Soutenance de thèse Benoit Ferté

The goal of this thesis is to study how quantum information spreads from a microscopic system to a macroscopic environment. To do so, we define the notion of phases of information propagation, which differentiate how much information an observer with access to some part of the environment is able to learn on the microscopic system, a notion close to the concept of objectivity in Quantum Darwinism. Phase transitions of information are then probed in several solvable models of expanding quantum circuits, where, by tuning the ratio between broadcasting and scrambling of information, we are able to set objectivity on or off in the system and to recover the behaviour of a functioning or broken measurement apparatus. In a second part, this notion of objectivity is compared to the notion of decoherent histories. While these two formalisms both aim at describing the emergence of « classicality » in quantum systems, we show that they in fact define two different kinds of classicalities: while the first one is related to the particular micro-to-macro process involved in a quantum measurement, the second one emerges solely from coarse-graining in macro-systems.