Anomalous Bell curve shape on ibm_marrakesh — has anyone seen this on other backends?
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Running a full angle-sweep Bell correlation measurement (37 angles, 0°–180°, 8192 shots) on ibm_marrakesh and ibm_fez, I'm seeing a consistent crossover shift in P_disagree(δ) — the 50% point lands at ~88.3° instead of 90°. The deformation model fits significantly better than QM+visibility (Δχ² = 124.5, 1 dof). Tested and ruled out: readout asymmetry, Ry gate offset (~20% contribution but wrong shape), T2 decoherence (acts in the opposite direction), angle-dependent gate duration, qubit-qubit crosstalk. What's left open: pulse-level gate miscalibration, which I can't test without pulse access. The effect is consistent across both chips (α = 0.467 vs 0.470), which is what makes it interesting — hardware artifacts are usually chip-specific. Has anyone run a similar full-angle Bell sweep on Heron-era chips or other backends (Eagle, Osprey, trapped ion) and seen something like this? Or know of a known IBM systematic that produces a smooth sinusoidal residual? Preprint + data + scripts: https://doi.org/10.5281/zenodo.18949735 Repo: https://github.com/3axap4eHko/bell-curve-asymmetry submitted by /u/3axap4eHko [link] [comments]