Dispersive readout plays a central role in superconducting quantum computing, enabling quantum nondemolition measurements through a coupled microwave resonator. While stronger resonator drives can improve measurement speed and fidelity, they can also activate multi-photon resonances that trigger measurement-induced state transitions (MIST) out of the computational subspace. We develop a driven-dissipative framework for MIST that retains the quantum resonator response absent in semiclassical treatments and yields analytically derived transition-rate expressions. The upward and downward transition-rate profiles determine the steady-state populations and finite-time dynamics, enabling quantitative exploration across drive strength and detuning, while capturing the crossover between quantum-resolved and semiclassical-like dynamics. This framework reproduces the quantum dynamics and identifies a strongly driven regime beyond semiclassical Landau--Zener predictions, where delayed population transfer opens a finite-time readout window with high resonator photon population. These results establish MIST as a predictive driven-dissipative process and characterize strongly driven regimes that can be leveraged for measurement optimization.
Dispersive readout plays a central role in superconducting quantum computing, enabling quantum nondemolition measurements through a coupled microwave resonator. While stronger resonator drives can improve measurement speed and fidelity, they can also activate multi-photon resonances that trigger measurement-induced state transitions (MIST) out of the computational subspace. We develop a driven-dissipative framework for MIST that retains the quantum resonator response absent in semiclassical treatments and yields analytically derived transition-rate expressions. The upward and downward transition-rate profiles determine the steady-state populations and finite-time dynamics, enabling quantitative exploration across drive strength and detuning, while capturing the crossover between quantum-resolved and semiclassical-like dynamics. This framework reproduces the quantum dynamics and identifies a strongly driven regime beyond semiclassical Landau--Zener predictions, where delayed population transfer opens a finite-time readout window with high resonator photon population. These results establish MIST as a predictive driven-dissipative process and characterize strongly driven regimes that can be leveraged for measurement optimization.
Superconducting qubits are most often measured using dispersive readout, which, ideally, implements a projective quantum non-demolition (QND) measurement. While a larger readout drive can increase the signal and, thus, reduce discrimination errors in the readout, strong microwave drives may also cause non-QND errors by driving the qubit to a state outside the computational subspace. In this work, we experimentally characterize measurement-induced state transitions (MIST) in a fluxonium qubit over its full external flux range. We further numerically calculate the MIST errors, and find that the theory accurately predicts eleven experimentally identified regions with increased MIST. In addition to transitions to higher fluxonium levels, we also find that, at certain flux points, MIST errors are dominated by transitions that include the transmission-line-like array modes of the fluxonium's superinductor. The excellent match between theory and experiment validates that the models accurately predict the occurrence of MIST in these systems, and further highlights the influence of array modes in fluxonium readout.
Fast and high-fidelity qubit measurement plays a key role in quantum error correction. In superconducting qubits, measurement is typically performed using a resonant microwave drive on a readout resonator dispersively coupled to the qubit. Shorter measurement times require larger numbers of photons populating the readout resonator, which ultimately leads to undesired measurementinduced state transitions (MIST) of the qubit. MIST can be particularly problematic because these transitions often leave the qubit in a high energy state, and the MIST locations in readout parameter space drift as a function of qubit offset charge. In transmon qubits, these drifts have been avoided using very large qubit-resonator detunings or dedicated offset charge biases. In this work, we take an alternative approach and add an inductive shunt to the transmon to eliminate the offset charge dependence and stabilize the MIST. We experimentally characterize MIST in several different inductively-shunted transmons, in agreement with quantum and semiclassical models for MIST. These results extend to other inductively-shunted qubits.
High-fidelity and rapid qubit readout is essential for superconducting quantum processors, typically realized through the quantum non-demolition (QND) dispersive interaction within a qubit-resonator architecture. However, the achievable readout speed and fidelity are fundamentally limited by measurement-induced state transitions (MIST). For a transmon qubit, MIST is highly sensitive to the offset charge ng due to the charge dispersion of its higher-lying energy levels. In this work, we systematically investigate ng-dependent MIST dynamics governed by the diabaticity and symmetry of pulse shaping within a charge-sensitive transmon architecture. We engineer fast-load and fast-clear pulses that effectively suppress resonator photon overshoots, thereby demonstrating a highly practical strategy to mitigate MIST without requiring complex waveforms or real-time feedback. Utilizing active gate-voltage control and rapid feedback, the measurement-induced transition probability is precisely mapped against ng and the steady-state resonator photon number, exhibiting strong agreement with numerical Floquet branch analysis. Ultimately, we evaluate the ng-averaged total error probabilities for both readout and post-readout stages, verifying that a straightforward three-step pulse scheme consistently minimizes overall readout errors. Within the framework of large-scale superconducting quantum processors, this practical, hardware-free approach inherently offers a better trade-off between the readout signal-to-noise ratio and QND preservation.
Fast, precise, and quantum-non-demolition (QND) readout of superconducting qubits is a fundamental component of high-fidelity quantum sensing and computation. Conventional approaches typically operate in the dispersive regime, where the qubit-resonator coupling g is weak compared to the detuning Δ. While exhibiting good QND properties, the readout rate is limited to ∼g2N/Δ≪g, where N is the number of photons in the resonator. QND readout in the nondispersive regime, where the readout rate reaches its full potential ∼g, relies on parameter sweeps that may encounter resonances, leading to measurement-induced state transitions (MIST). In this work, we study a nondispersive readout protocol that replaces these sweeps by sudden quenches of the coupling constant, using a resonator that is preloaded with photons. We call this protocol bang-bang readout, and show that it realizes single-shot projective measurements. The fidelity and QNDness of the qubit post-measurement are remarkably high, with an error that decreases like 1/N. We show that the protocol can also be implemented without preloading the resonator by instead strongly driving the qubit, e.g., with a classical flux drive.
Dispersive readout of the transmon qubit in circuit QED is known to lose its quantum non-demolition character at small to moderate measurement drive amplitudes. This phenomenon is understood to originate from Laundau-Zener transitions at accidental multi-photon resonances, where n drive photons can promote the transmon by m levels. This interpretation has been shown to be in agreement with experiments characterizing the dispersive readout of a single transmon. The impact of these measurement-induced state transition (MIST) of a transmon embedded in a multi-qubit chip, however, remains largely unexplored. Here, we show that the presence of other components, such as qubits and couplers, can affect the MIST threshold of a measured transmon. To arrive at these results, we present a general method to characterize measurement-induced transition when the qubit under readout is coupled to other circuit elements, a ubiquitous situation in circuit QED-based quantum processors. As an example, we consider the case of two transmon qubits, and we show that the spectator qubit can be impacted by the measurement-induced transition of the readout qubit and, conversely, that the presence of the spectator qubit can lower the MIST threshold of the readout qubit. Finally, we explore how adding a coupler mode between the two qubits further modifies these effects.
Driven-dissipative qubit-resonator dynamics, which are the basis of most dispersive superconducting qubit measurement schemes, are often modeled with Lindblad master equations built from subsystem local jump operators, even when the qubit and resonator are appreciably hybridized. In this work we revisit this setting using a microscopic Bloch-Redfield approach, where dissipation is constructed in the eigenbasis of the coupled qubit-resonator Hamiltonian with a complete, frequency dependent, open system description of the transmission line environment. Here, we show that the Lindblad and Bloch-Redfield decay rates can be quantitatively different in the absence of driving, while in the driven case we demonstrate that the time-independent Redfield dissipator and its time-dependent generalization can show qualitatively different behaviors as a function of driving strength. Finally, we investigate the effects of driving in structured spectral densities, recovering the suppression of measurement-induced relaxation in the presence of a so called Purcell filter.