Separable ellipsoids around multipartite states

TL;DR

This paper introduces a new ellipsoidal criterion for identifying separable states around multipartite states, expanding the known separable region and aiding in entanglement detection in quantum systems.

Contribution

It presents a novel ellipsoidal separable region around multipartite states, larger than previous bounds, and generalizes this to a trace formula for broader separability criteria.

Findings

Ellipsoids contain the separable ball and are typically exponentially larger.

The criteria enable rigorous numerical separability detection.

Applications include 3-qubit X states, noisy 4-qubit states, and the transverse field Ising model.

Abstract

We show that, in finite dimensions, around any $m$-partite product state $\rho_{\rm prod}=\rho_1\otimes...\otimes\rho_m$, there exists an ellipsoid of separable states centered around $\rho_{\rm prod}$. This separable ellipsoid contains the separable ball proposed in previous works, and the volume of the ellipsoid is typically exponentially larger than that of the ball, due to the hierarchy of eigenvalues in typical states. We further generalize this ellipsoidal criterion to a trace formula that yields separable region around all separable states, and further study biseparability. Our criteria not only help numerical procedures to rigorously detect separability, but they also lead to a nested hierarchy of SLOCC-stable subsets that cover the separable set. We apply the procedure for separability detection to 3-qubit X states, genuinely entangled 4-qubit states mixed with noise, and the 1d transverse field Ising model at finite temperature to illustrate the power of our procedure for understanding entanglement in physical systems.