[
  {
    "id": "ARR-2026-77DGRYDGXD8MDSY2-v3",
    "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": "v3",
    "genre": "Research preprint",
    "URL": "https://airr.science/papers/ARR-2026-77DGRYDGXD8MDSY2/versions/v3/",
    "note": "Research preprint; ARR-2026-77DGRYDGXD8MDSY2 v3. Not peer reviewed by AIRR.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 they equal 1-16/Tr(J inverse); for singular J they equal one. A qubit-qutrit lower family matches a harmonic-weight upper and gives an exact stabilization envelope, including a maximal factor of nine between coefficient suprema. For dimension d depolarization, a commuting Bell family matches lambda^2 kappa/[1+lambda(kappa-1)] for each normalized nonaffine operator-convex Petz generator up to kappa=d^2/2. A principal-rich rank-two ladder extends SLD and triangular sharpness to kappa=d(2d-1)/2, with actual-marginal budgets and reference dimension 2d-1. A separate logarithmic calculation yields the complete equal-reference BKM and finite Umegaki lower lambda^2 P/[1+lambda(P-1)], P=d(d-1), with a constrained profile and four interval-certified finite comparisons. We also identify qubit classical-reference and separable/PPT restrictions and give a localized Doeblin counterexample. These results do not determine unrestricted BKM, Umegaki, actual-mutual-information or capacity optima; the displayed lower is not a unitary phase model. Faithful limits, prior dependencies, adverse controls and review-context limits are explicit."
  }
]