W-state graphs: Structure and Algorithms

TL;DR

This paper characterizes W-state graphs, a class of edge-coloured graphs representing quantum photonic experiments for generating multipartite W-states, providing structural insights and recognition algorithms.

Contribution

It offers a complete structural characterization of W-state graphs, linking them to matching-covered graphs and introducing W-cones as fundamental building blocks.

Findings

A graph is a W-state graph iff each 3-connected component is a W-cone.

Recognition of W-state graphs can be done efficiently by verifying matching-coveredness.

Generalizing to Dicke states is coNP-complete, indicating a complexity barrier.

Abstract

We study the class of edge-coloured graphs arising from the graph-theoretic representation of quantum photonic experiments that generate multipartite W-states. Abstracting away physical amplitudes and phases, we introduce W-state graphs: matching-covered graphs equipped with a half-edge 2-colouring such that every perfect matching contains exactly one bichromatic edge and every vertex is incident with a red half-edge. Our main contribution is a complete structural characterization of W-state graphs. We show that a graph is a W-state graph if and only if each of its 3-connected components is a W-cone, a simple and rigid building block defined by a universal vertex and a factor-critical base. This characterization implies that no W-state graph is simple and yields a recognition algorithm running as fast as verifying whether a graph is matching-covered. We also show that the natural generalization to Dicke states encounters a complexity barrier: verifying one of the two Dicke state conditions is itself coNP-complete, resolving an open problem of Vardi and Zhang [IJCAI 2023]. Our results place W-state graphs firmly within classical matching theory and precisely delineate the combinatorial structures capable of realizing idealized W-states in the experiment-graph framework.