# Classification of Quantum Cellular Automata

## TL;DR

This paper extends index theory for classifying one- and two-dimensional quantum cellular automata, revealing topological classifications and limitations in higher dimensions, especially regarding fermionic systems and complex manifolds.

## Contribution

It demonstrates how index theory can classify quantum cellular automata in 1D and 2D, introduces a topological classification in higher dimensions, and discusses limitations and group theoretical aspects.

## Key findings

- Index theory classifies 1D automata.
- Extension to 2D automata includes torsion, achieving full classification.
- Higher dimensions pose challenges due to nontrivial automata in 3D.

## Abstract

There exists an index theory to classify strictly local quantum cellular automata in one dimension. We consider two classification questions. First, we study to what extent this index theory can be applied in higher dimensions via dimensional reduction, finding a classification by the first homology group of the manifold modulo torsion. Second, in two dimensions, we show that an extension of this index theory (including torsion) fully classifies quantum cellular automata, at least in the absence of fermionic degrees of freedom. This complete classification in one and two dimensions by index theory is not expected to extend to higher dimensions due to recent evidence of a nontrivial automaton in three dimensions. Finally, we discuss some group theoretical aspects of the classification of quantum cellular automata and consider these automata on higher dimensional real projective spaces.