Edge Of The Set Of No-Signaling Assemblages

Following recent advancements, we consider a scenario of multipartite postquantum steering and general no-signaling assemblages. We introduce the notion of the edge of the set of no-signaling assemblages and we present its characterization. Next, we use this concept to construct witnesses for no-signaling assemblages without a local hidden state model. Finally, in the simplest nontrivial case of steering with two untrusted subsystems, we discuss the possibility of quantum realization of assemblages on the edge. In particular, for three-qubit states, we obtain a no-go-type result, which states that it is impossible to produce an assemblage on the edge using measurements described by positive operator valued measures as long as the rank of a given state is greater than or equal to 3.