Improved nonorthogonal decoy state method in quantum key distribution with a parametric down-conversion source

With parametric down-conversion sources (PDCSs), the nonorthogonal decoy state protocol based on one vacuum and two weak decoy states is presented. The detection events on Bob’s side are divided into two groups depending on whether Alice gets a trigger or not: triggered components and nontriggered components. The triggered components are used to estimate the fractions and error rates of single-photon and two-photon pulses, and then the final secure key rate is deduced. Besides, both triggered and nontriggered components are used to deduce a more accurate value of the key generation rate. The simulation of the final key generation rate over transmission distance shows that the first method can obtain a key generation rate close to the theoretical limit of the infinite decoy state protocol, while the second method is better.