How do isolated quantum systems approach an equilibrium state? In collaboration with Prof. Matthias Weidemüller (Physics Institute, University of Heidelberg) and Prof. Jürgen Berges (Institute for theoretical physics, University of Heidelberg) we experimentally and theoretically addressed this question for a prototypical spin system formed by ultracold atoms prepared in two Rydberg states with different orbital angular momenta. By coupling these states with a resonant microwave driving, we realize a dipolar XY spin-1/2 model in an external field. Compared to laser-dressed Rydberg gases, this system allows for highly coherent dynamics over timescales much larger than the typical decoherence time. Starting from a spin-polarized state, we suddenly switch on the external field and monitor the subsequent many-body dynamics. Our key observation was density dependent relaxation of the total magnetization much faster than typical decoherence rates. To determine the processes governing this relaxation, we employed different theoretical approaches that treat quantum effects on initial conditions and dynamical laws separately. This allowed us to identify an intrinsically quantum component to the relaxation attributed to primordial quantum fluctuations. Our findings identify a fundamental component governing the relaxation of isolated many-body quantum systems and will motivate more efficient theoretical approaches for addressing non-equilibrium problems.
