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  • Open Access

Benchmarking the quality of multiplexed qubit readout beyond assignment fidelity

Andras Di Giovanni1,*, Adrian Skasberg Aasen2,3, Jürgen Lisenfeld1, Martin Gärttner3, Hannes Rotzinger1,4, and Alexey V. Ustinov1,4

  • *Contact author: andras.digiovanni@gmail.com
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Phys. Rev. Applied 24, 044043 – Published 15 October, 2025

DOI: https://doi.org/10.1103/dpft-lxtx

Abstract

The accurate measurement of quantum two-level objects (qubits) is crucial for developing quantum computers. Over the last decade, the measure of choice for benchmarking readout routines for superconducting qubits has been assignment fidelity; however, this method only focuses on the preparation of computational basis states and therefore does not provide a complete characterization of the readout. Here, we expand the focus to the use of detector tomography to fully characterize multiqubit readout of superconducting transmon qubits. The impact of different readout parameters on the rate of information extraction is studied using quantum-state reconstruction infidelity as a proxy. The results are then compared with assignment fidelities, showing good agreement for separable two-qubit states. We therefore propose the rate of infidelity convergence as a validation tool for assignment fidelity and a more comprehensive benchmark for single- and multiqubit readout optimization. To make the best use of experimental resources, we investigate the most efficient distribution of a limited shot budget between detector tomography and state reconstruction within the context of single- and two-qubit experiments. To address the growing interest in three-qubit gates and test the scalability of the validation tool, we perform three-qubit quantum-state tomography that goes beyond conventional readout-error-mitigation methods and find a factor-of-30 reduction in quantum infidelity. Our results demonstrate that qubit readout correlations are not induced by a significantly reduced state distinguishability. Consequently, correlation coefficients can serve as a valuable tool in qubit readout optimization.

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Phys. Rev. Applied 24, 044043– Published 15 October, 2025
  • Received 21 February 2025
  • Revised 30 May 2025
  • Accepted 15 September 2025
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