[
  {
    "id": "ARR-2026-77DGRYDGXD8MDSY2-v2",
    "type": "article",
    "title": "Complete contraction of unital qubit channels and order-constrained depolarization",
    "author": [
      {
        "literal": "Lluis Eriksson"
      }
    ],
    "issued": {
      "date-parts": [
        [
          2026,
          10,
          27
        ]
      ]
    },
    "publisher": "AIRR.SCIENCE",
    "archive": "AIRR.SCIENCE",
    "archive_location": "ARR-2026-77DGRYDGXD8MDSY2",
    "version": "v2",
    "genre": "Research preprint",
    "URL": "https://airr.science/papers/ARR-2026-77DGRYDGXD8MDSY2/versions/v2/",
    "note": "Research preprint; ARR-2026-77DGRYDGXD8MDSY2 v2. Not peer reviewed by AIRR. Working paper; not admitted to the AIRR accepted collection.",
    "abstract": "We determine the complete equal-reference contraction coefficients of the minimal quantum Fisher metric, its quadratic chi-square divergence and the finite triangular Petz divergence for every unital qubit channel. For a faithful normalized Choi state J, all three equal 1-16/Tr(J inverse); for singular J they equal one. An explicit qubit-qutrit lower family matches a harmonic-weight upper bound and gives the exact stabilization envelope, including a maximal factor of nine between coefficient suprema. For depolarization in dimension d, a commuting Bell family matches lambda^2 kappa/[1+lambda(kappa-1)] for each normalized nonaffine operator-convex Petz generator when 1<kappa<=d^2/2. A rank-two ladder extends SLD and triangular sharpness to kappa<=d(d+1)/2, using the actual reference marginal for the budget. We identify qubit classical-reference and separable/PPT restrictions throughout the physical noise range. A localized rational counterexample addresses one printed Doeblin equivalence, with its Hermitian primal and equality dual stated correctly. The results do not determine unrestricted BKM, relative-entropy, actual-mutual-information or capacity coefficients, and the lower constructions are not unitary phase models. Faithful limits, adverse controls, prior-work dependencies and model-review limitations are explicit."
  }
]