FA-90958 / Quantum circuit simulation / Member archive

Pauli expectation silently accepts lowercase letters as identity · case 03

The string "z" is evaluated as identity (+1) instead of being rejected as "bad-pauli".

Member previewVariant 3 · 3 implementations · 6 checks per implementation

Case contract

Input [n, amps, pauli]; pauli is an n-character string over I,X,Y,Z whose rightmost character acts on qubit 0 (LSB). Return the real expectation <psi|P|psi>/<psi|psi> rounded to 6 decimals. Errors: "length-mismatch", "bad-pauli" (any character outside uppercase IXYZ).

Why this case matters

Pauli expectation values are the observable layer of variational algorithms; string-order or phase slips bias every energy estimate.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
regression: lowercase pauli	`1.0`	`"bad-pauli"`	Failed

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