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Glossary

Quantum glossary

Surface code

A quantum error-correcting code laid out on a 2D grid, needing only nearest-neighbour interactions.

Its high threshold (around 1% per operation) and hardware-friendly layout make it the leading candidate for fault-tolerant machines. In 2024 Google demonstrated logical error falling as the code grew — operation below threshold.

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Related terms

  • Quantum error correction — Encoding quantum information redundantly so errors can be detected and fixed without measuring — and destroying — the data.
  • Error threshold — The physical error rate below which adding more qubits to an error-correcting code makes logical errors rarer, not more common.
  • Logical qubit — A qubit encoded redundantly across many physical qubits so that errors can be detected and corrected.
  • Stabilizer formalism — A way to describe a quantum state by the Pauli operators that leave it unchanged, instead of by its amplitudes.
  • Code distance — The smallest number of qubits an error must touch to change the encoded information without being detected.