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Quasi-twisted Codes: Classical, Quantum, and Entanglement-Assisted Codes

Akram Saleh

Published Apr 20, 2026• 0 citations

Abstract

This thesis investigates the algebraic structure, construction, encoding, and decoding of quasi-cyclic (QC) and quasi-twisted (QT) codes and their applications to quantum error correction. For selected repeated-root and simple-root QC codes, generator-matrix-based construction methods are developed using the Generalized Discrete Fourier Transform (GDFT) and Discrete Fourier Transform (DFT), respectively. These methods enable efficient encoding by matrix-vector multiplication and provide computational advantages over codeword-based constructions. Spectral decoding procedures are also studied, including a GDFT-based folding framework that relates decoding of repeated-root QC codes to that of associated simple-root codes. Furthermore, a correspondence between certain repeated-root QC codes over finite fields and simple-root QT codes over finite chain rings is established. Building on these classical structures, one-generator QT codes of index 3 are investigated for quantum-code construction. Their Hermitian duals are characterized, and sufficient conditions for Hermitian self-orthogonality are derived, yielding stabilizer quantum error-correcting codes with good and, in several cases, optimal parameters. Structured syndrome-based decoding is developed by relating stabilizer measurements to classical decoding of the underlying QT codes. An extension technique preserving Hermitian self-orthogonality is introduced, producing additional stabilizer codes and corresponding decoding procedures. Finally, entanglement-assisted quantum error-correcting codes are constructed from 1-generator QT codes through Hermitian rank conditions, yielding maximal-entanglement codes with competitive parameters. Overall, the thesis establishes QC and QT codes as a unified algebraic framework for efficient classical coding, stabilizer quantum error correction, and entanglement-assisted quantum coding.

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This thesis investigates the algebraic structure, construction, encoding, and decoding of quasi-cyclic (QC) and quasi-twisted (QT) codes and their applications to quantum error correction. For selected repeated-root and simple-root QC codes, generator-matrix-based construction methods are developed using the Generalized Discrete Fourier Transform (GDFT) and Discrete Fourier Transform (DFT), respectively. These methods enable efficient encoding by matrix-vector multiplication and provide computational advantages over codeword-based constructions. Spectral decoding procedures are also studied, including a GDFT-based folding framework that relates decoding of repeated-root QC codes to that of associated simple-root codes. Furthermore, a correspondence between certain repeated-root QC codes over finite fields and simple-root QT codes over finite chain rings is established. Building on these classical structures, one-generator QT codes of index 3 are investigated for quantum-code construction. Their Hermitian duals are characterized, and sufficient conditions for Hermitian self-orthogonality are derived, yielding stabilizer quantum error-correcting codes with good and, in several cases, optimal parameters. Structured syndrome-based decoding is developed by relating stabilizer measurements to classical decoding of the underlying QT codes. An extension technique preserving Hermitian self-orthogonality is introduced, producing additional stabilizer codes and corresponding decoding procedures. Finally, entanglement-assisted quantum error-correcting codes are constructed from 1-generator QT codes through Hermitian rank conditions, yielding maximal-entanglement codes with competitive parameters. Overall, the thesis establishes QC and QT codes as a unified algebraic framework for efficient classical coding, stabilizer quantum error correction, and entanglement-assisted quantum coding.

Abstract The development of quantum codes and entanglement-assisted quantum codes with improved parameters through classical codes has been a critical area in both quantum communications and quantum information theory. In this paper, we introduce a novel approach using a family of one-generator quasi-twisted codes of index 3 and provide an extended version of them that maintain self-orthogonality, enabling the creation of stabilizer quantum codes. Computational findings include the construction of binary stabilizer codes with good parameters. Additionally, we propose techniques to design maximal-entanglement entanglement-assisted quantum codes based on this class of quasi-twisted codes. As a practical outcome, we find some optimal quantum codes and good maximal-entanglement entanglement-assisted quantum codes derived from these class of quasi-twisted codes and their extension.

Quantum information processing promises exponential advantages for certain computational and communication tasks, but practical quantum devices are inherently vulnerable to decoherence and operational imperfections. This thesis develops new algebraic and analytical tools for quantum error correction (QEC) aimed at unifying entanglement-assisted coding theory with operator-algebra frameworks, while also providing mathematically rigorous insights for continuous-variable photonic implementations. First, we introduce the framework of entanglement-assisted operator algebra quantum error correction (EA-OAQEC), extending entanglement-assisted QEC in an operator-algebra context. Built on a stabilizer-based formalism, EA-OAQEC unifies and generalizes subspace, subsystem, and hybrid classical-quantum entanglement-assisted codes. We establish a master error-correction theorem, define a generalized distance notion, and develop constructive techniques including gauge fixing, clean-qubit transformations, and entanglement-assisted gauge-fixing procedures. Second, we study general logical operators: unitaries that preserve a stabilizer codespace beyond the traditional Pauli normalizer. We derive explicit structure theorems for the centralizer and normalizer in the full unitary setting and show how normalizer transformations can be parameterized via classical bit matrix multiply complement (BMMC) permutations. Finally, motivated by photonic quantum computing, we analyze finite-rank compressions of the continuous position and momentum operators relevant to Gottesman-Kitaev-Preskill (GKP) codes. We prove that the spectra of these compressions are governed by the roots of classical Hermite polynomials and connect the compressed operators to physically realizable approximations of displacement operations and finite-energy GKP states.

Construction of quantum codes and entanglement-assisted quantum codes with good parameters via classical codes is an important task for quantum computing and quantum information. In this paper, by a family of one-generator quasi-cyclic codes, we provide quasi-cyclic extended constructions that preserve the self-orthogonality to obtain stabilizer quantum codes. As for the computational results, some binary and ternary stabilizer codes with good parameters are constructed. Moreover, we present methods to construct maximal-entanglement entanglement-assisted quantum codes by means of the class of quasi-cyclic codes and their extended codes. As an application, some good maximal-entanglement entanglement-assisted quantum codes are obtained and their parameters are compared.

Building large-scale quantum computers is a very active area of research. A critical part of this effort is the ability to control quantum errors. Recently, many good quantum error correcting codes (QECC) over various finite fields Fq have been constructed from codes over extension rings or mixed alphabet rings via some version of a Gray map. We show that most of these codes can be obtained more directly from classical codes, including cyclic, constacyclic, quasicyclic (QC), quasi-twisted (QT), and polycyclic codes using the CSS construction, Hermitian construction, and Lisonek-Singh methods. We believe direct and more elementary methods should be preferred.