Equation 1 · Error Correction Is the Whole Problem in Quantum Computing
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Symbol U
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Symbol psi
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=
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The classical repetition code is the first thing anyone reaches for, and quantum mechanics forbids it. A unitary machine that maps an arbitrary unknown state alongside a blank register onto two copies, . cannot exist for all | , 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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