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Glossary

Quantum glossary

Error threshold

Also: threshold theorem

The physical error rate below which adding more qubits to an error-correcting code makes logical errors rarer, not more common.

Above threshold, a bigger code only collects more errors. Below it, logical error rate falls exponentially with code distance. Crossing it is the milestone that makes large-scale quantum computing an engineering problem rather than a hope.

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Related terms

  • Fault tolerance — Running a quantum computation reliably even though every physical component is faulty, by encoding in error-correcting codes.
  • Surface code — A quantum error-correcting code laid out on a 2D grid, needing only nearest-neighbour interactions.