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Equation 20 · Error Correction Is the Whole Problem in Quantum Computing

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dd

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dd

Symbol d

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It does not promise that operating below threshold is sufficient. Below threshold, increasing d suppresses logical error. The suppression is exponential in d but the prefactor matters enormously in practice, and the useful regime is reached only when p is well below pthp_{\mathrm{th}} , not marginally below it. Near threshold, the exponent is small and the overhead explodes.

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