Round 2 verification audit for C052

Model: gpt-5

Claim: Correlated bit/phase flips are modelled by U_i(epsilon) = exp(i (epsilon/2) sigma_{i,down e}) tensor exp(i (epsilon/2) sigma_{i,down e}) (i=x bit-flip, i=z phase-flip), applied once per cycle after drive (A).

Source alignment: source/supp_content.tex:70-74 (Supp. Mat. S2)

Prior official verdict: verified with failure_reason None.

Executable evidence: run.py. Sandbox rerun logs: run.log.

Independent audit: I scanned the copied script for imports/shared helper dependencies and reran it through the sandbox. The code is self-contained in this attempt directory and targets the claim strategy: Confirm U_i(epsilon) is a symmetric two-ion rotation on the {down,e} subspace representing a correlated coherent error.. I checked the relevant family model rather than relying only on exit status; the rerun is treated as one reproducibility input.

Decision: Round 2 verdict is verified with failure_reason None and limitations []. Notes: U_i(eps)=exp(i(eps/2)sigma_{i,de}) (x) exp(i(eps/2)sigma_{i,de}) is unitary, swap-symmetric, and of identical-factor kron(u,u) form (same single-ion rotation applied coherently to both ions => correlated coherent error). At eps=pi the single-ion factor = iX (x, bit-flip) / iZ (z, phase-flip) on {down,e} and leaves |up> fixed, matching the x=bit-flip / z=phase-flip labelling. Clean mathematical fact -> verified.