Protecting sing-photon multi-mode W state from photon loss
  Single-photon entanglement is of major importance in current quantum communications. However, it is sensitive to photon loss. In this paper, we discuss the protection of single-photon multi-mode W state with noiseless linear amplification. It is shown that the amplification factor is only decided with the transmission coefficient of the variable fiber beam splitters, and it does not change with the number of the spatial mode. This protocol may be useful in current quantum information processing. 
Yu-Bo Sheng, Yang Ou-Yang, Lan Zhou, and Lei Wang
quant-ph