  Dipolar interactions are ubiquitous in nature and rule the behavior of a
broad range of systems spanning from energy transfer in biological systems to
quantum magnetism. Here, we study magnetization-conserving dipolar induced
spin-exchange dynamics in dense arrays of fermionic erbium atoms confined in a
deep three-dimensional lattice. Harnessing the special atomic properties of
erbium, we demonstrate control over the spin dynamics by tuning the dipole
orientation and changing the initial spin state within the large 20 spin
hyperfine manifold. Furthermore, we demonstrate the capability to quickly turn
on and off the dipolar exchange dynamics via optical control. The experimental
observations are in excellent quantitative agreement with numerical
calculations based on discrete phase-space methods, which capture entanglement
and beyond-mean field effects. Our experiment sets the stage for future
explorations of rich magnetic behaviors in long-range interacting dipoles,
including exotic phases of matter and applications for quantum information
processing.
