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

**Abstract:**

We introduce a property of a matrix-valued linear map Φ 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}⊗Φ. Equivalently, the non-mP dimension of Φ tells us the maximal number of negative eigenvalues that the adjoint map I_{m}⊗Φ∗ 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 Φ 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.

**Authors:**

- Nathaniel Johnston
- Benjamin Lovitz
- Daniel Puzzuoli

**Download:**

- Official publication at Quantum
- Preprint from arXiv:1906.04517 [quant-ph]
- Local preprint [pdf]

**Cite as:**

- N. Johnston, B. Lovitz, and D. Puzzuoli. The non-m-positive dimension of a positive linear map.
*Quantum,*3:172, 2019.

**Related Papers:**

- Non-positive partial transpose subspaces can be as large as any entangled subspace – an earlier paper containing many preliminary results that form a basis for this paper