Quantum isomorphism is equivalent to equality of homomorphism counts from planar graphs

TL;DR

This paper characterizes quantum isomorphism of graphs through homomorphism counts from planar graphs, revealing new connections between quantum graph theory, combinatorics, and the Four Color Theorem.

Contribution

It establishes that quantum isomorphism corresponds to equal planar homomorphism counts and introduces graph categories and quantum groups based on graph intertwiners.

Findings

Quantum isomorphism equals equality of planar homomorphism counts

Homomorphism counts from planar graphs do not determine graph isomorphism

Decidability of planar homomorphism differences is proven to be undecidable

Abstract

Over 50 years ago, Lov\'{a}sz proved that two graphs are isomorphic if and only if they admit the same number of homomorphisms from any graph [Acta Math. Hungar. 18 (1967), pp. 321--328]. In this work we prove that two graphs are quantum isomorphic (in the commuting operator framework) if and only if they admit the same number of homomorphisms from any planar graph. As there exist pairs of non-isomorphic graphs that are quantum isomorphic, this implies that homomorphism counts from planar graphs do not determine a graph up to isomorphism. Another immediate consequence is that determining whether there exists some planar graph that has a different number of homomorphisms to two given graphs is an undecidable problem, since quantum isomorphism is known to be undecidable. Our characterization of quantum isomorphism is proven via a combinatorial characterization of the intertwiner spaces of the quantum automorphism group of a graph based on counting homomorphisms from planar graphs. This result inspires the definition of "graph categories" which are analogous to, and a generalization of, partition categories that are the basis of the definition of easy quantum groups. Thus we introduce a new class of "graph-theoretic quantum groups" whose intertwiner spaces are spanned by maps associated to (bi-labeled) graphs. Finally, we use our result on quantum isomorphism to prove an interesting reformulation of the Four Color Theorem: that any planar graph is 4-colorable if and only if it has a homomorphism to a specific Cayley graph on the symmetric group $S_4$ which contains a complete subgraph on four vertices but is not 4-colorable.