Exact Steering Bound for Two-Qubit Werner States

TL;DR

This paper proves that for two-qubit Werner states, projective measurements and POVMs are equally effective for quantum steering, resolving a long-standing open problem and clarifying measurement roles in quantum nonlocality.

Contribution

The paper demonstrates that projective measurements and POVMs have equal power in steering Werner states, closing the Werner gap and providing new local hidden state models.

Findings

POVMs and projective measurements are equally powerful for Werner states.

Constructed local hidden state models for Werner states with radius r ≤ 1/2.

Revealed differences in noise tolerance between measurement types for fixed state ensembles.

Abstract

We investigate the relationship between projective measurements and positive operator-valued measures (POVMs) in the task of quantum steering. A longstanding open problem in the field has been whether POVMs are more powerful than projective measurements for the steerability of noisy singlet states, which are known as Werner states. We resolve this problem for two-qubit systems and show that the two are equally powerful, thereby closing the so-called Werner gap. Using the incompatible criteria for noisy POVMs and the connection between quantum steering and measurement incompatibility, we construct a local hidden state model for Werner states with Bloch sphere radius $r\leq 1/2$ under general POVMs. This construction also provides a local hidden variable model for a larger range of Werner states than previously known. {In contrast, we also show that projective measurements and POVMs can have inequivalent noise tolerances when using a fixed state ensemble to build different local hidden state models.} These results help clarify the relationship between projective measurements and POVMs for the tasks of quantum steering and nonlocal information processing.