Quantum entanglement without eigenvalue spectra:multipartite case

TL;DR

This paper introduces eigenvalue-independent algebraic invariants for multipartite mixed states, providing a new separability criterion and constructing states with unique entanglement properties, challenging eigenvalue-based entanglement characterization.

Contribution

It develops algebraic sets as non-local invariants for multipartite states, offering a novel separability criterion and examples of complex entanglement structures.

Findings

New algebraic invariants for multipartite states independent of eigenvalues

A separability criterion based on algebraic set properties

Construction of multipartite states with specific entanglement features

Abstract

We introduce algebriac sets in the products of complex projective spaces for multipartite mixed states, which are independent of their eigenvalues and only measure the "position" of their eigenvectors, as their non-local invariants (ie. remaining invariant after local untary transformations). These invariants are naturally arised from the physical consideration of checking multipartite mixed states by measuring them with multipartite separable pure states. The algebraic sets have to be the sum of linear subspaces if the multipartite mixed state is separable, and thus we give a new separability criterion of multipartite mixed states. A continuous family of 4-party mixed states, whose members are separable for any 2:2 cut and entangled for any 1:3 cut (thus bound entanglement if 4 parties are isolated), is constructed and studied from our invariants and separability criterion. Examples of LOCC-incomparable entangled tripartite pure states are given to show it is hopeless to characterize the entanglement properties of tripartite pure states by only using the eigenvalue speactra of their partial traces. We also prove that at least $n^2+n-1$ terms of separable pure states, which are "orthogonal" in some sence, are needed to write a generic pure state in $H_A^{n^2} \otimes H_B^{n^2} \otimes H_C^{n^2}$ as a linear combination of them.