Collective and optical phenomena in equilibrium and nonequilibrium interacting quantum systems

Abstract

In this thesis we study collective, emergent and optical properties of interacting quantum systems both in equilibrium and nonequilibrium situations from a microscopic modelling. This orientation steams from both the fact there is a profound need to design, characterise and set up control strategies for realistic systems in which quantum technologies could be conceived and the interest to grasp and identify fundamental principles for the emergence of macroscopic behaviour. The thesis is divided into three parts: I Optical and Collective Phenomena; II Equilibrium many-body systems and III Nonequilibrium many-body systems. Part I includes four complementary contributions to the optics emerging from the collective behaviour of microscopic quantum systems. In part II (Equilibrium many-body systems) of the thesis I have addressed the physics of quantum phase transitions from the perspective of nonequilibrium thermodynamics. We have shown that such an approach captures the essential features of finite order transitions that have a strong connection to thermodynamical and energetic figures of merit, but does not capture infinite order transitions that are of a much more subtle nature. Motivated by these exotic infinite order transitions we have looked at quantum phases and phase transitions through an informational and operational perspective based on pure state conversions restricted by local operations. In the third and last part (Nonequilibrium many-body systems) of the thesis I have laid out a project on the closed evolution of quantum spin chains focussing on the emergent nonequilibrium laws that depart from equilibrium physics.