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Quantum error correction protects a logical qubit by encoding its quantum state into a larger Hilbert space, where a specified family of physical errors can be diagnosed and reversed by a common recovery quantum channel. The requirement is to restore every encoded quantum superposition, including its relative phase, rather than only the computational basis states. A quantum error-correcting code specifies which errors have this protection; it does not promise recovery from every possible disturbance.

The beautiful idea is to measure relations between physical qubits that reveal an error syndrome while revealing nothing about the protected logical qubit. For example, the bit-flip repetition code compares neighboring qubits without reading their individual values. Its parity checks can locate one Pauli X gate error while leaving the unknown coefficients intact. Encoding is a linear isometry, consistent with the no-cloning theorem: redundancy in a shared quantum state supplies protection without making independent copies of the unknown input.

Peter W. Shor's 1995 paper ^[ref], published on 1 October, introduced the nine-qubit Shor code to reduce quantum decoherence in memory. Andrew M. Steane's ^[ref], published on 29 July 1996 and ^[ref], connected quantum error correction to classical linear codes; the seven-qubit Steane code protects against arbitrary errors on one physical qubit. These constructions protect relative phase as well as computational basis values, going beyond the elementary bit-flip repetition code.

For an authored account of the mechanism, see John Preskill's ^[ref], especially section 7.1 on collective parity measurements and section 7.9 on stabilizer codes. Andrew Steane's A tutorial on quantum error correction develops the distinction between correcting a stored quantum state and operating with imperfect quantum gates.