Quantum Graph Homomorphisms via Operator Systems

TL;DR

This paper investigates quantum graph homomorphisms using operator systems and C*-algebras, introducing a new quantum chromatic number and exploring their structure through completely positive maps.

Contribution

It defines a C*-algebra capturing quantum homomorphisms, introduces a quantum chromatic number, and studies quantum graph cores, linking algebraic structures with computational complexity.

Findings

Defined a C*-algebra encoding quantum homomorphisms

Introduced a new quantum chromatic number with basic properties

Proposed a framework for quantum graph cores

Abstract

We explore the concept of a graph homomorphism through the lens of C$^*$-algebras and operator systems. We start by studying the various notions of a quantum graph homomorphism and examine how they are related to each other. We then define and study a C$^*$-algebra that encodes all the information about these homomorphisms and establish a connection between computational complexity and the representation of these algebras. We use this C$^*$-algebra to define a new quantum chromatic number and establish some basic properties of this number. We then suggest a way of studying these quantum graph homomorphisms using certain completely positive maps and describe their structure. Finally, we use these completely positive maps to define the notion of a "quantum" core of a graph.