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Marginalize two-qubit pauli error on lost partners - #284
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🟡 Changes recommended
A normalized channel can panic in debug builds after partner loss or leakage because floating-point rounding pushes a derived marginal above 1, violating a strict assertion.
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What changed in this PR
This PR makes two-qubit Pauli noise apply the marginal channel to a surviving qubit when its partner is lost, updating the tableau and Pauli-sum backends.
Changes:
- Apply marginal Pauli and depolarizing noise to survivors, including leaked partners in the tableau backend.
- Add regression tests for survivor behavior and loss-aware coefficient scaling.
- Update comparison tests to document TableauSum’s unchanged no-op behavior.
| File | Description |
|---|---|
crates/ppvm-tableau/src/tableau_like.rs |
Compute and apply survivor marginals; update channel documentation. |
crates/ppvm-tableau/src/noise.rs |
Test lost and leaked partners and survivor flip rates. |
crates/ppvm-tableau-sum/tests/sampler_vs_pure.rs |
Adjust expectations for differing backend loss behavior. |
crates/ppvm-pauli-sum/tests/loss.rs |
Test marginal coefficient scaling and both-lost cases. |
crates/ppvm-pauli-sum/src/sum/noise.rs |
Treat lost slots as identity when scaling survivor terms. |
Review effort: Balanced
Contributor
|
Summed marginals of a normalized two-qubit channel can round to 1 + eps, tripping the debug probability check on a lost/leaked partner. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
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## 🤖 New release * `ppvm-traits`: 0.2.0 -> 0.2.1 * `ppvm-pauli-word`: 0.2.0 -> 0.2.1 * `ppvm-pauli-sum`: 0.2.0 -> 0.2.1 (✓ API compatible changes) * `ppvm-sym`: 0.2.0 -> 0.2.1 * `ppvm-tableau`: 0.2.0 -> 0.2.1 (✓ API compatible changes) * `stim-parser`: 0.2.0 -> 0.2.1 * `ppvm-stim`: 0.2.0 -> 0.2.1 * `ppvm-tableau-sum`: 0.2.0 -> 0.2.1 (✓ API compatible changes) * `vihaco-circuit-isa`: 0.2.0 -> 0.2.1 * `ppvm-vihaco`: 0.2.0 -> 0.2.1 * `ppvm-tui`: 0.2.0 -> 0.2.1 * `ppvm-cli`: 0.2.0 -> 0.2.1 <details><summary><i><b>Changelog</b></i></summary><p> ## `ppvm-cli` <blockquote> ## [0.2.1](v0.2.0...v0.2.1) - 2026-11-06 ### Other - Marginalize two-qubit pauli error on lost partners ([#284](#284)) </blockquote> </p></details> --- This PR was generated with [release-plz](https://github.com/release-plz/release-plz/). Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
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