Constructing locally indistinguishable orthogonal product bases in an $m \otimes n$ system

TL;DR

This paper constructs minimal and small locally indistinguishable orthogonal product bases in general bipartite quantum systems, advancing understanding of their structure and local indistinguishability.

Contribution

It introduces new constructions of orthogonal product bases with minimal sizes that are locally indistinguishable in arbitrary $m imes n$ systems, including the smallest known such bases.

Findings

Constructed a basis with $4p-4$ members, generalizing previous results.

Identified a smallest known completable basis with 8 members that cannot be distinguished locally.

Presented a small, possibly uncompletable basis with $2p-1$ members, including a 5-member uncompletable basis.

Abstract

Recently, Zhang et al [Phys. Rev. A 92, 012332 (2015)] presented $4d-4$ orthogonal product states that are locally indistinguishable and completable in a $d\otimes d$ quantum system. Later, Zhang et al. [arXiv: 1509.01814v2 (2015)] constructed $2n-1$ orthogonal product states that are locally indistinguishable in $m\otimes n$ ($3\leq m \leq n$). In this paper, we construct a locally indistinguishable and completable orthogonal product basis with $4p-4$ members in a general $m\otimes n$ ($3\leq m \leq n$) quantum system, where $p$ is an arbitrary integer from $3$ to $m$, and give a very simple but quite effective proof for its local indistinguishability. Specially, we get a completable orthogonal product basis with $8$ members that cannot be locally distinguished in $m\otimes n$ ($3\leq m \leq n$) when $p=3$. It is so far the smallest completable orthogonal product basis that cannot be locally distinguished in a $m\otimes n$ quantum system. On the other hand, we construct a small locally indistinguishable orthogonal product basis with $2p-1$ members, which is maybe uncompletable, in $m\otimes n$ ($3\leq m \leq n$ and $p$ is an arbitrary integer from $3$ to $m$). We also prove its local indistinguishability. As a corollary, we give an uncompletable orthogonal product basis with $5$ members that are locally indistinguishable in $m\otimes n$ ($3\leq m \leq n$). All the results can lead us to a better understanding of the structure of a locally indistinguishable product basis in $m \otimes n$.