FA-90894 / Quantum circuit simulation / Member archive

Marginal accepts qubit index equal to the register width · case 04

Asking for qubit 2 of a two-qubit state returns {"0": 1.0} instead of the "bad-qubit" error.

Member previewVariant 4 · 3 implementations · 6 checks per implementation

Case contract

Input [n, amps, qubits]; amps are 2**n [re, im] pairs (qubit 0 = LSB), possibly unnormalized. Return the marginal outcome distribution over the listed qubits as {bitstring: probability} where the first listed qubit is the rightmost character, probabilities are normalized by the total squared norm, rounded to 6 decimals after summation, and zero entries are omitted. Errors: "bad-length", "duplicate-qubit", "bad-qubit", "zero-state" (total squared norm <= 1e-12).

Why this case matters

Marginal readout distributions are what users compare against hardware counts; ordering or normalization slips mislabel every histogram.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression: out of range qubit equals n	`{"0": 1.0}`	`"bad-qubit"`	Failed

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