What is Google Quantum AI Surface-Code Below-Threshold Logical Qubit?

Quantum computers store information in qubits, which are fragile and make frequent errors, so useful machines will need far lower error rates than single qubits can deliver. Quantum error correction tackles this by spreading information across many physical qubits to form one more resilient "logical" qubit. This result is an experiment by the Google Quantum AI team using a surface code, a checkerboard arrangement of qubits that checks for errors without revealing the stored data directly. The team reported that enlarging the code from 17 to 49 physical qubits lowered the logical qubit's error rate, though only slightly (about 4%, by the team's own account), which they described as a first-ever for any quantum computing platform, and published it in Nature in 2023.

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