Simple proof of conditions requiring maximal entanglement for local state verification and discrimination

Abstract

For deterministic state verification by separable measurements, the Schmidt rank of an entangled resource must be at least that of the state being verified [N. Yu, R. Duan, and M. Ying, IEEE Trans. Inf. Theory 60, 2069 (2014)], and if the two are equal, the resource must be maximally entangled [S. Akibue, J. Miyazaki, and H. Osaka, Lett. Math. Phys. 116, 110 (2026)]. The latter was derived from a general theorem proved using algebraic geometry. In this paper, we give a simple, self-contained proof of both statements using only linear algebra. We then use these results to show that among pure entangled resources of local dimension at most $d$, a maximally entangled state of $\mathbb{C}^d\otimes\mathbb{C}^d$ is necessary and sufficient for perfect local discrimination of a generic orthogonal basis of $\mathbb{C}^d\otimes\mathbb{C}^d$, however weakly entangled. The same holds for local implementation of a generic orthogonal measurement.