Analysis of coherent quantum cryptography protocol vulnerability to an   active beam-splitting attack
  We consider a new type of attack on a coherent quantum key distribution protocol [coherent one-way (COW) protocol]. The main idea of the attack consists in measuring individually the intercepted states and sending the rest of them unchanged. We have calculated the optimum values of the attack parameters for an arbitrary length of a channel length and compared this novel attack with a standard beam-splitting attack. 
D.A. Kronberg, E.O. Kiktenko, A.K. Fedorov, and Y.V. Kurochkin
quant-ph cs.IT math.IT