*2.2.1 Quantum teleportation implementation*

QT is a quantum information transmission method using the uncertainty of quantum entanglement to realize the remote transmission of quantum states, which is one of the main technologies for constructing quantum communication network [15]. In 1993, Bennett et al. first proposed a theoretical protocol based on Einstein-Podolsky-Rosen entangled photons for teleportation [16]. The main idea is that the communication parties share a pair of entangled particles to establish a quantum

channel, and the sender will transmit the unknown. After the quantum state and the shared particle perform a specific measurement on the local particle, the measurement result is notified to the receiving end, and the receiving end user performs a quantum gate operation on the particles owned based on the measurement result to obtain the quantum state to be transmitted by the sending end. It is worth noting that in the process of QT, the physical particles at the sender are not transmitted to the receiver but always stay in the sender. What is transmitted is only the quantum state, and the sender can even have nothing to do with this quantum state.

In 1997, the Zeilinger Research Group in Austria first reported the QT experiment in "Nature" [17]. The experimental results confirmed the feasibility of QT with a success rate of 25%. Since then, many scholars have developed theories of QT, exploring how to use different entangled states to construct quantum channels in the process of teleportation or how to transmit multi-qubit quantum states. In the current teleportation network experiment, the challenge is taken and a 30 km optical-fiber-based quantum network distributed over a 12.5 km area is constructed, which is robust against noise in real world with active stabilization strategies, allowing us to realize QT with all the ingredients simultaneously [18]. In Calgary fiber network, QT is reported from a telecom photon at 1532 nm wavelength, interacting with another telecom photon onto a photon at 795 nm wavelength. It improves the teleportation distance to 6.2 km [19].

#### *2.2.2 Entanglement swapping and quantum repeaters*

Quantum entanglement is a unique property of quantum systems, and it is also an important communication resource in QCNs. In principle, quantum entanglement is based on quantum superposition state [20]. Since quantum superposition experiments only reflect the indistinguishability of physical processes and are not limited to any specific physical quantities (such as momentum, energy, position, polarization, etc.), quantum entanglement is essentially not necessarily related to any specific physical quantities. The characteristics of quantum superposition have led many scholars to use a variety of methods to successfully prepare entangled states in experiments. For example, there are two typical methods for entangled photon generation technology based on parametric down conversion. The first type of entanglement source is the II-type phase-matched nonlinear crystal entanglement source [21]. The second entanglement source uses collinear nonlinear crystals to generate entanglement [22]. In addition, there is also the use of photonic crystal fibers to generate entangled photon pairs [23].

### **3. Quantum key distribution network**

QKD generates and distributes symmetrical cryptographic keys with information theoretical security based on the fundamental laws of quantum physics, i.e., the security is independent of all future advances of algorithm or computational power. Quantum key distribution network (QKDN) is a network comprising two or more QKD nodes connected through QKD links, which allows sharing keys between the QKD nodes by key relay when they are not directly connected by a QKD link.

Although the international research on QKD is getting more and more in-depth, the research focus has always been on the performance improvement of the "pointto-point" QKD system, that is, how to increase the rate of quantum key generation, reduce the qubit error rate, and improve quantum key transmission distance, etc. It is difficult to use point-to-point QKD technology to support encryption requirements of various services from many nodes, and the security of services cannot be

guaranteed. Therefore, it is urgent to establish a QKDN that supports multipoint interconnection. This part will introduce the existed research about QKDNs, including the QKDN architecture, trusted repeater node structure, routing and resource allocation, key pool construction, resilience, and machine learning application.
