Relativistic locality can imply subsystem locality

Locality is a central notion in modern physics, but different disciplines understand it in different ways. Quantum field theory focusses on relativistic locality, enforced by microcausality, while quantum information theory focuses on subsystem locality, which regulates how information and causal influences propagate in a system, with no direct reference to spacetime notions. Here we investigate how microcausality and subsystem locality are related. The question is relevant for understanding whether it is possible to formulate quantum field theory in quantum information language, and has bearing on the recent discussions on low-energy tests of quantum gravity. We present a first result in this direction: in the quantum dynamics of a massive scalar quantum field coupled to two localised systems, microcausality implies subsystem locality in a physically relevant approximation.