The out-of-equilibrium dynamics of interacting many-body systems presents one of the most challenging problems in modern many-body physics with implications ranging from thermalization dynamics over transport properties to novel transient effects and the formation of order. During the last years, ultracold atoms in optical lattices have emerged as a very versatile platform to study quantum many-body physics in a clean and well-controlled environment with tunable dimensions.
Traditionally, however, only the transient dynamics were of real interest, since the system would typically relax back into a well-understood thermal state in the long-time limit. In this talk, I will demonstrate that the exquisite control over the parameters of these synthetic man-body systems enables not only surprising equilibrium states, such as the observation of stable states at negative absolute temperatures, but furthermore offers access to intrinsically non-ergodic dynamics, where the system never relaxes to a thermal state.
One example, connected to ‘classical’ integrability, is the sudden expansion of hard-core bosons, where we could experimentally observe ballistic mass transport in a strongly interacting system leading to the spontaneous emergence of non-ground-state coherence. The second example is the realization of Many-Body Localization of interacting fermions, where the presence of disorder creates a non-ergodic insulating state with vanishing transport and subthermal entanglement spreading at all energies.