Spinor Dynamics Of A Freely Expanding F=1 Bose-Einstein Condensate

Spin-exchange collisions drive coherent population oscillations between $m_F$ ground states in an optically trapped $F=1$ Bose-Einstein condensate that depend on the density and quadratic Zeeman shift. Due to the slow expansion of the condensate, spinor dynamics persist after release from the trap when time-of-flight detection is used, and the changing density during free expansion must be factored into the analysis, particularly if the effective magnetic field is changed during this time. The recent adoption of microwave dressings in spinor BEC experiments has improved the control over the quadratic Zeeman shift, $q$, and allowed for negative shifts and fast switching times for quench experiments. By switching the parameter $q$ at a variable time during the free expansion of a sodium $F=1$ Bose-Einstein condensate, the effects of a changing density and magnetic field on the spinor evolution were investigated. Our measurements agree well with a mean-field simulation under the single-mode approximation, and both experimental and simulation data are presented.