Logical qubit

An error-protected qubit whose information is spread across many physical qubits, so individual faults can be found and fixed.

• The information lives in correlations across the group, so no single physical qubit holds it and one failing does not destroy it.
• The best physical qubits err roughly once per 1,000 operations; useful algorithms need about one in a million to one in a billion or better.
• Overhead depends on hardware quality and the code used: from a dozen or so to over a thousand physical qubits per logical one.
• Check claims: some only detect errors or run a single round; the key figure is the logical error rate per cycle, and whether it falls as the code grows.