Decoherence of collective atomic spin states due to inhomogeneous   coupling
  We investigate the decoherence of a superposition of symmetric collective internal states of an atomic ensemble due to inhomogeneous coupling to external control fields. For asymptotically large system, we find the characteristic decoherence rate scales as $\sqrt{N}$ with $N$ being the total number of atoms. Our results shed new light on attempts for quantum information processing and storage with atomic ensembles. 
C. P. Sun, S. Yi, and L. You