Multicopy Adaptive Local Discrimination: Strongest Possible Two-Qubit Nonlocal Bases

Ensembles of composite quantum states can exhibit nonlocal behavior in the sense that their optimal discrimination may require global operations. Such an ensemble containing N pairwise orthogonal pure states, however, can always be perfectly distinguished under an adaptive local scheme if (N - 1) copies of the state are available. In this Letter, we provide examples of orthonormal bases in two-qubit Hilbert space whose adaptive discrimination require three copies of the state. For this composite system, we analyze multicopy adaptive local distinguishability of orthogonal ensembles in full generality which, in turn, assigns varying nonlocal strength to different such ensembles. We also come up with ensembles whose discrimination under an adaptive separable scheme require less numbers of copies than adaptive local schemes. Our construction finds important application in multipartite secret sharing tasks and indicates toward an intriguing superadditivity phenomenon for locally accessible information.

Automated summary

The research focuses on the discrimination of composite quantum states, revealing that an ensemble containing N pairwise orthogonal pure states can be perfectly distinguished under certain conditions. Examples of orthonormal bases in two-qubit Hilbert space requiring three copies for adaptive discrimination were provided, and the varying number of copies needed for discrimination under different schemes were analyzed.