Entanglement witnessing by arbitrarily many independent observers recycling a local quantum shared state

TL;DR

This paper demonstrates that certain entangled states, which do not violate Bell inequalities, can still have their entanglement witnessed sequentially by multiple independent observers using unsharp measurements, expanding the understanding of quantum correlations.

Contribution

It introduces a method to detect entanglement in states that do not violate Bell inequalities, allowing an arbitrary number of observers to witness entanglement sequentially.

Findings

Entangled states without Bell violation can be used to witness entanglement sequentially.

A larger set of states can be used for entanglement witnessing than for Bell violation.

Multiple independent observers can detect entanglement without violating Bell inequalities.

Abstract

We investigate the scenario where an observer, Alice, shares a two-qubit state with an arbitrary number of observers, Bobs, via sequentially and independently recycling the qubit in possession of the first Bob. It is known that there exist entangled states which can be used to have an arbitrarily long sequence of Bobs who can violate the Clauser-Horne-Shimony-Holt (CHSH) Bell inequality with the single Alice. We show that there exist entangled states that do not violate the Bell inequality and whose entanglement can be detected by an arbitrary number of Bobs by suitably choosing the entanglement witness operator and the unsharp measurement settings by the Bobs. This proves that the set of states that can be used to witness entanglement sequentially is larger than those that can witness sequential violation of local realism. There exist, therefore, two-party quantum correlations that are Bell "classical", but whose entanglement "nonclassicality" can be witnessed sequentially and independently by an arbitrarily large number of observers at one end of the shared state with the single observer at the other end.