 Error syndrome

An error syndrome is the list of outcomes obtained by measuring a chosen ordered family of stabilizer generators. Write $s_j=0$ for eigenvalue $+1$ and $s_j=1$ for eigenvalue $-1$. For a definite Pauli error $E$ on a code state, the rule isThe commuting checks admit joint orthogonal projectorsFor density operator $\rho$, the Born rule gives outcome probability $\mathrm{tr}({\Pi }_s\rho )$ and conditional state ${\Pi }_s\rho {\Pi }_s/\mathrm{tr}({\Pi }_s\rho )$ when that probability is nonzero. A definite Pauli error has one deterministic error syndrome independent of the encoded amplitudes. A coherent linear combination of Pauli errors may occupy several sectors, so measuring the error syndrome selects a sector; recovery preserves the logical qubit when the entire error family is jointly correctable. ^[ref].

An error syndrome locates an error only relative to an assumed family and a recovery rule. Two Pauli errors differing by a logical operator can share an error syndrome; see stabilizer-syndrome coset. In the three-qubit bit-flip syndrome table, $X_3$ and $X_1{X}_2$ both give $01$. Recovering with $X_3$ corrects the first but converts the second into the logical operator $X_1{X}_2{X}_3$. Likewise, $Z_1$ has error syndrome $00$ yet changes the encoded relative phase.