Dynamics in spinor condensates controlled by a microwave dressing field

We experimentally study spin dynamics in a sodium antiferromagnetic spinor condensate as a result of spin-dependent interactions $c$ and microwave dressing field interactions characterized by the net quadratic Zeeman effect ${q}_{\mathrm{net}}$. In contrast to magnetic fields, microwave dressing fields enable us to access both negative and positive values of ${q}_{\mathrm{net}}$. We find an experimental signature to determine the sign of ${q}_{\mathrm{net}}$ and observe harmonic spin population oscillations at every ${q}_{\mathrm{net}}$ except near each separatrix in phase space where spin oscillation period diverges. No spin domains and spatial modes are observed in our system. Our data in the negative ${q}_{\mathrm{net}}$ region exactly resembles what is predicted to occur in a ferromagnetic spinor condensate in the positive ${q}_{\mathrm{net}}$ region. This observation agrees with an important prediction derived from the mean-field theory: spin dynamics in spin-1 condensates substantially depends on the sign of ${q}_{\mathrm{net}}/c$. This work uses only one atomic species to reveal mean-field spin dynamics, especially the remarkably different relationship between each separatrix and the magnetization, of spin-1 antiferromagnetic and ferromagnetic spinor condensates.