  In this paper, a photon-number-resolving decoy state quantum key distribution
scheme is presented based on recent experimental advancements. A new upper
bound on the fraction of counts caused by multiphoton pulses is given. This
upper bound is independent of intensity of the decoy source, so that both the
signal pulses and the decoy pulses can be used to generate the raw key after
verified the security of the communication. This upper bound is also the lower
bound on the fraction of counts caused by multiphoton pulses as long as faint
coherent sources and high lossy channels are used. We show that Eve's coherent
multiphoton pulse (CMP) attack is more efficient than symmetric individual (SI)
attack when quantum bit error rate is small, so that CMP attack should be
considered to ensure the security of the final key. finally, optimal intensity
of laser source is presented which provides 23.9 km increase in the
transmission distance. 03.67.Dd
