Nonconvex optimization strategy for computing convex-roof entanglement

We develop a numerical methodology for the computation of entanglement measures for mixed quantum states. Using the well-known Schr"odinger-Hughston-Jozsa-Wootters theorem, the computation of convex-roof entanglement measures is reframed as a search for unitary matrices: a nonconvex optimization problem. To address this nonconvexity, we modify a genetic algorithm, known in the literature as differential evolution, constraining the search space to unitary matrices by using a QR factorization. We then refine results using a quasi-Newton method. We benchmark our method on simple test problems and, as an application, compute entanglement between a system and its environment over time for pure dephasing evolutions. We also study the temperature dependence of Gibbs state entanglement for a class of block-diagonal Hamiltonians to provide a complementary test scenario with a set of entangled states that are qualitatively different. We find that the method works well enough to reliably reproduce entanglement curves, even for comparatively large systems. The modified genetic algorithm is a derivative-free, nonconvex computational method that broadly applies to the computation of convex-roof entanglement measures.