  How do isolated quantum systems approach an equilibrium state? We
experimentally and theoretically address 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. Starting
from a spin-polarized state we suddenly switch on the external field and
monitor the subsequent many-body dynamics. Our key observation is density
dependent relaxation of the total magnetization much faster than typical
decoherence rates. To determine the processes governing this relaxation we
employ different theoretical approaches which treat quantum effects on initial
conditions and dynamical laws separately. This allows us to identify an
intrinsically quantum component to the relaxation attributed to primordial
quantum fluctuations.
