Approximating the Set of Separable States Using the Positive Partial Transpose Test

TL;DR

This paper demonstrates that the positive partial transpose test and similar criteria do not effectively approximate the set of separable states in high-dimensional quantum systems, as the distance between PPT states and separable states can be maximal.

Contribution

It proves that the PPT test and other criteria do not bound the distance from separable states in high dimensions, revealing their limitations as approximations.

Findings

Maximum trace distance from PPT states to separable states tends to 1 as dimension increases.

Similar results hold for reduction, majorization, and symmetric extension criteria.

PPT states and separable states have fundamentally different geometric shapes.

Abstract

The positive partial transpose test is one of the main criteria for detecting entanglement, and the set of states with positive partial transpose is considered as an approximation of the set of separable states. However, we do not know to what extent this criterion, as well as the approximation, are efficient. In this paper, we show that the positive partial transpose test gives no bound on the distance of a density matrix from separable states. More precisely, we prove that, as the dimension of the space tends to infinity, the maximum trace distance of a positive partial transpose state from separable states tends to 1. Using similar techniques, we show that the same result holds for other well-known separability criteria such as reduction criterion, majorization criterion and symmetric extension criterion. We also bring an evidence that the sets of positive partial transpose states and separable states have totally different shapes.