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Gauge-Coded Quantum Cellular Automata with an Explicit Continuum Limit to the 1D Dirac Equation

Mohamed Sacha

Published Dec 8, 2025• 0 citations
Publisher

Abstract

We construct and analyze an explicit one-dimensional quantum cellular automaton (QCA) that provides a controlled lattice regularization of the free Dirac equation in (1+1) dimensions, and we extend it to a locally U(1) gauge-covariant version. The microscopic model is strictly local, causal in the Lieb–Robinson sense, and unitarily evolves a chain of finite-dimensional quantum systems by a translation invariant, discrete-time rule. At the representation-theoretic level, it realizes a chiral spin-1/2 degree of freedom per site. We prove that, in the joint limit of small lattice spacing and small time step with fixed physical scales, the QCA dynamics converges to the continuum Dirac evolution, with an explicit bound on the error valid for smooth, band-limited initial states. We then couple the QCA to a compact U(1) gauge field living on links, in a way that is exactly gauge-covariant at the microscopic level. A local Gauss constraint defines a gauge-invariant code subspace, and we show that the QCA update preserves this subspace and implements a gauge-covariant discrete Dirac dynamics. This provides a concrete example of a gauge-coded QCA with a controlled continuum limit, which can serve as a microscopic building block for more elaborate SU(3) × SU(2) × U(1) constructions. All results are stated with modest but explicit hypotheses and proofs, so that the framework is mathematically well-defined and amenable to further generalization.

DOI
10.20944/preprints202511.2241.v2

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