Sodium Bose-Einstein Condensates in the F=2 State in a Large-volume   Optical Trap
  We have investigated the properties of Bose-Einstein condensates of sodium atoms in the upper hyperfine ground state in a purely optical trap. Condensates in the high-field seeking $| F=2, m_F=-2>$ state were created from initially prepared $| F=1,m_F=-1>$ condensates using a one-photon microwave transition at 1.77 GHz. The condensates were stored in a large-volume optical trap created by a single laser beam with an elliptical focus. We found condensates in the stretched state $| F=2, m_F=-2>$ to be stable for several seconds at densities in the range of $10^{14}$ atoms/cm$^{3}$. In addition, we studied the clock transition $| F=1, m_F=0>$ $\to$ $| F=2, m_F=0>$ in a sodium Bose-Einstein condensate and determined a density-dependent frequency shift of $(2.44\pm 0.25) \times 10^{-12}$ Hz cm$^{3}$. 
A. G\"orlitz, T. L. Gustavson, A. E. Leanhardt, R. L\"ow, A. P.   Chikkatur, S. Gupta, S. Inouye, D. E. Pritchard and W. Ket...