# Quantum Key Distribution Networks: Challenges and Future Research Issues in Security
## 3 Key Results of Existing QKD Networks  
This section surveys existing research studies and reports to summarize the key results in existing QKD networks. To prove the practicality of QKD networks, some countries and research institutes have invested enormous resources in practical field experiments on QKD networks. BBN Technologies and Harvard and Boston Universities [20; 21; 22] proposed the first QKD network--the DARPA quantum network--which has 10 quantum nodes and adopts a hybrid network type (i.e., active optical switch and trusted node networks). The DARPA network adopts the BB84 protocol to generate unconditional security keys and achieved the best performance of key generating rate of 400 bps over 29 km. Subsequently, in 2004, the European Commission's (EC) integrated FP6 Project Secure Communication based on Quantum Cryptography (SECOQC) launched a major project--SECOQC QKD Network [23; 24; 25]--to define the practical applications of QKD networks and to further analyze the issues associated with QKD networks, including their security, design and architecture, communications protocols, and implementation methods. SECOQC clearly indicates that QKD networks are the infrastructure for providing key distribution and secure communication in future Internet environments. The SECOQC QKD network adopts a trusted node network framework and has six quantum nodes. In addition, six different technologies (including attenuated laser pulse, one-way weak coherent pulse, entanglement photons, and free space) are used to establish quantum links, and five different QKD protocols are adopted to distribute the local keys. The best performance in terms of the key generation rate of 3.1 kbps over 33 km was achieved by the SECOQC QKD network. The SECOQC network lays the groundwork and provides a guide for IP and routing protocols for implementing QKD networks. The Tokyo UQCC (Updating Quantum Cryptography and Communication) QKD testbed network was launched in Japan [26; 27] since 2010. It employs four access nodes and six repeater nodes to form the infrastructure of the QKD network and distribute the local keys via both the BB84 and BBM92 [5] protocols. A live demonstration of secure TV conferencing using the key distribution service of this QKD network was presented in October 2010. The best performance in terms of the key generation rate achieved by the Tokyo UQCC QKD network was 304 kbps over 45 km. The QKD networks have been implemented and tested on a large scale in China, and four main QKD network trials have been performed--the Beijing-Shanghai QKD network [28; 29; 30], the Jinan Government Private QKD Network [31; 32; 33], the Wuhan QKD Network [29], and the Hefei-Chaohu-Wuhu QKD Network [28; 29; 35]. The four QKD networks all adopted the trusted node network and the BB84 protocol to provide services of QKD networks. The numbers of quantum nodes in these QKD networks were 32, 32, 71, and 9, respectively. Notably, the Jinan government private QKD network and Wuhan QKD network use the client-server architecture to organize quantum nodes, that is, a central control node to manage the routing tables and services. The best performance in terms of key generation rate achieved among the four QKD networks was 250 kbps over 43 km by the Beijing-Shanghai QKD network. Table 1 summarizes the key results of the above-mentioned QKD networks. In addition to improving the key generation rate and communication distance, enhancement of the routing algorithm and QoS are important and interesting research issues. The DARPA QKD network adopts the open shortest path first (OSPF) algorithm [67] to design the routing protocol. To accelerate the development process of the SECOQC QKD Network, a modified OSPFv2 protocol [67] was adopted, even though QoS cannot be supported by the OSPFv2 protocol. In another study [36], [PERSON]'s algorithm was used to design the routing protocol of the QKD network, and two performance indicators were proposed to evaluate the proposed routing protocol. [PERSON] et al.