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The pure-state ECQC conjecture: counterexamples and a prime-dimensional classification

Ya Zhang

Published Oct 9, 2026• 0 citations
Publisher

Abstract

Complete written proof and exact checkers for the pure-state prime-dimensional ECQC question: it holds universally exactly at p=2; every odd prime has a full-Schmidt-rank pure-state counterexample; dimensions 3 and 5 attain the sharp pure-state ratios 3/2 and 5/2. The three-dimensional witness has a separate complete Lean proof, but the full prime-dimensional classification is not claimed Lean-complete. This record explicitly distinguishes prior mixed-state counterexamples and does not claim first priority. The fixed immutable GitHub Release is https://github.com/mxym/math/releases/tag/ecqc-pure-state-classification-v1. AI-assisted research and writing; no external funding. Existing licenses and third-party notices are retained.

DOI
10.5281/zenodo.23256947

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The generalized Pauli group and its normalizer, the Clifford group, have a rich mathematical structure which is relevant to the problem of constructing symmetric informationally complete POVMs (SIC-POVMs). To date, every known SIC-POVM fiducial vector is an eigenstate of an order 3 unitary in the Clifford group. I show that every order 3 unitary in prime dimensions p > 3 lies in the same conjugacy class of the Clifford group and construct a class representative for all such dimensions. It follows that if even one such SIC-POVM fiducial vector is an eigenvector of such a unitary, then all of them are (for a given such dimension). I also conjecture that in all dimensions d, the number of conjugacy classes is bounded above by 3 and depends only on d mod 9, and I support this claim with numerical evidence.
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I construct a positive-operator-valued measure (POVM) which has 2d2d2d rank-1 elements and which is informationally complete for generic pure states in ddd dimensions, thus confirming a conjecture made by Flammia, Silberfarb, and Caves (e-print quant-ph∕0404137). I show that if a rank-1 POVM is required to be informationally complete for all pure states in ddd dimensions, it must have at least $3d\ensuremath{-}2$ elements. I also show that, in a POVM which is informationally complete for all pure states in ddd dimensions, for any vector there must be at least $2d\ensuremath{-}1$ POVM elements which do not annihilate that vector.