Passive Decoy State Quantum Key Distribution

The use of decoy states enhances the performance of practical quantum key distribution systems significantly by monitoring the quantum channel in a more detailed way. While active modulation of the intensity of the pulses is an effective way of preparing decoy states in principle, in practice passive preparation might be desirable in some scenarios. Known passive methods involve parametric down-conversion. In this paper we show how phase randomized coherent states can be used for the same purpose. Our method involves only linear optics together with a simple threshold photon detector. The performace is comparable to the active decoy methods.