Separable ellipsoids around multipartite states

We show that in finite dimensions, around any m-partite product state ρprod=ρ1⊗⋯⊗ρm, there exists an ellipsoid of separable states centered around ρprod. 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 stochastic local operations and classiccal communication (SLOCC)-stable subsets that cover the separable set. We apply the procedure for separability detection to three-qubit X states, genuinely entangled four-qubit states mixed with noise, and the one-dimensional transverse-field Ising model at finite temperature to illustrate the power of our procedure for understanding entanglement in physical systems.