Research On The Key Technologies Of Fiber Quantum Key Distribution

For thousands of years, cryptography always plays an important and unique role in defense, military, and diplomatic fields. With the development of computer and internet technology, cryptography also plays an indispensable role in our daily life.The basic purpose of cryptography is securely transmitting confidential information over an insecure channel, which depends on the apllied cryptosystem. There are two main cryptosystems, one is symmetric key cryptosystem, the other one is public cryptosystem. The idea of quantum computer and quantum algorithms bring tremendous potential threaten to these two cyptosytems.The only proved secure cryptosystem is one-time pad, but it has the problem of key distribution. Quantum key distribution (QKD) can perfectly solve this problem. Base on the quantum theory, QKD can make enables two participants to share secure keys over an insecure public channel in real time. Therefore, combing QKD with the one-time pad cryptosystem, we can perfectly achieve unconditional secure communication.Since 1984, QKD has attracted a widespread concern, and many countries have put a lot of effort into the QKD research field. After the past two decades of developments, QKD has achieved significant improvements, both in theory and in experiments. The current QKD research is towards practicability. During my Ph.D period, key technologies of optical fiber QKD are the main research contents, including high-speed QKD system over long distance and networking technology of QKD network. The major results are outlined as follows:1. Based on the previous decoy state QKD experiment with the heralded single-photon source, we recalculated the secure rates in the case of intensity fluctuations.Compared with the weak coherent source, we found that the heralded single-photon source was a good source for decoy state QKD, since it not only has larger upper bound of transmission distance, but also more robust against intensity fluctuations.2. We improved the Faraday-Michelson QKD system, including the optical fiber system and the electronic control system. The speed of the improved QKD system increased to 20 MHz, and we applied the improved QKD system in the wavelength-saving QKD network.3. Based on the Faraday-Michelson interferometer, a differential phase shift (DPS) QKD system with the pattern generator, the superconducting single photon detector and the time-to-digit convertor was implemented. The clock rate of the system is 2 GHz, and the key was distributed over 260 km standard single mode fiber with 52.9 dB channel loss, of which the loss is one order of magnitude larger than previous record.4. Using the continuous wave ,a DPS QKD system was implemented ,and the transmission distance is 205 km with standard single mode fiber,which is the longest transmission distance with continuous wave.5. A robust hierarchical metropolitan QKD network was designed and built on the commercial fiber network of China Telecom Corporation Limited, in Wuhu. There are seven nodes in the QKD network, and five of them are in the government department. According to the priority, the network was divided into two layers, the QKD backbone network and the QKD subnet. The QKD backbone network is a real-time full connectivity network with QKD router in the center, the QKD subnet is a time division multiplexing full connectivity with QKD switcher in the center. And the two layers blend together using the trusted delay way.6. A wavelength-saving real-time full connectivity QKD router was proposed. Compared with our previous real-time full connectivity QKD router, the new router can save 50% to 75% wavelength source. The corresponding graph model of the wavelength-saving QKD router was found, and two internal connection modes of the router were proposed. Then we field tested the wavelength-saving QKD network over the commercial fiber network of China Telecom Corporation Limited, in Wuhu. The real-time full connectivity QKD network with five nodes was only supported by two wavelengths and the clock rate of the QKD link was 20 MHz. In addition, we proposed a simple corresponding mode to analysis the insertion loss and crosstalk of the QKD network.