**Abstract**

As quantum computers with sufficient computational power are becoming mature, the security of classical communication and cryptography may compromise, which is based on the mathematical complexity. Quantum communication technology is a promising solution to secure communication based on quantum mechanics. To meet the secure communication requirements of multiple users, multipointinterconnected quantum communication networks are specified, including quantum key distribution networks and quantum teleportation networks. The enabling technologies for quantum communication are the important bases for multipointinterconnected quantum communication networks. To achieve the better connection, resource utilization, and resilience of multipoint-interconnected quantum communication networks, the efficient network architecture and optimization methods are summarized, and open issues in quantum communication networks are discussed.

**Keywords:** multipoint-interconnected, quantum communication networks, quantum key distribution, quantum teleportation

#### **1. Introduction**

Quantum communication such as Quantum Key Distribution (QKD) and Quantum Teleportation (QT) is capable of exploiting the principles of quantum mechanics to transport classical, or even quantum, bits of information. quantum communication networks extend the concept of quantum communications, since they can transport, elaborate, and store quantum information (qubits) between different node pairs. Quantum communication networks leverage the principles of quantum mechanics including no-cloning, quantum measurement, entanglement, and teleporting. Hence, the new networking and computing capabilities emerge. At the same time, new and challenging constraints are imposed on the design and operations of quantum communication networks. This chapter firstly introduces the quantum communication enabling technologies including QKD and QT; then focuses on the research about QKD networks and QT networks to enable multipoint interconnection such as the architecture and service provisioning algorithms; finally, pays attention to problems and challenges of QT networking.

## **2. Quantum communication enabling technologies**

The realizations of quantum communication network mainly include quantum key distribution technology and quantum teleportation technology.

#### **2.1 Quantum key distribution**

Quantum cryptography, which applies quantum properties to design the secure communication system, is the subset of quantum communication. QKD technology is a realization of quantum cryptography. It 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. QKD has the characteristic of "point-to-point" implementation. Thanks to the developments of quantum relay and switching technologies, the long-distance QKD is enabled. The following two subsections briefly introduce the QKD implementation and the related quantum relay and switching technologies to realize long-distance quantum communication.

#### *2.1.1 Quantum key distribution implementation*

The first quantum key distribution (QKD) protocol, the famous BB84 protocol, was proposed by Charles Bennett and Gilles Brassard in 1984 [1]. Since then, a series of QKD protocols such as E91, B92, SARG04, COW, DPS, GG02, MDI-QKD have been proposed one after another. There are three main implementation technologies of QKD: Discrete-Variable Quantum Key Distribution (DV-QKD), Continuous-Variable Quantum Key Distribution (CV-QKD), and Measurement Device-Independent Quantum Key Distribution (MDI-QKD). DV-QKD encodes information on a single photon and uses a single-photon detector for detection. DV-QKD originated earlier and is more mature, with a longer safe transmission distance. Besides, multi-node quantum network has been successfully established. The disadvantage is that single-photon sources are tricky to prepare [2]. Unlike DV or qubit-based QKD, the secret keys in CV-QKD are encoded in quadrature of the quantized electromagnetic field and decoded by coherent detections, which is lower cost and more practical. Under the same conditions, the output key rate of CV-QKD is much higher than that of the DV-QKD, and it is highly integrated with traditional optical communication networks. However, the current CV-QKD technology is not as good as the DV-QKD technology in terms of safe transmission distance, and the problem of working bandwidth also needs to be further resolved [3]. The security of MDI-QKD does not depend on whether the quantum device is trusted or not. MDI-QKD completely removes all security loopholes in the detection system and ensures a QKD network security with untrusted relays. Compared with CV-QKD, MDI-QKD can obtain higher security key rate, but the communication distance is shorter, and the channel is required to be asymmetric (that is, the measurement equipment is required to be close to the user on one side) [4].

In the past 10 years, a series of small-scale QKD technology verification networks have been built abroad, covering local area networks, metropolitan area networks, and intercity networks [5–9]. At the same time, a number of major technical research studies have been carried out in China to address quantum secure communication. Local area networks, metropolitan area networks, intercity networks, and wide area networks have carried out related work, including the quantum communication Beijing-Shanghai trunk line project for connecting metropolitan area networks, and the planned satellite-ground integrated wide-area quantum communication network. To keep faint quantum signals apart from intensive classical data signals, traditional QKD networks utilize low-noise dedicated fibers, such as dark fibers, which will significantly increase QKD deployment cost. Also, researchers have studied how to combine QKD deployment onto existing optical networks [10].
