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  • Open Access

Suppression of Measurement-Induced State Transitions in cosφ-Coupling Transmon Readout

Cyril Mori1, Francesca D’Esposito1, Alexandru Petrescu2, Lucas Ruela1, Shelender Kumar1, Vishnu Narayanan Suresh1, Waël Ardati1, Dorian Nicolas1, Giulio Cappelli1 et al.

Arpit Ranadive1,*, Gwenael Le Gal1, Martina Esposito1,†, Quentin Ficheux1, Nicolas Roch1, and Olivier Buisson1,‡

  • *Present address: Google Quantum AI, Goleta, California 93117, USA.
  • †Present address: CNR-SPIN, Complesso di Monte S. Angelo, 80126 Napoli, Italy.
  • ‡Contact author: olivier.buisson@neel.cnrs.fr
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PRX Quantum 7, 020369 – Published 25 June, 2026

DOI: https://doi.org/10.1103/ylsw-t92p

Abstract

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 cosφ-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 cosφ-coupling is very robust to readout photons compared to the usual transverse coupling.

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Popular Summary

The fidelity and quantum non-demolition (QND) properties of qubit readout are central to the development of quantum computers. For transmons, measurement is usually achieved through a linear coupling between the qubit and the readout mode. Although this coupling is preeminent, it has many limitations. In fact, at sufficiently high photon numbers, required for fast and high-fidelity readout, the measurement loses its QND nature. This is due to measurement-induced state transitions (MIST), which are intrinsic to the coupling between the qubit and the measurement. They appear at low photon counts and lead to structural instabilities in the qubit. More generally, MIST limit all operations using microwave drives such as readout and quantum gates.

In this work, we implemented a nonlinear cosφ-coupling between the transmon qubit and the readout mode. This coupling suppresses MIST and maintains the stability of the qubit up to high photon counts above 300. The observation and suppression of MIST can be explained by the symmetry of the coupling, which is tunable with a magnetic field. All of these results were corroborated by theoretical analysis and simulations, showing that the cosφ-coupling is more robust to measurement photons than the usual linear coupling, making it a compelling alternative for high fidelity and nondestructive qubit readout.

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PRX Quantum 7, 020369– Published 25 June, 2026
  • Received 23 September 2025
  • Accepted 22 April 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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