The Quantum Homomorphism Orders are Universal

TL;DR

This paper proves that quantum homomorphism orders for finite graphs are universal, meaning they can embed any countable partial order, highlighting their rich structural complexity.

Contribution

It establishes the universality of quantum homomorphism orders for both directed and undirected finite graphs, expanding understanding of their structural properties.

Findings

Quantum homomorphism orders are universal for finite graphs.

Directed graph orders embed classical universality results.

Explicit constructions demonstrate embedding of directed cycles into undirected graphs.

Abstract

Man\v{c}inska and Roberson introduced quantum graph homomorphisms as the existence of perfect quantum strategies for graph homomorphism games. The resulting relation is a quasi-order on finite graphs, and hence gives a partial order after quotienting by quantum homomorphic equivalence. We prove that the quantum homomorphism orders of both finite directed graphs and finite undirected graphs are universal: every countable partial order embeds into them. For directed graphs, the proof uses the classical universality of the homomorphism order on finite disjoint unions of clockwise directed cycles, together with the fact that quantum homomorphisms between such directed cycles coincide with classical homomorphisms. For undirected graphs we construct an explicit ordered undirected indicator whose terminal vertices are quantum endpoint-forcing. Replacing each directed edge by this indicator embeds the directed-cycle order into the quantum homomorphism order of finite undirected graphs.