[
  {
    "id": "ARR-2026-77DGRYDGXD8MDSY2-v1",
    "type": "article",
    "title": "Sharp reference-assisted contraction for the minimal quantum Fisher metric",
    "author": [
      {
        "literal": "Lluis Eriksson"
      }
    ],
    "issued": {
      "date-parts": [
        [
          2026,
          10,
          27
        ]
      ]
    },
    "publisher": "AIRR.SCIENCE",
    "archive": "AIRR.SCIENCE",
    "archive_location": "ARR-2026-77DGRYDGXD8MDSY2",
    "version": "v1",
    "genre": "Research preprint",
    "URL": "https://airr.science/papers/ARR-2026-77DGRYDGXD8MDSY2/versions/v1/",
    "note": "Research preprint; ARR-2026-77DGRYDGXD8MDSY2 v1. Not peer reviewed by AIRR. Working paper; not admitted to the AIRR accepted collection.",
    "abstract": "We derive a harmonic-weight contraction bound for random-unitary channels on pairs with the same uniform-twirl image. It applies to every operator-convex Petz divergence on faithful states and is uniform over finite quantum reference systems. For a qubit depolarizing channel with retained amplitude 0<lambda<1, we identify the exact complete equal-reference contraction coefficient of both the minimal quantum Fisher metric and the triangular Petz divergence: 3lambda^2/(1+2lambda). The lower bound uses an explicit qubit-qutrit ladder state, a partial-trace-zero tangent, and an ordered faithful regularization. Thus the general Petz upper bound is sharp as a bound uniform over that family. At lambda=1/2 it equals 3/8, compared with the ordinary value 1/4. This does not determine the relative-entropy or mutual-information coefficient. We include a finite rational triangular-divergence witness above 1/3, explain a limitation of scalar-centering proofs, and preserve failed auxiliary variance and numerical routes. The supplement is public and pinned to an immutable commit."
  }
]