Quantum source-channel coding and non-commutative graph theory

TL;DR

This paper extends classical graph theory concepts to quantum information using non-commutative graphs, providing new bounds and tools for quantum source-channel coding and demonstrating the importance of entanglement in quantum communication.

Contribution

It introduces homomorphisms on non-commutative graphs to analyze quantum source-channel coding, generalizes classical graph invariants, and constructs a quantum channel illustrating entanglement's role.

Findings

Generalized Lovász number as a homomorphism monotone

Extended Schrijver and Szegedy numbers as monotones

Constructed a quantum channel requiring non-maximally entangled states for zero-error capacity

Abstract

Alice and Bob receive a bipartite state (possibly entangled) from some finite collection or from some subspace. Alice sends a message to Bob through a noisy quantum channel such that Bob may determine the initial state, with zero chance of error. This framework encompasses, for example, teleportation, dense coding, entanglement assisted quantum channel capacity, and one-way communication complexity of function evaluation. With classical sources and channels, this problem can be analyzed using graph homomorphisms. We show this quantum version can be analyzed using homomorphisms on non-commutative graphs (an operator space generalization of graphs). Previously the Lov\'{a}sz $\vartheta$ number has been generalized to non-commutative graphs; we show this to be a homomorphism monotone, thus providing bounds on quantum source-channel coding. We generalize the Schrijver and Szegedy numbers, and show these to be monotones as well. As an application we construct a quantum channel whose entanglement assisted zero-error one-shot capacity can only be unlocked by using a non-maximally entangled state. These homomorphisms allow definition of a chromatic number for non-commutative graphs. Many open questions are presented regarding the possibility of a more fully developed theory.