  Recent works in foundations of quantum (field) theory and relativistic
quantum information try to better grasp the interplay between the structure of
quantum correlations and the constraints imposed by causality on physical
operations. Chapter 1 is dedicated to the study of the conceptual implications
of quantum nonlocality, a concept that subsumes that of entanglement in a
certain way. We detail the recent information-theoretic approaches to
understanding the structure of nonlocal correlations, and the issues the latter
raise concerning the ability of local observers to isolate a system from its
environment. Chapter 2 reviews in what sense imposing Poincare invariance
affects entanglement detection and quantification procedures. This invariance
ultimately forces a description of all quantum systems within the framework of
quantum field theory, which leads to the impossibility of localized
finite-energy states and to the divergence of all entanglement measures for
local observers. We provide a solution to these two problems by showing that
there exists a generic equivalence between a finite spatial resolution of the
measurement apparatus and the exclusion of high-energy degrees of freedom from
the definition of the observed system. This equivalence allows for an epistemic
interpretation of the standard quantum formalism describing nonrelativistic
localized systems and their correlations, hence a clarification of the origin
of the finite measures of entanglement between such systems. Chapter 3 presents
a recent theoretical framework that predicts the existence of correlations with
indefinite causal order. In analogy to the information-theoretic approaches to
nonlocal correlations, we introduce some principles that constrain the
structure of such correlations, which is a first step toward a clear
understanding of their physical origin.
