Research Of Parametric Conversion Source Based Decoy-state Quantum Key Distribution

Cryptography plays an increasingly important role in finance, communication and military affairs, which come from the growth of demand for information security. How-ever, with the propose and development of quantum cumputer, classic cryptography would be impacted tremendously, since its complexity-based cryprography algorithms turn out to be pale before the computing ability and algrithms of quantum computers.Fortunately,quantum ky disribution (QKD) afford us an alternative to distribute secure keys, which is unconditionally secure and attracts extensive attentions. Combined with the classic proved unconditional security "one time pad", one can be ensured uncondi-tional secure communications.The unconditional security of QKD is based on the laws of quantum mechanics,the theories and experiments have been studied for more than 30 years, since the pro-posal of QKD in 1984. However, there is room to be improved for the performance of QKD systems at present, it is necessary to seek for new approaches to improve the performance of practical QKD systems, which can meet the increasing demand of se-cret communication. For instance, week coherent pulse generated from a laser usually used as the light source of a QKD system, owing to a Possion distribution of photon numbers, it suffer a high probability of vacuum state and relatively low probability of single photon, which could limits the transmission distance of a QKD system. Hence,we turned our attention to the sub-Possion photon number distribution, which almost has no vacuum state, and the probability of single photon is relatively high when the probability is normalised. When such source applied into a QKD system, combining the decoy-state method, one can obtain a comparable result with the ideal case.This dissertation focuses on my major research results in practical QKD systems,which are arranged as follows:1. We applied a sub-Possion source into the BB84 QKD and MDI-QKD protocol,which called the single-photon-added coherent state (SPACS). This source has no vacuum state in theory, and possess a rather high probability of single-photon than HSPS or weak coherent state (WCS). Following the decoy-state method, we numerically simulated the key rate and transmission distance for both protocol,and the results show an obvious advantage than the other two sources.2. HSPS generated from the spontaneous parametric down-conversion (SPDC) pro-cess, when measure the photon in one mode of the two mode state, the photons in the other mode can be heralded, then the vacuum pulse can be almost elimi-nated. We built a HSPS source based on the PPLN crystal with high brightness,applied the passive decoy-state into BB84 QKD protocol, and finally obtained secure keys at 200 km through our system, the key rate is comparable with WCS regime.3. The photon number distribution of WCS is Possion, the probability of vacuum state is high and the probability of single photon is relatively low when attenuated heavily, which could limits the transmission distance of a QKD system. To solve this problem, researchers have proposed a scheme which only based on the linear optical device, combined with the passive decoy-state method. We proposed an advanced scheme based on it, with which allowed us to precisely estimate the contribution of single photon, consequently promoted the performance of QKD system.