Microwave Control of Spin Dynamics in F=1 Sodium Spinor Bose-Einstein Condensates*

We present our latest experimental data on controlling spin dynamics in F=1 sodium spinor Bose-Einstein condensates via microwave dressing. By applying quenches and time-dependent microwave pulse sequences, we implement nonlinear atom interferometry in spin-space in the long evolution time regime, $t \gg h/c$, where $c$ is the spin-dependent interaction energy. We also investigate the breakdown of commonly made approximations such as the single-mode approximation and the undepleted pump approximation for certain parameters.