Witnessing network steerability of every bipartite entangled state without inputs

TL;DR

This paper demonstrates that in quantum networks, every bipartite entangled state can exhibit a form of quantum steering without inputs, using linear witnesses and tomography.

Contribution

It introduces a method to activate and witness the steerability of all bipartite entangled states in network scenarios without inputs.

Findings

Linear witnesses for NPT bipartite states established

Swap-steering can be observed for all bipartite entangled states

Linear inequalities enable detection of swap-steerability with tomography

Abstract

Quantum steering is an asymmetric form of quantum nonlocality where one can detect whether a measurement on one system can steer or change another distant system. It is well-known that there are quantum states that are entangled but unsteerable in the standard quantum steering scenario. Consequently, a long-standing open problem in this regard is whether the steerability of every entangled state can be activated in some way. In this work, we consider quantum networks and focus on the swap-steering scenario without inputs and find linear witnesses of network steerability corresponding to any negative partial transpose (NPT) bipartite state and a large class of bipartite states that violate the computable cross-norm (CCN) criterion. Furthermore, by considering that the trusted party can perform tomography of the incoming subsystems, we construct linear inequalities to witness swap-steerability of every bipartite entangled state. Consequently, for every bipartite entangled state one can now observe a form of quantum steering.