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  • Open Access

Probing Excited-State Dynamics of Transmon Ionization

Zihao Wang1,2,*, Benjamin D’Anjou3, Philippe Gigon3, Alexandre Blais3,4, and Machiel S. Blok1,2,†

  • *Present address: AWS Center for Quantum Computing, Pasadena, California, USA.
  • †Contact author: machielblok@rochester.edu
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Phys. Rev. X 16, 021033 – Published 12 May, 2026

DOI: https://doi.org/10.1103/8tdv-hgmb

Abstract

The fidelity and quantum nondemolition character of the dispersive readout in circuit QED are limited by unwanted transitions to highly excited states at specific photon numbers in the readout resonator. This observation can be explained by multiphoton resonances between computational states and highly excited states in strongly driven nonlinear systems, analogous to multiphoton ionization in atoms and molecules. In this work, we utilize the multilevel nature of high-EJ/EC transmons to probe the excited-state dynamics induced by strong drives during readout. With up to ten resolvable states, we quantify the critical photon number of ionization, the resulting state after ionization, and the fraction of the population transferred to highly excited states. Moreover, using pulse shaping to control the photon number in the readout resonator in the high-power regime, we tune the adiabaticity of the transition and verify that transmon ionization is a Landau-Zener-type transition. We further extend these methods to a typical transmon with EJ/EC≈55 and probe the offset-charge dependence of ionization dynamics in a timed-resolved manner. Our experimental results agree well with the theoretical prediction from a semiclassical driven transmon model and may guide future exploration of strongly driven nonlinear oscillators.

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Popular Summary

Maintaining high-fidelity readout is essential for fault-tolerant quantum computation, yet strong measurement drives can trigger unwanted transitions where superconducting transmon qubits “ionize” into highly excited states. We utilize the multilevel nature of transmons to directly probe these ionization dynamics, using the fact that a high Josephson-energy–to–charging-energy ratio transmon enables high-fidelity control and readout of up to ten energy eigenstates. By quantifying critical photon numbers and characterizing postionization populations, we demonstrate that ionization occurs through pairwise multiphoton resonances between computational and highly excited states. We further verify that these transitions follow Landau-Zener-type dynamics, where the ionization probability depends on the adiabaticity of the drive. These findings establish a rigorous framework for understanding drive-induced errors and provide a pathway for optimizing measurement protocols to preserve qubit coherence in superconducting architectures.

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Phys. Rev. X 16, 021033– Published 12 May, 2026
  • Received 9 May 2025
  • Revised 25 November 2025
  • Accepted 20 February 2026
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