Quantum-Relay-Assisted Key Distribution over High Photon Loss Channels
  The maximum distance of quantum communication is limited due to the photon loss and detector noise. Exploiting entanglement swapping, quantum relay could offer ways to extend the achievable distance by increasing the signal to noise ratio. In this letter we present an experimental simulation of long distance quantum communication, in which the superiority of quantum relay is demonstrated. Assisted by quantum relay, we greatly extend the distance limit of unconditional secure quantum communication. 
An-Ning Zhang, Yu-Ao Chen, Chao-Yang Lu, Xiao-Qi Zhou, Zhi Zhao, Qiang   Zhang, Tao Yang, Jian-Wei Pan