FA-90930 / Quantum circuit simulation / Member archive

Collapse keeps amplitudes of the unobserved branch · case 05

After observing 0 on |+> the returned state still has a 0.707 amplitude on |1>.

Member previewVariant 5 · 3 implementations · 7 checks per implementation

Case contract

Input [n, amps, q, outcome]; measure qubit q (0 = LSB) of the possibly unnormalized state. Return {"probability": p, "state": post} with p normalized by the total squared norm and the post-measurement state renormalized to unit norm with the other branch zeroed; values rounded to 6 decimals. Return "impossible" when p < 1e-12.

Why this case matters

Mid-circuit measurement and collapse drive dynamic circuits; wrong renormalization breaks every later probability.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression: random collapse 14	`{"probability": 0.2153, "state": [[-0.892275, 0.451491], [0.0, 0.0], [0.0, 0.0], [-2.588, 1.898]]}`	`{"probability": 0.2153, "state": [[-0.892275, 0.451491], [0.0, 0.0], [0.0, 0.0], [0.0, 0.0]]}`	Failed

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