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Equation 5 · Part 2 · Error Correction Is the Whole Problem in Quantum Computing

Symbol psi_L

Si ∣ψL⟩=+∣ψL⟩,i=1,…,n−k.S_i \, |\psi_L\rangle = + |\psi_L\rangle , \qquad i = 1, \dots, n - k .
ψL\psi_L

What this part means

psiLi_L is part of the quantity the equation computes from the expression on the right.

Its job in the formula

psiLi_L is part of the quantity the equation computes from the expression on the right.

The passage around this formula

The technique that makes this work is the stabiliser formalism, given its general fault-tolerant treatment by Gottesman [ 4 ] . A code is defined not by listing its states but by naming a set of commuting Pauli operators SiS_i — the stabiliser generators — whose joint +1 eigenspace is the code space: Si ∣ψL⟩=+∣ψL⟩,i=1,…,n−kS_i \, |\psi_L\rangle = + |\psi_L\rangle , \qquad i = 1, \dots, n - k . An [[n, k, d]] code encodes k logical qubits into n physical qubits with distance d . The stabiliser generators are measured repeatedly. Because each SiS_i commutes with every other and with the logical operators, measuring them extracts no information about which encoded state is present. What it extracts is whether an error has anticommuted with a given check — a single…

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