Аннотация:The analysis of the security of quantum key distribution systems with respect to an attack withnondemolishing measurement of the number of photons (photon number splitting—PNS attack)is carried out under the assumption that in the communication channel in each parcel there is apure Fock state with a different number of photons, and the distribution of states by number ofphotons has Poisson statistics. In reality, in the communication channel in each parcel there arenot individual Fock states, but a pure coherent state with a random phase—a superposition ofFock states with different numbers of photons. The paper analyzes the necessary experimentalresources necessary to prepare individual Fock states with a certain number of photons from thesuperposition of Fock states for a PNS attack. Optical schemes for implementing such an attackare given, and estimates of experimental parameters at which a PNS attack is possible are made.