# Indistinguishability of bipartite states by positive-partial-transpose operations in the many-copy scenario

## TL;DR

This paper introduces a criterion for indistinguishability of bipartite states by PPT operations in multiple copies, revealing limitations in distinguishing entangled states and their orthogonal complements.

## Contribution

It establishes a simple test for PPT indistinguishability and determines the minimal dimension of strongly PPT-unextendible subspaces in bipartite systems.

## Key findings

- Any entangled pure state and its orthogonal complement are indistinguishable by PPT operations in many copies.
- The minimal dimension of strongly PPT-unextendible subspaces in an m×n system is m+n-1.
- A criterion based on PPT-definite operators effectively verifies indistinguishability.

## Abstract

A bipartite subspace $S$ is called strongly positive-partial-transpose-unextendible (PPT-unextendible) if for every positive integer $k$, there is no PPT operator supporting on the orthogonal complement of $S^{\otimes k}$. We show that a subspace is strongly PPT-unextendible if it contains a PPT-definite operator (a positive semidefinite operator whose partial transpose is positive definite). Based on these, we are able to propose a simple criterion for verifying whether a set of bipartite orthogonal quantum states is indistinguishable by PPT operations in the many copy scenario. Utilizing this criterion, we further point out that any entangled pure state and its orthogonal complement cannot be distinguished by PPT operations in the many copy scenario. On the other hand, we investigate that the minimum dimension of strongly PPT-unextendible subspaces in an $m\otimes n$ system is $m+n-1$, which involves a generalization of the result that non-positive-partial-transpose (NPT) subspaces can be as large as any entangled subspace [N. Johnston, Phys. Rev. A 87: 064302 (2013)].