Entanglement in the Dicke subspace

TL;DR

This paper develops a comprehensive mathematical framework linking entanglement properties of Dicke state mixtures to tensor theory, enabling explicit construction of PPT entangled states and advancing understanding of multipartite entanglement.

Contribution

It introduces a tensor-based parametrization for Dicke states, connecting entanglement to convex cones of tensors and providing new tools for entanglement detection and state classification.

Findings

Constructed explicit PPT entangled states for three or more qutrits.

Proved PPT entanglement exists in all multipartite systems with three qutrits or more.

Established that PPT with respect to the most balanced bipartition implies PPT for all bipartitions.

Abstract

In this paper, we provide a complete mathematical theory for the entanglement of mixtures of Dicke states. These quantum states form an important subclass of bosonic states arising in the study of indistinguishable particles. We introduce a tensor-based parametrization where the diagonal entries of these states are encoded as a symmetric tensor, enabling a direct translation between entanglement properties and well-studied convex cones of tensors. Our results bridge multipartite entanglement theory with semialgebraic geometry and the theory of completely positive and copositive tensors. This dictionary maps separability to completely positive tensors, the PPT property to moment tensors, entanglement witnesses to copositive tensors, and decomposable witnesses to sum of squares tensors. Using this framework, we construct explicit PPT entangled states in three or more qutrits. In this class of states, we establish that PPT entanglement exists for all multipartite systems with three qutrits or more, disproving a recent conjecture in [J. Math. Phys. 66, 022203 (2025)]. We also show that, for mixtures of Dicke states, the PPT condition with respect to the most balanced bipartition implies PPT with respect to any other bipartition. We further connect bosonic extendibility of mixtures of Dicke states to the duals of known hierarchies for non-negative polynomials, such as the ones by Reznick and Polya. We thus provide semidefinite programming relaxations for separability and entanglement testing in the Dicke subspace.