Entanglement quantification made easy: Polynomial measures invariant under convex decomposition

TL;DR

This paper introduces a geometric approach to simplify the calculation of certain polynomial entanglement measures, making it easier to evaluate entanglement in specific quantum states.

Contribution

It proves that polynomial entanglement measures of degree 2 are decomposition-independent under certain conditions, enabling analytical evaluation for some mixed states.

Findings

Polynomial measures of degree 2 are invariant under pure-state decompositions.

Analytical formulas are derived for convex roof measures in specific rank-two states.

Several classes of four-qubit states have marginals satisfying the invariance condition.

Abstract

Quantifying entanglement in composite systems is a fundamental challenge, yet exact results are only available in few special cases. This is because hard optimization problems are routinely involved, such as finding the convex decomposition of a mixed state with the minimal average pure-state entanglement, the so-called convex roof. We show that under certain conditions such a problem becomes trivial. Precisely, we prove by a geometric argument that polynomial entanglement measures of degree 2 are independent of the choice of pure-state decomposition of a mixed state, when the latter has only one pure unentangled state in its range. This allows for the analytical evaluation of convex roof extended entanglement measures in classes of rank-two states obeying such condition. We give explicit examples for the square root of the three-tangle in three-qubit states, and show that several representative classes of four-qubit pure states have marginals that enjoy this property.