Repeat-Until-Success Generation of Symmetric States without External   Control
  In the present paper, we propose a "repeat-until-success" scheme induced by single particle measurement to generate arbitrary symmetric states based on spin network. This protocol requires no modulated controls during the whole process and it provides a persistent approach towards the desired symmetric state. As a special case, we demonstrate that W state can be created with unit probability within this framework. 
Qing Chen, Jianhua Cheng, Ke-Lin Wang, Jiangfeng Du