Experimental passive decoy-state quantum key distribution

The decoy-state method is widely used in practical quantum key distribution systems to replace ideal single photon sources with realistic light sources of 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 experimentally demonstrate the passive decoy-state method over a 50 km long optical fiber and obtain a key rate of about 100 bit s−1. Our result suggests that the passive decoy-state source is a practical candidate for future quantum communication implementation.