Unified picture of measurement-induced ionization in the transmon

Circuit quantum electrodynamics (cQED) has emerged as a powerful platform for quantum computation and for the investigation of quantum optics at microwave frequencies. A critical part of all cQED experiments is qubit readout, which relies on microwave drives. In principle, higher drive amplitudes should lead to faster and more accurate readout. However, experiments have consistently shown that as the drive amplitude increases, the readout quality rapidly deteriorates, something that severely limits qubit readout in the laboratory. We begin by reviewing the basics of qubit measurement in circuit QED, followed by presenting numerical simulations that capture the dynamics of the readout process. Our findings reveal signatures of 'qubit ionization', where the qubit is brought to highly excited states by the readout drive, leading to a breakdown of the measurement fidelity. Building on previous theoretical and experimental advances, we present a comprehensive theoretical framework providing a physical picture of the origin of transmon ionization, together with a set of tools which can readily be used to predict its occurrence. We further discuss how this phenomenon is not limited to qubit readout but also manifests itself in strongly driven nonlinear circuits across various settings. Finally, we compare our results with recent experimental data.