial atoms such as the fluxonium, where strong anharmonicity and all-to-all matrix elements can lead to complex multi-level dynamics. In this work, we develop a more comprehensive open systems model to understand MISTs in fluxonium qubits, which characterizes new features not captured by existing methods. We experimentally explore flux-dependent regimes of MISTs in fluxonium and examine how conventional theoretical predictions hold. We compare measured behavior with this theoretical model to identify trends and regimes of agreement. The results provide insight into pathways toward more robust, high-fidelity readout in complex superconducting qubits. (Part 2 of 2)