We are implementing a quantum key distribution system, suitable for long distances and high key bit rates, which can be used either with weak coherent pulses or real single photon sources as for example quantum dots.
Single photon source is indispensable for realization of photon based quantum communication technologies such as quantum key distribution (QKD). Weak coherent pulse source is a practical, efficient and robust solution to generate stream of single photons, probabilistically. In this paper, a methodology for design and development of weak coherent pulse (WCP) source with high count rate of upto 10 Mcps is proposed and implemented. The generation of optical pulses with narrow pulse width and repetition rate of up to 100 MHz is achieved by developing a miniaturized pulse laser module. The weak coherent pulses are generated by heavily attenuating the laser pulses so as to reach the mean photon no. per pulse, μ, close to 0.1. WCP source performance in terms of mean photon number, second order coherence and photon count rate is also investigated. The measured g(2)[0] of ~0.98 shows that the developed WCP source has a very good quality.
Since the single photon is easy to attenuate and be disturbed, its key generation rate and transmission distance in quantum communication systems are generally limited. In contrast, weak coherent pulse (WCP) and heralded single photon source (HSPS) exhibit higher feasibility than the single photon source in quantum key distributions (QKD). We compare the performances of QKD using these two kinds of photon sources and decoy state method, incorporating data-postprocessing methods including the Lütkenhaus scheme and Gottesman-Lo-Lütkenhaus- Preskill (GLLP) scheme. Simulation results indicate that QKD using HSPS can transmit longer distance than using WCP with lower overall quantum bit error rate (QBER) at the same transmission distance, albeit with relatively low key generation rate.
Recently a long distance and high key rate quantum key distribution (QKD) has become possible by the idea of the decoy state method. We show that a longer distance QKD is possible by utilizing a heralded single photon source (utilizing spontaneous parametric down-conversion) as a source instead of a weak coherent pulse (WCP) as proposed in the original decoy state method. Moreover, the key rate is improved by utilizing a presently available photon number resolving detector as a trigger detector of the heralded single photon source and it is shown to approach the key rate of the WCP.