# Quantum-Key-Distribution (QKD) Networks Enabled by Software-Defined Networks (SDN)
## 1 Introduction  
With rapid developments in Internet of Things technology, more secure communication for users is required with increasing demands in information networks, so as to overcome perceived security threats as much as possible. As a promising technology, quantum key distribution (QKD) has been proven to provide users with secure keys exploiting the laws of quantum physics, i.e., [PERSON]'s uncertainty principle and no-cloning theorem [1]. These features allow two users to know if there is any eavesdropping during the communication process between them [2]. To extend QKD for multiple users, QKD networks have been studied and developed around the world in the past decades, which mainly uses laying fibers as basic transmission medium to serve security demands with secret-key provisioning [3; 4; 5]. Furthermore, the construction of QKD backbones and metro-area networks currently has been launched with large investment, and their development is attracting great attention around world.  
Traditional QKD is limited to point-to-point connectivity in the physical layer, using resources like wavelengths and time slices having different capacities depending on the point-to-point QKD demand. A QKD network however, needs the ability to allocate different resources in a global manner using a unified control plane for the easier operation. To address this problem, software-defined networking (SDN) has gained popularity by dividing networks into data plane and control plane and supporting programmability of network functionalities [6; 7]. The core idea of SDN is to realize flexible control of traffic and make the network more intelligent by separating control and data planes. The control plane can grasp the global network view and make it convenient for operators to manageand upgrade the network efficiently. In the coming years, QKD networks enabled by SDN will be an important scenario for developing multi-user cases. On the one hand, QKD networks can be controlled by SDN for the unified interaction of network devices and protocols [8]; on the other, QKD can be a secure solution for SDN-based networks [9]. Therefore, there are some researches focused on the topics of QKD networks with SDN. A QKD-enabled optical network architecture is proposed to add an additional layer, i.e., QKD layer, for secret keys in software-defined optical networks (SDONs) [10]. Moreover, some key-assignment schemes are developed to secure control signals and data services and enhance their security in SDONs [11; 12; 13] with wavelength division multiplexing (WDM) [14] and optical time division multiplexing (OTDM) [15; 16]. However, there is a lack of studies addressing systematic secret-key allocation with centralized control and coordination of the QKD resources.  
In this paper, by introducing SDN technology into the management of QKD networks, we carefully described the architecture of QKD networks enabled by SDN, including available interfaces and protocols in the networks. To solve three important issues in QKD networking, we have designed multi-resources allocation, secret-key management and survivability guarantee to provide reference results. To explain these with specific details, we have structured the paper as follows. Section 2 introduces recent progresses of QKD networks, and Section 3 describes the architecture of QKD networks enabled by SDN. The related interfaces and protocols in QKD networks enabled by SDN are shown in Section 4. Section 5 presents three useful cases in QKD networks enabled by SDN. Section 6 finally concludes this paper.