Intercept-resend attacks in the Bennett-Brassard 1984 quantum key   distribution protocol with weak coherent pulses
  Unconditional security proofs of the Bennett-Brassard protocol of quantum key distribution have been obtained recently. These proofs cover also practical implementations that utilize weak coherent pulses in the four signal polarizations. Proven secure rates leave open the possibility that new proofs or new public discussion protocols obtain larger rates over increased distance. In this paper we investigate limits to error rate and signal losses that can be tolerated by future protocols and proofs. 
Marcos Curty, Norbert L\"utkenhaus