**5. Open issues and challenges**

## **5.1 Survivability for QKD over SDONs**

QKD can provide secret keys for end-to-end paths and improve the security of SDONs. However, how to guarantee survivability in a QKD over SDON is an important topic. QCh and PCh should be protected simultaneously in a QKD over SDON. Especially due to the utilization of key-updating period (security level) with different time slots, protection action will occur at a subwavelength level. Synchronization might also be a difficult problem for QCh, PCh, and DCh.

### **5.2 Cost optimization for QKD over SDONs**

In a QKD network, two types of nodes should be deployed, i.e., QKD node and intermediate node with trusted repeaters. Also, several wavelength channels in existing WDM optical networks should be planned as QChs and PChs. In practice, different number of nodes and QChs/PChs may produce different costs and performance for QKD over SDONs. Accordingly, how to optimize the cost of deploying QKD over SDONs while satisfying the performance requirements is another open issue.

### **5.3 Key on demand (KoD) for QKD over SDONs**

The secret key rate (i.e., the generation of secret keys in bits per second) in current advanced QKD systems is extremely low compared with the gigabit data transmission over each wavelength in WDM optical networks. Increasing the number of nodes and QChs/PChs can further increase the secret key rate, but it will also drastically increase the system complexity and power consumption. Thus, the use of an efficient key on demand (KoD) scheme to achieve efficient secret key resource usage while satisfying security requirements of CChs and DChs is also essential for QKD over SDONs.

## **6. Conclusions**

This chapter provides a brief introduction to the basic principles and enabling technologies of QKD. Based on the QKD-enabling technologies, an architecture of QKD over SDONs is presented. Resource allocation problem is elaborated in detail and is classified into wavelength allocation, time-slot allocation, and secret key allocation problems in QKD over SDONs. Finally, several open issues and challenges are discussed.

**21**

**Author details**

provided the original work is properly cited.

Yongli Zhao\*, Yuan Cao, Xiaosong Yu and Jie Zhang

\*Address all correspondence to: yonglizhao@bupt.edu.cn

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

State Key Laboratory of Information Photonics and Optical Communications,

Beijing University of Posts and Telecommunications, Beijing, China

*Quantum Key Distribution (QKD) over Software-Defined Optical Networks*

*DOI: http://dx.doi.org/10.5772/intechopen.80450*

*Quantum Key Distribution (QKD) over Software-Defined Optical Networks DOI: http://dx.doi.org/10.5772/intechopen.80450*
