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  • Open Access

Impact of Josephson-Junction Array Modes on Fluxonium Readout

Shraddha Singh1,2,3,*,†, Gil Refael4,3, Aashish Clerk5,3, and Emma Rosenfeld3,‡

  • *Contact author: shraddha.singh@yale.edu
  • †Present address: IBM Quantum, IBM T. J. Watson Research Center, Yorktown Heights, NY 10598, USA.
  • ‡Present address: Google Research.
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PRX Quantum 6, 040304 – Published 6 October, 2025

DOI: https://doi.org/10.1103/brdj-ggfj

Abstract

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.

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Popular Summary

Fluxonium qubits, known for their high coherence and fast gates, are a promising candidate for superconducting architectures. High-fidelity measurement of these qubits is a crucial component in employing a fluxonium-based architecture for fault-tolerant quantum computing. We present an analysis of dispersive readout in fluxonium qubits, specifically considering the “parasitic,” collective modes of a Josephson-junction array (JJA), which constitutes the inductive shunt in the circuit.

Measurement of superconducting circuits is currently limited by state transitions, so called measurement-induced state transitions (MISTs), in the qubit when increasing photons in the readout mode. Our analysis reveals that coupling to the parasitic modes of a JJA introduces additional state transitions during fluxonium readout. Consequently, such parasitic-mode-assisted MIST processes, which we refer to as PMISTs, can lower the onset of MIST processes to as low as ∼10 average photons in the readout mode, severely impacting the readout performance. Even when neglecting nonlinearities in the JJA, a significant number of these parasitic transitions, which are mediated by the coupling of the parasitic mode to the qubit mode, occur at considerable rates.

We also find that detrimental effects of PMISTs continue after a readout excitation is complete. Specifically, any remaining population in the parasitic modes from PMIST processes also causes subsequent dephasing of the fluxonium qubit. Without a judicious choice of readout frequency to avoid PMISTs, this can limit the performance of the qubit for further use in a quantum circuit. We also pay special attention to the role of symmetries in quantum circuits on PMISTs. To further identify mitigation strategies, we extend our findings across various fluxonium circuits, analyzing the dependency of parasitic frequencies and coupling between qubit and parasitic modes on different circuit parameters.

Our results underscore the substantial impact a JJA can have on the readout fidelity and the coherence of highly anharmonic superconducting circuits. While our analysis is performed using experimentally motivated parameters for fluxonium circuits, with further study, our approach and findings may be extended to the general case of a qubit in the presence of a spurious mode.

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Outline

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PRX Quantum 6, 040304– Published 6 October, 2025
  • Received 10 January 2025
  • Revised 21 May 2025
  • Accepted 26 August 2025
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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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