Security of quantum key distribution with a laser reference coherent state, resistant to loss in the communication channel

The problem of quantum key distribution security in channels with large losses is still open. Quasi-single-photon sources of quantum states with losses in the quantum communication channel open up the possibility of attacking with unambiguous state discrimination (USD) measurements, resulting in a loss of privacy. In this letter, the problem is solved by counting the classic reference pulses. Conservation of the number of counts of intense coherent pulses makes it impossible to conduct USD measurements. Moreover, the losses in the communication channel are considered to be unknown in advance and are subject to change throughout the series parcels. Unlike other protocols, differential phase shift (Inoue et al 2002 Phys. Rev. Lett. 89 037902, Inoue et al 2003 Phys. Rev. A 68 022317, Takesue et al 2007 Nat. Photon. 1 343, Wen et al 2009 Phys. Rev. Lett. 103 170503) and coherent one way (Stucki et al 2005 Appl. Phys. Lett. 87 194108, Branciard et al 2005 Appl. Phys. Lett. 87 194108, Branciard et al 2008 New J. Phys. 10 013031, Stucki et al 2008 Opt. Express 17 13326), the simplicity of the protocol makes it possible to carry out a complete analysis of its security. (letter)