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arXiv:2607.02170

Generalized Extended Codes with Applications in Entanglement-Assisted Qubit and Qutrit Codes

Yang Li, Martianus Frederic Ezerman, Shitao Li, San Ling, Zhonghua Sun

cs.ITquant-phSubmitted Jul 2, 2026• Announced Jul 3, 2026• 0 citations
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Abstract

We prove that any generalized extended code is monomially equivalent to the Hermitian dual of a code which is closely related to a second kind of extended code of $\C^{\perp_{\rm H}}$. Every $[n+1,k+1]_{q^2}$ linear code $\D$ with $d(\D^{\perp_{\rm H}})>1$ is monomially equivalent to the generalized extended code $\C({\bf u},a)$ of an $[n,k]_{q^2}$ linear code $\C$ for a fixed $a\in\F_{q^2}^{*}$ and some ${\bf u}\in\F_{q^2}^{n}$. We then characterize the Hermitian hull and Hermitian dual distance of $\C({\bf u},a)$ in terms of the position of ${\bf u}$ relative to $\C+\C^{\perp_{\rm H}}$ and the interaction between ${\bf u}$ and the minimum weight codewords of $\C^{\perp_{\rm H}}$, respectively. We obtain explicit criteria to independently control the expected Hermitian hull dimension and Hermitian dual distance of $\C({\bf u},a)$. In particular, several conditions for simultaneously increasing the Hermitian hull dimension and the Hermitian dual distance of $\C({\bf u},a)$ are derived. Applying these results to the Hermitian construction for EAQECCs gives us $267$ new EA qubit codes of lengths $n \leq 40$ and $14$ new EA qutrit codes of lengths $n \leq 25$ compared to the best-known codes in Grassl's code tables and the imporvements recorded in very recent works in the literature. Among the new parameter sets, we confirm improvements for $236$ qubit and $8$ qutrit codes.

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Letq=peq=p^{e}q=pebe a prime power and$\ell $be an integer with0≤ℓ≤e−10\leq \ell \leq e-10≤ℓ≤e−1. The$\ell −GaloishullofclassicallinearcodesisageneralizationoftheEuclideanhullandHermitianhull.WeprovideanecessaryandsufficientconditionunderwhichacodewordofaGRScodeoranextendedGRScodebelongstoits-Galois hull of classical linear codes is a generalization of the Euclidean hull and Hermitian hull. We provide a necessary and sufficient condition under which a codeword of a GRS code or an extended GRS code belongs to its−GaloishullofclassicallinearcodesisageneralizationoftheEuclideanhullandHermitianhull.WeprovideanecessaryandsufficientconditionunderwhichacodewordofaGRScodeoranextendedGRScodebelongstoits\ell −Galoisdualcode,generalizingboththeEuclideancaseandHermitiancaseintheliterature.Byusingfourdifferenttools:1)thenormmappingfrom-Galois dual code, generalizing both the Euclidean case and Hermitian case in the literature. By using four different tools: 1) the norm mapping from−Galoisdualcode,generalizingboththeEuclideancaseandHermitiancaseintheliterature.Byusingfourdifferenttools:1)thenormmappingfrom\mathbb {F}_{q}^{\ast }tototo\mathbb {F}_{p^{\ell }}^{\ast };2)thedirectproductoftwocyclicsubgroups;3)thecosetdecompositionofacyclicgroup;4)anadditivesubgroupof; 2) the direct product of two cyclic subgroups; 3) the coset decomposition of a cyclic group; 4) an additive subgroup of;2)thedirectproductoftwocyclicsubgroups;3)thecosetdecompositionofacyclicgroup;4)anadditivesubgroupof\mathbb {F}_{q}anditscosets,weconstructelevenfamiliesofand its cosets, we construct eleven families ofanditscosets,weconstructelevenfamiliesofq−aryMDScodeswith-ary MDS codes with−aryMDScodeswith\ell −Galoishullsofarbitrarydimensions,andgivetherelatedelevenfamiliesof-Galois hulls of arbitrary dimensions, and give the related eleven families of−Galoishullsofarbitrarydimensions,andgivetherelatedelevenfamiliesof[[n,k,d;c]]_{q}$entanglement-assisted quantum error-correcting codes (EAQECCs) with relatively large minimum distance in the sense that2d=n−k+2+c2d=n-k+2+c2d=n−k+2+c. We show that developing the theory on$\ell −Galoishullsof-Galois hulls of−Galoishullsofq−aryMDScodesinthispaperenablesustoobtainnew-ary MDS codes in this paper enables us to obtain new−aryMDScodesinthispaperenablesustoobtainnewq−aryEAQECCswithdifferentkindsoflengthsetsviadifferent-ary EAQECCs with different kinds of length sets via different−aryEAQECCswithdifferentkindsoflengthsetsviadifferent\ell $, where2ℓ∣e2\ell \mid e2ℓ∣e.
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