  Solid state systems derive their richness from the interplay between
interparticle interactions and novel band structures that deviate from those of
free particles. Strongly interacting systems, where both of these phenomena are
of equal importance, exhibit a variety of theoretically interesting and
practically useful phases. Systems of ultracold atoms are rapidly emerging as
precise and controllable simulators, and it is precisely in this strongly
interacting regime where simulation is the most useful. Here we demonstrate how
to hybridize Bloch bands in optical lattices to introduce long-range
ferromagnetic order in an itinerant atomic system. We find spontaneously broken
symmetry for bosons with a double-well dispersion condensing into one of two
distinct minima, which we identify with spin-up and spin-down. The density
dynamics following a rapid quench to the ferromagnetic state confirm quantum
interference between the two states as the mechanism for symmetry breaking.
Unlike spinor condensates, where interaction is driven by small spin-dependent
differences in scattering length, our interactions scale with the scattering
length itself, leading to domains which equilibrate rapidly and develop sharp
boundaries characteristic of a strongly interacting ferromagnet.

Quantum Gases