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  • Open Access

Readout-induced leakage of the fluxonium qubit

Aayam Bista1,*, Matthew Thibodeau1, Ke Nie1, Kaicheung Chow2, Bryan K. Clark1, and Angela Kou1,2,3

  • *Contact author: bista2@illinois.edu
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Phys. Rev. Applied 25, 034058 – Published 18 March, 2026

DOI: https://doi.org/10.1103/wjdb-4814

Abstract

Dispersive readout is widely used to perform high-fidelity measurement of superconducting qubits. Much work has been focused on the qubit readout fidelity, which depends on the achievable signal-to-noise ratio and the qubit relaxation time. As groups have pushed to increase readout fidelity by increasing readout photon number, dispersive readout has been shown to strongly affect the postmeasurement qubit state. Such effects hinder the effectiveness of quantum error correction, which requires measurements that both have high readout fidelity and are quantum nondemolition (QND). Here, we experimentally investigate non-QND effects in the fluxonium. We map out the state evolution of fluxonium qubits in the presence of resonator photons and observe that these photons induce transitions in the fluxonium both within and outside the qubit subspace. We numerically model our system and find that transitions to higher-excited states and coupling to an external spurious mode are necessary to explain observed non-QND effects.

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Outline

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Phys. Rev. Applied 25, 034058– Published 18 March, 2026
  • Received 31 January 2025
  • Revised 15 December 2025
  • Accepted 9 February 2026
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