Collective excitations in spinor Bose-Einstein condensate

The multiple broken symmetries in a ferromagnetic Bose-Einstein condensate lead to two Nambu-Goldstone bosons: a phonon and a magnon. A magnon --- a spin impurity excitation --- is a gapless mode with a quadratic dispersion relation. We introduce a new spin imaging technique to study the transport and dispersion of weak spin excitations in a $F=1$ ${}^{87}$Rb spinor condensates. We use this technique to measure the magnon mass, magnetic moment, and energy gap.