Coherent manipulation of spin wave vector for polarization of photons in   an atomic ensemble
  We experimentally demonstrate the manipulation of two-orthogonal components of a spin wave in an atomic ensemble. Based on Raman two-photon transition and Larmor spin precession induced by magnetic field pulses, the coherent rotations between the two components of the spin wave is controllably achieved. Successively, the two manipulated spin-wave components are mapped into two orthogonal polarized optical emissions, respectively. By measuring Ramsey fringes of the retrieved optical signals, the \pi/2-pulse fidelity of ~96% is obtained. The presented manipulation scheme can be used to build an arbitrary rotation for qubit operations in quantum information processing based on atomic ensembles. 
Shujing Li, Zhongxiao Xu, Haiyan Zheng, Xingbo Zhao, Yuelong Wu, Hai   Wang, Changde Xie and Kunchi Peng
physics.atom-ph