# Quantum Stream Cipher Based on Holevo-Yuen Theory
## 6 Future Outlook and Conclusions  
The current optical network was not laid out in a planned manner, but was configured by extending the existing communication lines for adapting the demand. In the future, the configuration and specifications of the optical network will be determined following to new urban planning. An actual example is the smart city that Toyota Motor Corporation et al. have disclosed as a future plan. Many ideas are also being discussed in other organizations. Recently, NTT has announced a future network concept so called IOWN. In these systems, the security of the all optical network with ultra-high speed is also important issue. The group of QKD and the group of Y-00 are promoting their respective technologies. However, recently, NSA and others announced the international stance on QKD [31]. They have a negative view of QKD, because the communication performance of QKD based on weak signal is not sufficient for applications to real situations. So, we do not employ QKD for key distribution of the initial key of Y-00, as shown in Figure 3 (Appendix B).  
**Examples of research reports on Y-00**  
**from the People's Republic of China**  
* **Army Engineering University of PLA, China** _IEEE Photonics J._**12**(4), 7904114 (2020). _Opt. Commun._**461**, 125151 (2020). _Opt. Express_**25** (10), 10947 (2017). _Quant. Inf. Process._**16**(8), 189 (2017).
* **Beijing University of Post and Telecommunications, China** _Opt. Fiber Technol._**52**, 101939 (2019). _Opt. Commun._**445**, 29 (2019). _OECC Technical Digest,_**501-3 (2018).**
* **Huazhong University of Science and Technology, China** _IEEE Access_**8, 63585 (2020).**  
On the other hand, the Y-00 quantum stream cipher is a technology that can realize the specification of high speed and long communication distance. In addition, the signals of Y-00 cipher with ultra-multiple-valued scheme for coherent state signal, so called  
Figure 7: Research activities on the Y-00 quantum stream cipher in China.  
quantum modulation, can have stronger quantum properties than QKD in the sense of quantum detection theory. So, the security is protected by many quantum no-go theorems (Appendix C). Although it is difficult to make an accurate prediction, there is a good chance that such a new technology will be used in the future. In view of the situation described in this paper, the Y-00 quantum stream cipher will contribute to real-world applications of quantum technology for Society 5.0, and new business development can be expected. Finally, we would like to note that Chinese research institutes have recently been actively working on Y-00 quantum stream cipher. Figure 7 shows a list of academic papers on their activities [32; 33; 34; 35; 36; 37; 38; 39]. It is expected that many research institutes will participate in this technological development.  
Conceptualization, M.S. and O.H.; methodology, M.S. and O.H.; validation, M.S. and O.H.; formal analysis, M.S. and O.H.; investigation, M.S. and O.H.; writing--original draft preparation, O.H.; writing--review and editing, M.S. and O.H. All authors have read and agreed to the published version of the manuscript.  
This research recieved no external funding.  
We are grateful to [PERSON], [PERSON] on experimental research, [PERSON], [PERSON] and [PERSON] for discussions on theory.  
The authors declare no conflict of interest.