Entanglement, EPR-correlations, Bell-nonlocality, and Steering

TL;DR

This paper rigorously defines quantum steering, a form of nonlocality distinct from entanglement and Bell-nonlocality, demonstrating its properties, relation to other quantum correlations, and providing new examples.

Contribution

It offers a rigorous, applicable definition of steering for mixed states, clarifies its position among quantum correlations, and expands understanding of its properties and examples.

Findings

Steerable states are a strict subset of entangled states.

Steerable states form a strict superset of Bell-nonlocal states.

New examples of steerable states are provided.

Abstract

In a recent work [Phys. Rev. Lett. {\bf 98}, 140402 (2007)] we defined ``steering'', a type of quantum nonlocality that is logically distinct from both nonseparability and Bell-nonlocality. In the bipartite setting, it hinges on the question of whether Alice can affect Bob's state at a distance through her choice of measurement. More precisely and operationally, it hinges on the question of whether Alice, with classical communication, can convince Bob that they share an entangled state, under the circumstances that Bob trusts nothing that Alice says. We argue that if she can, then this demonstrates the nonlocal effect first identified in the famous EPR paper [Phys. Rev. {\bf 47}, 777 (1935)] as a universal effect for pure entangled states. This ability of Alice to remotely prepare Bob's state was subsequently called steering by Schr\"odinger, whose terminology we adopt. The phenomenon of steering has been largely overlooked, and prior to our work had not even been given a rigorous definition that is applicable to mixed states as well as pure states. Armed with our rigorous definition, we proved that steerable states are a strict subset of the entangled states, and a strict superset of the states that can exhibit Bell-nonlocality. In this work we expand on these results and provide further examples of steerable states. We also elaborate on the connection with the original EPR paradox.