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Equation 2 · Error Correction Is the Whole Problem in Quantum Computing

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∣ψ⟩|\psi\rangle

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ψ\psi

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cannot exist for all |ψ\psi⟩\rangle , because unitarity preserves inner products and the required map does not. Wootters and Zurek established the point in 1982, in a paper whose title remains the cleanest statement of it [ 1 ] . There is no backup copy, no parity check computed by reading the data, no majority vote over three inspected replicas.

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