Drift-aware decoding of a repetition code: H7 boundary study, H8 decomposition and H9 co-drifter addendum

Abstract

Technical notes and complete deposit (code, synthetic data, preregistrations, decision log, independent audits) for three preregistered synthetic studies of drift-aware decoding of a distance-5 repetition code with one drifting data qubit. QP-02-H7, a kill test at equal information (1850 fresh trajectories): a hard keep/exclude switch beat neither equal-information baseline; verdict NO_SEPARATION. The soft reweighting policy P9 had an 8.6% [95% CI 7.1%, 10.0%] lower logical error rate than the switch, and a change-aware in-epoch estimator P10 3.1% [1.4%, 4.9%]. The hard action is not the source of the advantage measured in earlier runs. QP-02-H8, a preregistered 2x2 decomposition of the P9 versus P10 difference (1225 fresh trajectories): both main-effect labels POSITIVE; under the frozen attribution rule the estimator procedure is attributed to P9's advantage and the protocol regime offsets part of it. QP-02-H9 addendum: with a second qubit co-drifting with the first, the procedure contrast favours the structure-free estimator (run 002, labels NEGATIVE; an outcome-aware validation after run 001, which was INVALID_RUN and is reported beside it, never pooled). All data are simulated; no quantum hardware and no personal data are involved. Code (*.py, *.ps1) is MIT licensed, all other content CC BY 4.0. Start with DEPOSIT_README.md inside the archive.