slug: debrota-2020-orthocross-nonorthogonality bibkey: debrota2020varieties doi: 10.1142/S0219749920400055 url: https://arxiv.org/abs/1812.08762v5 triage: theorem motivation_gids:

• D5/S3/Quantum/Measurement/OrthocrossNonorthogonality.result

Orthocross MIC elements are never orthogonal, and their Gram entries tend to zero

Problem

J. B. DeBrota, C. A. Fuchs and B. C. Stacey (arXiv:1812.08762v5, quant-ph; Int. J. Quantum Inf. 19 (2021) 2040005) define the orthocross MIC of an orthonormal basis $\{|j⟩\}$ of ${\mathbb{C}}^d$:

• ${\Pi }_{\alpha }$ runs over the $d^2$ projectors onto $|j⟩$, $(|j⟩+|k⟩)/\sqrt{2}$ and $(|j⟩+i|k⟩)/\sqrt{2}$ ($j<k$);
• $\Omega ={\sum }_{\alpha }{\Pi }_{\alpha }$;
• $E_{\alpha }={\Omega }^{-1/2}{\Pi }_{\alpha }{\Omega }^{-1/2}$;
• the Gram matrix is $G_{\alpha \beta }=\mathrm{tr}{E}_{\alpha }{E}_{\beta }$.

The paper states:

The entries in $G$ for orthocross MICs can become arbitrarily small with increasing $d$, but no two elements of an orthocross MIC can be exactly orthogonal.

The verbatim statements are in the literature note. Issue #14213 reads the conjecture as two clauses, both for every $d$ and every orthonormal basis:

• (A) every off-diagonal Gram entry is a strictly positive real number;
• (B) every Gram entry tends to zero uniformly as $d\to \infty$.

Motivation

The orthocross MICs, introduced by Caves, Fuchs and Schack for the quantum de Finetti theorem, are the standard reference measurements built from a single basis. The authors report that the conjecture came from numerical investigations and that its proof had eluded them. Stacey (arXiv:1911.07386) notes that open conjectures about orthocross MICs remain.

Gap

Issue #14213 records the literature check before any Lean:

• arXiv metadata search for “orthocross”: 0 records;
• full-text search: only the source and Stacey’s two notes, which do not settle the conjecture;
• the 10 arXiv citers of 2024–2026 do not mention orthocross MICs;
• Zenodo: 0 records.

not-found-in-searched-scope.

Route

1. $G_{\alpha \beta }=w_{\alpha }{w}_{\beta }|v_{\alpha }^{†}M{v}_{\beta }{|}^2$ with $M={\Omega }^{-1}$ in the standard basis, the weights $w$ equal to $1$ or $\frac{1}{2}$, and $v_{\alpha }$ the unnormalized vectors. This uses basis covariance from the frozen half-integrality module.
2. Clause (B): Gershgorin gives $\Omega \ge \frac{d}{4}I$, hence $\Vert M\Vert \le 4/d$ and $|G_{\alpha \beta }|\le 256/d^2$.
3. Closed form: with $L=d-\frac{1-i}{2}$ and $q=\bar{L}/L$ ($|q|=1$), $M_{jk}=u{q}^{j-k+1}$ for $j<k$, the conjugate entries below the diagonal, and a positive real diagonal $t$. The constants are explicit, and $\Omega M=I$ is proved by geometric sums.
4. Clause (A):
   • multiplying by $(1-q)q^{d-2}/u$ turns $v_{\alpha }^{†}M{v}_{\beta }$ into the value at $q$ of a polynomial with Gaussian-integer coefficients of norm at most $16$;
   • its value and derivative at $1$ show that this polynomial is never zero;
   • $q=A/B$ with $A=(d-1)-di$ and $B=d-(d-1)i$, coprime by an explicit Bézout identity; a root at $q$ would make $B$ divide the leading coefficient, impossible for $d\ge 4$ since $N(B)=2d^2-2d+1>16$;
   • $d=2,3$ are checked exactly.

Falsifier

The statement concerns the orthocross vectors with phases $1$ and $i$ and the inverse square root ${\Omega }^{-1/2}$. The proof uses the specific Toeplitz-like structure of $\Omega$ ($d$ on the diagonal, $\frac{1\mp i}{2}$ off it). Other cross phases change $\Omega$ and are outside the claim.

Evidence

The canonical sources are:

• D5/S3/Quantum/Measurement/OrthocrossNonorthogonality.lean (public claim and result : claim);
• D5/S3/Quantum/Measurement/OrthocrossInverseFrame.lean (the closed inverse);
• D5/S3/Quantum/Measurement/OrthocrossPairingPolynomial.lean (the polynomial obstruction).

They reuse the frozen vec, weight, proj, frame, mic and gram of D5/S3/Quantum/Measurement/OrthocrossGramHalfInteger. That module exposes its basis covariance and trace reduction (proj_eq, frame_eq, posDef_one, gram_eq, frame_one) at their origin. The power-difference estimate norm_pow_sub_pow_le is owned by D5/S3/AnalyticClosure/ComplexPowerDifference; OrthocrossInverseFrame and D5/S3/AnalyticClosure/Polylogarithm/CompositionBanksLeadingClosure both apply it.

The axiom closure of result is exactly propext, Classical.choice and Quot.sound. There is no sorry, no native_decide and no new axiom.

Module statements:

• OrthocrossNonorthogonality sha256:f67dc87390a9c3aa80d2828495a74ac6df603d509f7e31176f589f9a79bdd29c, with result statement sha256:8643867bae425e6c54b5407b1010840f216c158c9fabdaa8e5b05360823d3efb and claim statement sha256:980463579004c56b5623f152b47e96fe5b8c5a7b8b4b0cbbeb028762100acd72;
• OrthocrossPairingPolynomial sha256:c081137237c75a072446bd68a26a574130bd89911498095c16b01fa3fa2078ad;
• OrthocrossInverseFrame sha256:fbf6675b37ea4634ae8326a91a34e11ce83e006b08822e5f62b80dc0ad9fa263.

Freeze events:

• OrthocrossNonorthogonality sha256:59d256a8952623235046390a724cee7423e042b7ea93f92625d21008964b41a8 (prerequisite: the pairing module);
• OrthocrossPairingPolynomial sha256:bd31a093508ad4fc75b55620f1dbf39411bae4cec64f737f02c5c358138b444c (prerequisite: the inverse module);
• OrthocrossInverseFrame sha256:b271cefafcc9f26c6d243d8108f5eee918c00ce4dbb600d39d09ff63fc689405 (prerequisites: the re-pinned OrthocrossGramHalfInteger, Freeze event sha256:5ffeb9a36ac6534935003bc70cb8e9bc0ab12173f5f19a1e4b7f75b6e9362d03, and ComplexPowerDifference, Freeze event sha256:5093366ae5c13b7912901d46ce6956c8acac00dfd98ac3eae30f255adefeb704, module statement sha256:b7edbb5cf79561007f44e5c761060c2fa581c7aaa1052a2f07ab94afa744ef71).

Triage

Tier 1 conjecture of a 2018 paper (still stated in v5), preregistered as research line #14213 before any Lean. theorem; resolution proved.

The other public theorems of the three modules lie on the proof path of result. Utility is none for all three. There is no digestion atom.

What the settlement shows

Proved by result: for every dimension $d$ and every orthonormal basis, every off-diagonal Gram entry of the orthocross MIC is a strictly positive real number, and every Gram entry tends to zero uniformly in the basis as $d$ grows.

Proved inside the proof, for every $d$.

• Every Gram entry, the diagonal included, is a strictly positive real number (gram_pos).
• Every Gram entry is at most $256/d^2$ in modulus (gram_norm_bound).
• ${\Omega }^{-1}$ has the closed form above: all off-diagonal entries have the same modulus $|u|$, their phases are powers of the unit $q=\bar{L}/L$, and the diagonal is a positive constant.
• No inverse-frame pairing $v_{\alpha }^{†}{\Omega }^{-1}{v}_{\beta }$ vanishes, equal indices included.

Argued, not formalized.

• Mechanism. Orthogonality would need a cancellation among at most four entries of ${\Omega }^{-1}$. After scaling, it becomes an algebraic relation for $q$ over $\mathbb{Z}[i]$ with small coefficients. The denominator $B$ of $q=A/B$ has norm $2d^2-2d+1$, so for $d\ge 4$ no such relation holds. The decay is the elementary fact that every eigenvalue of $\Omega$ is at least of order $d$, which the paper’s eigenvalue formula also shows.
• Sharper decay. The paper’s eigenvalues give ${\lambda }_{\min }(\Omega )=d-\frac{1}{2}(1+\cot \frac{3\pi }{4d})$ (the value in the paper’s bound on outcome probabilities), which is asymptotic to $(1-\frac{2}{3\pi })d$, so the constant $256$ is far from optimal. Numerically $d^2\max G$ equals $1.31$, $1.23$, $1.18$, $1.14$ and $1.12$ for $d=2,\dots ,6$; whether $d^2\max G$ has a positive limit is not determined here.
• Other phases. The obstruction uses that $q$ is a ratio of two coprime Gaussian integers of large norm. Replacing the phase $i$ by another root of unity changes the ring of coefficients; whether the non-orthogonality survives is not addressed.

Open. The exact asymptotics of the smallest Gram entry are not determined here: numerically $\min G\approx$ $4.1\times 10^{-2}$, $3.8\times 10^{-3}$, $7.8\times 10^{-4}$, $2.4\times 10^{-4}$ and $9.6\times 10^{-5}$ for $d=2,\dots ,6$. These finite values do not determine the asymptotic rate.

Effect on the paper. Both clauses of the conjecture hold. Together with the frozen half-integrality result, both orthocross conjectures of the paper are settled.

ASSUMED-UNVERIFIED

The published journal text was not read. The bounded literature check does not establish exhaustive worldwide novelty, priority, or the absence of an independent proof.