Magnon Interferometry in a ${}^{87}$Rb Spinor Condensate

Low temperature properties of a many-body system are typically governed by the lowest energy excitations. In many magnetic system, these low energy excitations are magnons, a Goldstone boson created by spontaneous symmetry breaking of the order parameter. We report on performing interferometry with coherent magnon waves to measure the magnon dispersion relation and gap in a spin-1 ferromagnetic rubidium spinor condensate. We find a magnon mass slightly heavier than predicted by mean-field calculations of condensate with contact interactions. We also observe a nonzero gap, consistent with magnetic dipole-dipole interactions.