The security of quantum key distribution (QKD) is based on the basic principles of quantum mechanics, and it thus has unconditional security in theory. In existing quantum key distribution systems, weak coherent sources (WCS) are often utilized as light sources, resulting in limited transmission distances, due to a high probability of vacuum pulses in these sources. Besides, there inevitably exist equipment defects in practical QKD systems, e.g., the phase modulator and intensity modulator have certain defects, causing distinguishability in higher dimensions of quantum states, and resulting in side-channel vulnerabilities. An eavesdropper can carry out corresponding attacks, and thus threaten practical security of QKD systems.
To overcome the above limitations, we propose an improved protocol on quantum key distribution based on monitoring heralded single-photon sources. Due to the simultaneity of parametric down-conversion photon pairs, we can precisely herald the arriving of one photon, by measuring the arrival time of another one. Through this way, we can greatly reduce the probability of vacuum states in the signal light, and increase the longest transmission distance of the QKD system. Moreover, a light source monitoring module is inserted into the sender’s side. By randomly select certain period to measure the Hong-Ou-Mandel interference between the signal light and the idle light through the source monitoring module, we can estimate the side-channel information leakage of the source, and then obtain the key generation rate.
Compared with the QKD protocol based on monitoring weak coherent sources, our present work can give a better performance in either the transmission distance or the key generation rate, especially when the interference error is large. In addition, in principle, our present work can also be extended to other quantum key distribution protocols, such as the measurement-device-independent protocols, to further improve the security and practicability of QKD systems. Therefore, our present work can provide valuable references for the large-scale application of quantum communication networks in the near future.