  We have experimentally observed the dynamics of an antiferromagnetic sodium
Bose-Einstein condensate (BEC) quenched through a quantum phase transition.
Using an off-resonant microwave field coupling the F = 1 and F = 2 atomic
hyperfine levels, we rapidly switched the quadratic energy shift q from
positive to negative values. At q = 0 the system undergoes a transition from a
polar to antiferromagnetic phase. We measured the dynamical evolution of the
population in the F = 1, m_F = 0 state in the vicinity of this transition point
and observed a mixed state of all 3 hyperfine components for q < 0. We also
observed the coarsening dynamics of the instability for q<0, as it nucleated
small domains that grew to the axial size of the cloud.
