PDF, English Download (4MB) | Terms of use

Abstract

The number of parameters needed to specify the state of a many-body quantum system grows exponentially with the number of its constituents. This fact makes it computationally intractable to exactly describe dynamics and characterize the state on a microscopic level. In this thesis, we employ quantum field theory concepts for experimentally characterizing a spinor Bose gas far from equilibrium. First, we introduce the relevant concepts, which provide efficient descriptions for emerging macroscopic phenomena, in a formulation matching the capabilities of ultracold atomic systems. For our experimental study we employ a 87Rb spinor Bose-Einstein condensate in a quasi-one-dimensional trap geometry. We explore the phase diagram as a function of the effective quadratic Zeeman shift by measuring the fluctuations in the spin degree of freedom and identify three distinct phases. With this knowledge, we study the instabilities which occur after an instantaneous quench across the quantum phase transition separating the different phases. These instabilities allow us to drive the system far from equilibrium in a highly controlled fashion. For long times after the quench we observe universal dynamics associated with the emergence of a non-thermal fixed point. The structure factor of the angular orientation of the transversal spin features rescaling in time and space with universal exponents as well as a universal scaling function. Using the experimental control, we probe the insensitivity of this phenomenon to details of the initial condition. Spatially resolved snapshots of the complex-valued transversal spin field allow for the extraction of one-particle irreducible correlation functions, the building blocks of the quantum effective action. We find a strong suppression of the 4-vertex at low momenta emerging in the highly occupied regime. The introduced concepts together with the presented experimental applicability give new means for studying many-body systems at all stages of their evolution: from the initial instabilities and across transient phenomena far from equilibrium to the eventual thermalization.