Quantum key distribution protocols using entangled state
  We present three quantum key distribution protocols using entangled state. In the first two protocols, all Einstein-Podolsky-Rosen pairs are used to distribute a secret key except those chosen for eavesdropping check, because the communication parties measure each of their particles in an invariable measuring basis. The first protocol is based on the ideal of qubit transmission in blocks. Although it need quantum memory, its theoretic efficiency approximates to 100%. The second protocol does not need quantum memory and its efficiency for qubits can achieve 100%. In the third protocol, we present a controlled quantum key distribution using three-particle entangled state to solve a special cryptographic task. Only with the controller's permission could the communication parties establish their sharing key and the sharing key is secret to the controller. We also analyze the security and the efficiency of the present protocols. 
Jian Wang, Quan Zhang, Chao-jing Tang