Entanglement Classification with Algebraic Geometry

TL;DR

This paper uses algebraic geometry to classify symmetric multipartite entangled states, revealing structural properties, invariants, and physical interpretations, and explicitly classifying states with up to four qubits.

Contribution

It introduces a novel algebraic geometric framework for entanglement classification, connecting secant varieties and tangents to physical properties and providing explicit classifications for small systems.

Findings

Symmetric separable states form a Veronese variety.

SLOCC classes are grouped into families via secant varieties.

Explicit classification for N ≤ 4 qubits is achieved.

Abstract

We approach multipartite entanglement classification in the symmetric subspace in terms of algebraic geometry, its natural language. We show that the class of symmetric separable states has the structure of a Veronese variety and that its $k$-secant varieties are SLOCC invariants. Thus SLOCC classes gather naturally into families. This classification presents useful properties such as a linear growth of the number of families with the number of particles, and nesting, i.e. upward consistency of the classification. We attach physical meaning to this classification through the required interaction length of parent Hamiltonians. We show that the states $W_N$ and GHZ$_N$ are in the same secant family and that, effectively, the former can be obtained in a limit from the latter. This limit is understood in terms of tangents, leading to a refinement of the previous families. We compute explicitly the classification of symmetric states with $N\leq 4$ qubits in terms of both secant families and its refinement using tangents. This paves the way to further use of projective varieties in algebraic geometry to solve open problems in entanglement theory.