# NPT bound entanglement: does undistillability imply PPT?

From many copies of a noisy entangled state, two parties using local operations and classical communication (LOCC) can often distill nearly perfect Bell pairs. States with a positive partial transpose (PPT) can never be distilled. The question is whether this one eigenvalue test tells the whole story, or whether some states that fail it are undistillable too.

## Problem

For a state $\rho$ on $\mathbb{C}^d\otimes\mathbb{C}^d$, write $\rho^{T_B}$ for its partial transpose on the second factor. The state is NPT if $\rho^{T_B}$ has a negative eigenvalue. It is $n$-distillable if some vector $\lvert\psi\rangle$ on $n$ copies, of Schmidt rank at most $2$ across the cut between Alice's $n$ systems and Bob's $n$ systems, satisfies

$$\langle\psi\rvert\,(\rho^{T_B})^{\otimes n}\,\lvert\psi\rangle<0 .$$

It is distillable if it is $n$-distillable for some $n$. By a criterion of M. and P. Horodecki, this is the same as being able to distill Bell pairs by LOCC [6].

**Question.** Is every NPT state distillable? Equivalently, is there a state that is NPT and yet $n$-undistillable for every $n\ge 1$?

Every NPT state can be turned into an NPT Werner state by LOCC, so it is enough to study [6]

$$\rho_\alpha=\frac{I+\alpha F}{d^2+\alpha d},\qquad -1\le\alpha\le 1,$$

where $F\lvert x\rangle\lvert y\rangle=\lvert y\rangle\lvert x\rangle$ is the swap. The state $\rho_\alpha$ is NPT exactly when $\alpha<-1/d$, and one-copy distillable exactly when $\alpha<-1/2$. For $d\ge 3$, the candidates are the Werner states with $-1/2\le\alpha<-1/d$.

## Background

In 1998 the Horodeckis proved that a distillable state must violate the PPT criterion, so entangled PPT states cannot be distilled; they called such entanglement "bound" [1]. In 2000 DiVincenzo, Shor, Smolin, Terhal and Thapliyal [2] and, independently, Dür, Cirac, Lewenstein and Bruß [7] gave evidence that some NPT Werner states are undistillable too; the first group also recast the question in terms of positive linear maps [2]. It is No. 2 in the Open Quantum Problems collection of IQOQI Vienna [6], and it decides whether distillability, a property of arbitrarily many copies, has a single-copy test.

## What is known

- PPT states are undistillable [1]. NPT states on $\mathbb{C}^2\otimes\mathbb{C}^n$ are distillable, and so are NPT Gaussian states [6].
- For $d\ge3$, every Werner state with $-1/2\le\alpha<-1/d$ is NPT and one-copy undistillable. For each fixed $n$, some NPT Werner states are proved $n$-copy undistillable, but these intervals shrink to nothing as $n$ grows [2, 6].
- In July 2026, two independent preprints proved that $\rho_\alpha$ is two-copy distillable exactly when $\alpha<-1/2$, in every dimension, so the whole window is two-copy undistillable [4, 5]. Bharti, Gajjala and Haug also give explicit $\gamma_k>0$ such that $\alpha\ge-\gamma_k$ implies $k$-copy undistillability [5].
- NPT states of rank at most four are distillable, so a counterexample has rank at least five. Chen and Đoković built a rank-five two-qutrit NPT family that is not 1-distillable, and not $n$-distillable for any given $n$ once its parameter is small enough; they conjecture it is undistillable [3].

## What would count as progress

- A proof that one fixed NPT state, such as a Werner state at $\alpha=-1/2$ with $d=3$ or $4$, is $k$-copy undistillable for every $k$. This settles the problem: NPT bound entanglement exists.
- Three-copy undistillability of the whole window, for instance through a sharper partial-trace inequality like those in [4, 5], or larger explicit $\gamma_k$.
- A distillability certificate inside the window: a Schmidt-rank-two vector on $k\ge3$ copies with $\langle\psi\rvert(\rho_\alpha^{T_B})^{\otimes k}\lvert\psi\rangle<0$ for some $-1/2\le\alpha<-1/d$, checkable in exact arithmetic.

## References

1. M. Horodecki, P. Horodecki and R. Horodecki, "Mixed-state entanglement and distillation: is there a 'bound' entanglement in nature?", *Physical Review Letters* 80, 5239 (1998). [doi:10.1103/PhysRevLett.80.5239](https://doi.org/10.1103/PhysRevLett.80.5239)
2. D. P. DiVincenzo, P. W. Shor, J. A. Smolin, B. M. Terhal and A. V. Thapliyal, "Evidence for bound entangled states with negative partial transpose", *Physical Review A* 61, 062312 (2000). [doi:10.1103/PhysRevA.61.062312](https://doi.org/10.1103/PhysRevA.61.062312)
3. L. Chen and D. Ž. Đoković, "Distillability of non-positive-partial-transpose bipartite quantum states of rank four", *Physical Review A* 94, 052318 (2016). [doi:10.1103/PhysRevA.94.052318](https://doi.org/10.1103/PhysRevA.94.052318)
4. T. C. Fraser, F. Huber, B. Pozsgay and I. Vona, "On the two-copy distillability of Werner states and a new partial trace inequality", preprint (2026). [arXiv:2607.24309](https://arxiv.org/abs/2607.24309)
5. K. Bharti, R. Gajjala and T. Haug, "Two-copy nondistillability of Werner states: sharp partial-trace inequalities and finite-copy extensions", preprint (2026). [arXiv:2607.24479](https://arxiv.org/abs/2607.24479)
6. D. Bruß (contact), "Undistillability implies PPT?", Open Quantum Problems, IQOQI Vienna, problem 2 (posted 2000). [Archived page](https://web.archive.org/web/20231029013418/https://oqp.iqoqi.oeaw.ac.at/undistillability-implies-ppt)
7. W. Dür, J. I. Cirac, M. Lewenstein and D. Bruß, "Distillability and partial transposition in bipartite systems", *Physical Review A* 61, 062313 (2000). [doi:10.1103/PhysRevA.61.062313](https://doi.org/10.1103/PhysRevA.61.062313)

## Source

Imported from [QIQCOP Zoo op_74c4ffb5b042baf2](https://qiqc-op.com/problem/op_74c4ffb5b042baf2/), curated by Bikun Li, Qicheng Tang, Changhao Li, Chengkai Zhu, Minbo Gao, Zhong-Xia Shang, Bin Cheng and Naixu Guo. Open according to the source as of 2026-10-20. The statement above is written in our own words; see the source for the original wording, history and discussion.