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arXiv:2606.05010

Measurement-induced state transitions in multi-qubit transmon processors

Baptiste Hoyau, Alexander McDonald, Boris M. Varbanov, Manuel H. Muñoz-Arias, Alexandre Blais

quant-phSubmitted Jun 3, 2026• Announced Jun 4, 2026• 0 citations
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Abstract

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.

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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 nnn drive photons can promote the transmon by mmm 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.
Drive-induced unwanted state transitions (DUST) are limiting both for microwave readout and parametric operations of superconducting qubits. Among them, measurement-induced state transitions (MIST) are due to intrinsic resonances described by the readout Hamiltonian. They were previously studied with a qubit linearly coupled to its readout mode, which constitutes the usual readout Hamiltonian. Since MIST can appear even at moderate powers, they limit the readout signal-to-noise ratio and the quantum non-demolition readout fidelity. In this work, we study the high-power readout regime in a different transmon readout scheme, implementing a nonlinear coupling called the cos  φ -coupling. This coupling stems from a transmon molecule circuit and has symmetry properties that suppress nonparity-conserving MIST. We succeed in performing multistate single-shot readout up to the fifth excited state of the transmon, which enables us to identify leakage pathways from the computational subspace. The measurements indicate that the system is free of MIST up to high powers, with more than 300 photons in the readout mode. The MIST can be controllably turned on by breaking the parity symmetry of the coupling using flux-tuning. These experimental results are corroborated by branch analysis and simulations of the classical chaotic dynamics, showing that the cos  φ -coupling is very robust to readout photons compared to the usual transverse coupling.
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.
Understanding the mechanisms that limit high-fidelity readout in circuit quantum electrodynamics is essential for its optimization. Multi-photon resonances are understood to be a limiting factor, causing population transfer from the computational states to higher-energy states under drive. This effect, known as measurement-induced state transitions, has been extensively studied for the transmon qubit. While this exploration has begun for the fluxonium qubit, a systematic study of this effect is lacking. Here, we bridge this gap by theoretically studying measurement-induced state transitions in the fluxonium qubit over a wide range of parameters, comprising essentially all experimentally explored ranges. We find that lighter fluxoniums are less susceptible to these state transitions when compared to their heavier counterparts. We attribute this effect to the combination of lower density of multi-photon resonances, a smaller requisite coupling for a given dispersive shift, and a more harmonic-like structure of the charge operator. We confirm the validity of our analysis by performing time-dependent readout simulations. Finally, we consider the impact of the superinductor's array modes on measurement-induced state transitions over a large range of parameters.
Dispersive readout of superconducting qubits is often limited by readout-drive-induced transitions between qubit levels. While there is a growing understanding of such effects in transmon qubits, the case of highly nonlinear fluxonium qubits is more complex. We theoretically analyze measurement-induced state transitions (MISTs) during the dispersive readout of a fluxonium qubit. We focus on a new mechanism: a simultaneous transition/excitation involving the qubit and an internal mode of the Josephson-junction array in the fluxonium circuit. Using an adiabatic Floquet approach, we show that these new kinds of MIST processes can be relevant when using realistic circuit parameters and relatively low readout drive powers. They also contribute to excess qubit dephasing even after a measurement is complete. In addition to outlining basic mechanisms, we also investigate the dependence of such transitions on the circuit parameters. We find that with a judicious choice of frequency allocations or coupling strengths, these parasitic processes can most likely be avoided.
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.
Repeated quantum non-demolition measurement is a cornerstone of quantum error correction protocols. In superconducting qubits, the speed of dispersive state readout can be enhanced by increasing the power of the readout tone. However, such an increase has been found to result in additional qubit state transitions that violate the desired quantum non-demolition character of the measurement. Recently, the readout of a transmon superconducting qubit was improved by using a tone with frequency much larger than the qubit frequency. Here, we experimentally identify the mechanisms of readout-induced transitions in this regime. In the dominant mechanism, the energy of an incoming readout photon is partially absorbed by the transmon and partially returned to the transmission line as a photon with lower frequency. Other mechanisms involve the excitation of unwanted package modes, decay via material defects, and, at higher qubit frequencies, the activation of undesired resonances in the transmon spectrum. Our work provides a comprehensive characterization of superconducting qubit state transitions caused by a strong drive.