Suppressing quantum errors by scaling a surface code logical qubit

TLDR

Practical quantum computing demands error rates far below those achievable with physical qubits, and while quantum error correction encodes logical qubits across many physical qubits to reduce errors, adding qubits also introduces more error sources, so the error density must remain sufficiently low for scaling to improve performance. The study measures logical qubit performance scaling across multiple code sizes to demonstrate that superconducting qubits can overcome the additional errors introduced by larger code sizes. By running a distance‑25 repetition code to probe low‑probability error sources and modeling the experiment to extract error budgets, the authors identify a logical error floor set by a single high‑energy event and highlight the biggest challenges for future systems. The results show that a distance‑5 surface code logical qubit modestly outperforms an ensemble of distance‑3 qubits, with logical error per cycle 2.914 % versus 3.028 %, and mark the first experimental demonstration that quantum error correction improves performance as qubit number increases, paving the way to required logical error rates.

Abstract

Abstract Practical quantum computing will require error rates well below those achievable with physical qubits. Quantum error correction 1,2 offers a path to algorithmically relevant error rates by encoding logical qubits within many physical qubits, for which increasing the number of physical qubits enhances protection against physical errors. However, introducing more qubits also increases the number of error sources, so the density of errors must be sufficiently low for logical performance to improve with increasing code size. Here we report the measurement of logical qubit performance scaling across several code sizes, and demonstrate that our system of superconducting qubits has sufficient performance to overcome the additional errors from increasing qubit number. We find that our distance-5 surface code logical qubit modestly outperforms an ensemble of distance-3 logical qubits on average, in terms of both logical error probability over 25 cycles and logical error per cycle ((2.914 ± 0.016)% compared to (3.028 ± 0.023)%). To investigate damaging, low-probability error sources, we run a distance-25 repetition code and observe a 1.7 × 10 −6 logical error per cycle floor set by a single high-energy event (1.6 × 10 −7 excluding this event). We accurately model our experiment, extracting error budgets that highlight the biggest challenges for future systems. These results mark an experimental demonstration in which quantum error correction begins to improve performance with increasing qubit number, illuminating the path to reaching the logical error rates required for computation.