Skip to main content
Glossary

Quantum glossary

Logical qubit

A qubit encoded redundantly across many physical qubits so that errors can be detected and corrected.

A surface-code logical qubit uses on the order of 2d² physical qubits for code distance d. Estimates for breaking RSA-2048 run to around a million physical qubits, which is why qubit counts alone say little about capability.

See it for yourself

Related terms

  • Quantum error correction — Encoding quantum information redundantly so errors can be detected and fixed without measuring — and destroying — the data.
  • Surface code — A quantum error-correcting code laid out on a 2D grid, needing only nearest-neighbour interactions.
  • Fault tolerance — Running a quantum computation reliably even though every physical component is faulty, by encoding in error-correcting codes.