Fig. 2
From: Device-independent certification of non-classical joint measurements via causal models
Physical realisation of Alice and Bob’s preparation devices. The box corresponds to the preparation device, and the vertical line emerging from it is the system Alice (Bob) sends Charlie. Here, on the left hand side of the diagram, x denotes the different possible ensembles and a denotes the specific state prepared from each ensemble. As Alice and Bob control both system and ancilla on the right hand side diagram, they know the measurement outcome a′. Hence, they know the exact state prepared on their original system. More specifically, the choice of measurement on their ancilla, denoted by x, specifies the two different ensembles the original system can be steered to. Moreover, the specific measurement outcome a′ prepares a fixed state a from the ensemble associated with that choice of measurement
