Experimental imperfections (dark counts and inefficiencies of detectors, multiphoton emissions of sources, losses in quantum channels ...) must be decreased for practical quantum key distribution. And the system must have required properties such as robustness, stability, and operability at an ambient temperature.

Here we propose a quantum key distribution (QKD) scheme utilizing one mode of spontaneous parametric down conversion gated (a heralded single photon source) by a photon number resolving detector. This photon number measurement is possible by using single-photon detectors operating at room temperature and optical fibers. By post-selection, the multiphoton probability in this scheme can be reduced to lower than that of a scheme using an attenuated coherent light and the key rate is increased.

We show the result of calculations which presents the advantage of our scheme and an experimental demonstration of quantum key distribution with a heralded single photon source.