The measurement of quantum entanglement and enumeration of graph coverings

TL;DR

This paper develops formulas for quantum entanglement invariants using tensor representations of unitary groups, linking them to graph coverings and providing tools to distinguish entangled states.

Contribution

It introduces new formulas for invariants that separate entangled from unentangled states and establishes a bijection between graph coverings and invariant bases.

Findings

Invariants can distinguish entangled states from unentangled states.

A graph-theoretic interpretation of invariant dimensions is provided.

A bijection between graph coverings and invariant bases is established.

Abstract

We provide formulas for invariants defined on a tensor product of defining representations of unitary groups, under the action of the product group. This situation has a physical interpretation, as it is related to the quantum mechanical state space of a multi-particle system in which each particle has finitely many outcomes upon observation. Moreover, these invariant functions separate the entangled and unentangled states, and are therefore viewed as measurements of quantum entanglement. When the ranks of the unitary groups are large, we provide a graph theoretic interpretation for the dimension of the invariants of a fixed degree. We also exhibit a bijection between isomorphism classes of finite coverings of connected simple graphs and a basis for the space of invariants. The graph coverings are related to branched coverings of surfaces.