Control of spin current in a Bose gas by bang-bang pulses
  We generate spin currents in an $^{87}$Rb spin-2 Bose-Einstein condensate by application of a magnetic field gradient. The spin current destroys the spin polarization, leading to a sudden onset of two-body collisions. In addition, the spin coherence, as measured by the fringe contrast using Ramsey interferometry, is reduced drastically but experiences a weak revival due to in-trap oscillations. The spin current can be controlled using periodic $\pi$ pulses (bang-bang control), producing longer spin coherence times. Our results show that spin coherence can be maintained even in the presence of spin currents, with applications to quantum sensing in noisy environments. 
Yujiro Eto, Mark Sadgrove, Sho Hasegawa, Hiroki Saito, and Takuya   Hirano
cond-mat.quant-gas