# The Non-m-Positive Dimension of a Positive Linear Map

## TL;DR

This paper introduces the non-m-positive dimension of a linear map as a measure of its ability to detect entanglement and analyzes its properties for various well-known quantum maps.

## Contribution

It defines the non-m-positive dimension, explores its properties, and derives bounds for key positive maps, extending results to multipartite quantum systems.

## Key findings

- Bounds for non-mP dimension of transpose, reduction, Choi, and Breuer--Hall maps.
- Construction of large multipartite subspaces with non-positive partial transpose.
- Development of multipartite entanglement witnesses with maximum negative eigenvalues.

## Abstract

We introduce a property of a matrix-valued linear map $\Phi$ that we call its "non-m-positive dimension" (or "non-mP dimension" for short), which measures how large a subspace can be if every quantum state supported on the subspace is non-positive under the action of $I_m \otimes \Phi$. Equivalently, the non-mP dimension of $\Phi$ tells us the maximal number of negative eigenvalues that the adjoint map $I_m \otimes \Phi^*$ can produce from a positive semidefinite input. We explore the basic properties of this quantity and show that it can be thought of as a measure of how good $\Phi$ is at detecting entanglement in quantum states. We derive non-trivial bounds for this quantity for some well-known positive maps of interest, including the transpose map, reduction map, Choi map, and Breuer--Hall map. We also extend some of our results to the case of higher Schmidt number as well as the multipartite case. In particular, we construct the largest possible multipartite subspace with the property that every state supported on that subspace has non-positive partial transpose across at least one bipartite cut, and we use our results to construct multipartite decomposable entanglement witnesses with the maximum number of negative eigenvalues.