Quantum error correction of a grid-state qubit with state preparation and measurement errors below $10^{-3}$

Sara Turcotte; Lucas St-Jean; Amélie L. Pessonneaux; Ross Shillito; Bohdan Kulchytskyy; Eliott Ouellet; Jean Olivier Simoneau; Florian Hopfmueller; Matthew Hamer; Pascal Lemieux; Dany Lachance-Quirion; Baptiste Royer; Nicholas E. Frattini

arXiv · 2607.06718

Quantum error correction of a grid-state qubit with state preparation and measurement errors below $10^{-3}$

Sara Turcotte·Lucas St-Jean·Amélie L. Pessonneaux·Ross Shillito·Bohdan Kulchytskyy·Eliott Ouellet·Jean Olivier Simoneau·Florian Hopfmueller·Matthew Hamer·Pascal Lemieux·Dany Lachance-Quirion·Baptiste Royer·Nicholas E. Frattini

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Abstract

Grid state qubits offer a hardware-efficient approach to large-scale fault-tolerant quantum computing. They access the information redundancy required for quantum error correction by exploiting the large Hilbert space naturally available in harmonic oscillators. Superconducting architectures are particularly suitable to implement grid state qubits due to their fast and high-fidelity operations. Grid states in superconducting circuits enable quantum error correction (QEC) with performance beyond break-even. However, the state preparation and measurements (SPAM) errors of grid states has been a significant limitation to computational performances. In this work, we leverage high-performance QEC to enable repeat-until-success state preparation of both cardinal and magic states of the single-mode grid-state qubit. We combine this with an improved measurement protocol that corrects for both finite-energy envelope and auxiliary qubit readout errors, and increases robustness to photon loss. Our experiments, using both techniques, achieve a combined state-preparation and measurement error below $10^{-3}$. This represents two orders-of-magnitude improvement over the state of the art, bringing this platform on par with standard SPAM error levels measured in transmon qubits.

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Sara Turcotte, Lucas St-Jean, Amélie L. Pessonneaux, Ross Shillito, Bohdan Kulchytskyy, Eliott Ouellet, Jean Olivier Simoneau, Florian Hopfmueller, Matthew Hamer, Pascal Lemieux, Dany Lachance-Quirion, Baptiste Royer, Nicholas E. Frattini. 2026-07-07. Quantum error correction of a grid-state qubit with state preparation and measurement errors below $10^{-3}$. https://arxiv.org/abs/2607.06718

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