Quantum error correction below the surface code threshold
Google Quantum AI demonstrated a superconducting surface-code memory whose logical error rate decreases as the code distance grows, crossing below the fault-tolerance threshold. Scaling from distance-3 to distance-5 to distance-7 codes, the logical qubit's error per cycle was suppressed by roughly a factor of two per increment, showing exponential error suppression. This provides experimental evidence that the surface code can reach the regime needed for scalable fault-tolerant quantum computing.