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Glossary

Quantum glossary

Ancilla qubit

Also: ancilla, helper qubit, scratch qubit

An extra qubit used as scratch space during a computation, usually returned to |0⟩ afterwards.

Ancillas hold intermediate results, mark states for an oracle, or collect syndromes in error correction. Leaving one entangled with the main register when you are done silently ruins interference, which is why “uncomputing” it matters.

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Related terms

  • Uncomputation — Running part of a circuit in reverse to return scratch qubits to |0⟩ and disentangle them from the result.
  • Syndrome — The pattern of parity-check results that tells an error-correcting code where an error probably happened.