Approximating Quantum States with Positive Partial Transposes in Multipartite System via Linearized Proximal Alternative Direction Method of Multipliers

Numerical approximation of quantum states via convex combinations of states with positive partial transposes (bi-PPT state) in multipartite systems constitutes a fundamental challenge in quantum information science. We reformulate this problem as a linearly constrained optimization problem. An approximate model is constructed through an auxiliary variable and a suitable penalty parameter, balancing constraint violation and approximation error. To slove the approximate model, we design a Linearized Proximal Alternating Direction Method of Multipliers (LPADMM), proving its convergence under a prescribed inequality condition on regularization parameters. The algorithm achieves an iteration complexity of $O(1/\epsilon^2)$ for attaining $\epsilon$-stationary solutions. Numerical validation on diverse quantum systems, including three-qubit W/GHZ states and five-partite GHZ and multiGHZ states with noises, confirms high-quality bi-PPT approximations and decomposability certification, demonstrating the utility of our method for quantum information applications.