*2.1.2 Quantum relay and switching*

There are two main ways to achieve long-distance QKD, namely quantum relay technology and quantum switching technology. On the one hand, quantum relay technology can solve the problem of exponential attenuation of photon signal transmission in optical fiber for long-distance QKD. There are currently two types of quantum relay technologies. One is based on trusted relay, and the other is based on quantum relay. The trusted relay scheme is to cache the key generated by the point-to-point link in the trusted relay node and then transmit the end-to-end key required by the user hop-by-hop through the multi-hop link using one-time pad. This scheme breaks through the transmission distance limitation of the QKD link, but the relay node for key transmission must be trusted [11]. The quantum relay scheme is to use the principle of quantum entanglement to realize the storage and forwarding of quantum states, so as to realize the long-distance distribution of quantum states [12]. In order to overcome the fading of quantum information during quantum channel transmission, using quantum nodes instead of optical nodes to transform quantum information can effectively increase the transmission distance. Quantum nodes with this function are usually called quantum repeaters. This technology does not require trustworthy relay nodes, but it is still in the stage of theoretical research. On the other hand, in quantum switching technology, trusted relay is mainly used by switching nodes of quantum secure communication network based on single core fiber. Through relay nodes, the "Beijing-Shanghai trunk line" passes through Beijing, Jinan, Hefei, and Shanghai, connecting Beijing and Shanghai's quantum key distribution metro-network, which can provide data transmission based on quantum encryption for government affairs, finance, and other fields [13]. In 2018, Travis S. Humble et al. designed and implemented software-defined quantum networking protocol and soft switch to support the integration of quantum communication and existing optical communication [14]. In 2020, by integrating the fiber and free-space QKD links, the QKD network in China has been extended to a total distance of 4600 km, where any user in the network is able to communicate with any other [9].
