Techniques for protecting quantum information from decoherence and gate errors by encoding logical qubits across many physical qubits — analogous to but vastly more complex than classical error correction.
QEC is THE central challenge in building useful quantum computers. Physical qubits are fragile (lose coherence in microseconds, gates have 0.1-1% error rates), but quantum algorithms often need millions of operations to complete. QEC encodes a single 'logical qubit' across thousands of physical qubits, using clever measurements that detect errors without disturbing the quantum state. The Surface Code is the leading approach. Achieving fault-tolerant quantum computing requires physical qubits with low enough error rates to make encoding 'profitable' (below the threshold theorem).
Encoding 1 logical qubit using 49 physical qubits in a distance-7 Surface Code — protects against any single error, requires error rates < 1% on the physical qubits.
QEC ratios (physical qubits per logical qubit) determine when quantum becomes practically useful — current ratios mean millions of physical qubits are needed for cryptographic-scale Shor's algorithm.
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