  The decoy-state method is widely used in practical quantum key distribution
systems to replace ideal single photon sources with realistic light sources by
varying intensities. Instead of active modulation, the passive decoy-state
method employs built-in decoy states in a parametric down-conversion photon
source, which can decrease the side channel information leakage in decoy state
preparation and hence increase the security. By employing low dark count
up-conversion single photon detectors, we have experimentally demonstrated the
passive decoy-state method over a 50-km-long optical fiber and have obtained a
key rate of about 100 bit/s. Our result suggests that the passive decoy-state
source is a practical candidate for future quantum communication
implementation.
