The evolution of an isolated quantum system is unitary. This is simple to probe for small systems consisting of few non-interacting particles. But what happens if the system becomes large and its constituents interact? In general one will not be able to follow the evolution of the complex many body eigenstates. Ultra cold quantum gases are an ideal system to probe these aspects of many body quantum physics and the related quantum fields. Our pet systems are one-dimensional Bose-gases. Interfering two systems allows studying coherence between the two quantum fields and the full distribution functions and correlation functions give detailed insight into the many body states and their non-equilibrium evolution. In our experiments we study how the coherence created between the two isolated one-dimensional quantum gases by coherent splitting slowly degrades by coupling to the many internal degrees of freedom available. We find that a one-dimensional quantum system relaxes to a pre-thermalisatized quasi steady state which emerges through a light cone like spreading of ’de-coherence’. The pre-thermalized state is described by a generalized Gibbs ensemble. Finally we investigate two distinct ways for subsequent evolution away from the pre-thermalized state. One proceeds by further de-phasing, the other by higher order phonon scattering processes. In both cases the final state is indistinguishable from a thermally relaxed state. We conjecture that our experiments points to a universal way through which relaxation in isolated many body quantum systems proceeds if the low energy dynamics is dominated by long lived excitations (quasi particles).

The Colloquium Ehrenfestii takes place Wednesday evenings starting at 19:30 hours in the main auditorium of the Oort building. Before the Colloquium, there is a common dinner in the canteen located on the ground floor of the Oort building. This dinner starts at 18:00 hours sharp and is free of charge, under the condition that one attends the colloquium and that one has made a reservation before noon on the Tuesday preceding the colloquium.