Impossible measurements require impossible apparatus

A well-recognized open conceptual problem in relativistic quantum field theory concerns the relation between measurement and causality. Naive generalizations of quantum measurement rules can allow for superluminal signaling ("impossible measurements"). This raises the problem of delineating physically allowed quantum measurements and operations. We analyze this issue in a recently proposed framework in which local measurements (in possibly curved spacetime) are described physically by coupling the system to a probe. We show that the state-update rule in this setting is consistent with causality provided that the coupling between the system and probe is local. Thus, by establishing a well-defined framework for successive measurements, we also provide a class of physically allowed operations. Conversely, impossible measurements can only be performed using impossible (nonlocal) apparatus.