The quantum entanglement contained in density matrices

TL;DR

This paper clarifies that density matrices inherently describe quantum states and their entanglement, emphasizing that bipartite density matrices generally contain quantum entanglement unless they are separable.

Contribution

It highlights the intrinsic quantum entanglement in density matrices and clarifies the conditions under which bipartite states are entangled.

Findings

Density matrices represent quantum states and their entanglement.

Bipartite density matrices contain quantum entanglement unless separable.

Entanglement is inherent in most bipartite density matrices.

Abstract

We point out that density matrices can only be used to describe quantum states, so the entanglement contained in a density matrix is just quantum entanglement. This means a bipartite state described by a density matrix contains quantum entanglement, unless the density matrix has the form $\rho_{AB}=\rho_A\otimes \rho_B$.