Multi-step quantum secure direct communication using multi-particle   Green-Horne-Zeilinger state
  A multi-step quantum secure direct communication protocol using blocks of multi-particle maximally entangled state is proposed. In this protocol, the particles in a Green-Horne-Zeilinger state are sent from Alice to Bob in batches in several steps. It has the advantage of high efficiency and high source capacity. 
Chuan Wang, Fu Guo Deng and Gui Lu Long