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  • Open Access

Improved Error Correction with Leakage Reduction Units Built into Qubit Measurement in a Superconducting Quantum Processor

Yuejie Xin1,2, Sean L. M. van der Meer1,2, Marc Serra-Peralta1,3, Tim H. F. Vroomans1,2, Matvey Finkel1,2, Hendrik M. Veen1,2,†, Marc W. Beekman4, and Leonardo DiCarlo1,2,*

  • *Contact author: l.dicarlo@tudelft.nl
  • †Present address: SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands.
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PRX Quantum 7, 033062 – Published 21 September, 2026

DOI: https://doi.org/10.1103/y66v-988s

Abstract

Leakage to non-computational states is a source of correlated errors in both time and space that limits the effectiveness of quantum error correction (QEC) with superconducting circuits. We present and experimentally demonstrate a high-fidelity, hardware-efficient leakage reduction unit (LRU) operating concurrently with transmon measurement without incurring time overhead for 2-level readout (2RO). Adapted from double-drive reset of population (DDROP), the protocol utilizes simultaneous drives on the transmon and its readout resonator, leveraging the dispersive shift to create a directional process that returns the transmon to the computational subspace. The LRU achieves a 98.4% leakage removal fraction without compromising the 2RO assignment fidelity (99.2%) and can also be configured for 3-level readout. We combine LRU-enhanced measurement and leakage-informed neural-network decoding to successfully suppress logical error rates in both memory and stability QEC experiments without any post-selection. Remarkably, the LRU preserves the logical error-suppression rate in stability experiments as injected leakage increases.

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Popular Summary

Quantum computers must self-correct errors to be useful. A popular platform, superconducting quantum circuits, treats many-level systems, such as the transmon as a quantum two-level system or qubit. However, transmons can sometimes leak to higher energy levels outside the qubit subspace, an error that standard error correction is not designed to handle. Current fixes require pausing the quantum computer, which wastes time and lets other errors creep in. We have developed a method to reduce leakage concurrently with readout and demonstrate its usefulness in two types of error-correction experiments, called memory and stability. Our method applies simultaneous microwave pulses to the transmon and its readout resonator during readout. These pulses create a one-way process pushing the transmon from a leakage state back to the normal qubit state. The readout also reveals the transmon state (including the leakage state), allowing a richer set of information to be fed to a neural network that computes the required logical corrections. We show that the logical error is suppressed with both leakage mitigation and leakage information. We have tested our method in independent memory and stability experiments with up to 9 qubits, so a logical next step will be to deploy it on larger scale error correction protocols involving more qubits and requiring both variants.

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PRX Quantum 7, 033062– Published 21 September, 2026
  • Received 2 December 2025
  • Revised 15 May 2026
  • Accepted 7 July 2026
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