In December 2025, Pan Jianwei, Zhu Xiaobo, and Peng Chengzhi at the University of Science and Technology of China (USTC) demonstrated below-threshold quantum error correction on the Zuchongzhi 3.2 superconducting processor. The result was published as an Editors' Suggestion in Physical Review Letters (Vol. 135, 260601, 2025-12-22).

The experiment encodes 1 logical qubit using a distance-7 surface code, requiring 107 physical qubits — 49 data qubits, 48 ancilla qubits, and additional leakage reduction unit (LRU) qubits. The system achieved an error suppression factor Λ = 1.40, confirming that the logical error rate decreases as code distance increases and that the processor operates below the fault-tolerance threshold.

A key innovation is a novel all-microwave leakage suppression architecture. Leakage — where qubits escape to non-computational states — causes long-lived correlated errors that severely degrade QEC performance. The USTC team suppresses leakage through existing microwave control lines without additional hardware, reducing wiring complexity and cryogenic packaging overhead, offering a scalability advantage for future large-scale fault-tolerant systems.

This result makes China the first non-US country to achieve below-threshold QEC, following Google's Willow (Λ ≈ 2.14, December 2024) as a landmark milestone in the global race toward fault-tolerant quantum computing.