With the rapid development of information technology, the security risks of communication networks are becoming more and more obvious. The computing power of electronic computers continues to increase, and traditional data encryption methods encounter great risks, and will be riddled with holes in the cracking of quantum computers.
The basic principle of secure communication of quantum information is to use the inaccurate measurement and non-copyability of quantum states. The information is encrypted with quantum keys on both sides of the communication cable. If the content of the information is captured or copied, the original quantum state will be destroyed. In order to make the transmitting party aware of the existence of the listener, quantum communication is also called a completely secure transmission data plan.
As everyone knows, quantum information is technically confidential, but it encounters many challenges in ultra-long-distance communication.
The secondary electrons in the quantum state are indispensable and cannot be copied. They cannot be copied and enlarged like traditional communication, which greatly limits the distribution distance of quantum keys in the optical fiber. Therefore, in the secure communication of quantum information based on the technology of secondary electrons, the transmission distance is quite large due to the loss in the route, and the optical fiber with lower attenuation coefficient is a reasonable method to increase the transmission distance.
Therefore, the application of ultra-low loss optical fibers in quantum communication will become more and more important.
So, what is very low loss fiber?
The loss of optical fiber mainly comes from the Rayleigh scattering loss and digestion and absorption loss of the core material. The traditional fiber needs to be mixed in the core to increase the refractive index of the core during production, but it will cause high Rayleigh scattering and fiber attenuation coefficient. The ultra-low loss fiber uses pure sio2 in the core, and the inclusion of the insulating layer reduces the refractive index, which not only reduces the attenuation coefficient caused by the core Rayleigh scattering, but also maintains the transmission of the reflection of the data signal light.
The refractive indices of basic germanium-doped core fibers and pure silicon-core fibers are spread. The application of pure silicon core technically maintains the reduction of the optical fiber attenuation coefficient, such as Corning's SMF-29 Lu ULL (Ultra LowLoss, extremely low loss) pure silicon core optical fiber, the attenuation coefficient at 1550 nm can be reduced to 0.20 dB/km (basic). 0.20 dB/km for fiber).
Application of ultra-low loss optical fiber in quantum communication
For quantum communication, improving the security communication distance, improving the security code rate, and improving the security factor of the system software are the three most critical technical goals for the application of quantum key distribution. So what is the main performance of ultra-low loss fiber at many levels here?
1) Improve safety driving distance
For long-distance and wide-area quantum key distribution, it needs to be maintained in two processes. First, the 100-kilometer quantum metro Internet is maintained based on optical fibers; then, the quantum intercity railway Internet is maintained based on reliable repeaters. This application field in my country is also a global precedent. The regional Beijing quantum trunk line opened in 2018 has a total length of 3,000 km and a total of 31 reliability relay stations. The average distance between each two relay stations is 62.5 km. However, if extremely low-loss optical fibers are used, the spacing between each relay station can be increased, and in theory, fewer and fewer reliable relay stations are required (as shown in Figure 3). The reduction of the total number of relay stations can not only reduce the capital investment of machinery and equipment; at the same time, it also reduces the potential security risks of all external chains (reliable relay stations are the stage where the security foundation of quantum information security technology is relatively weak), improving the external chain. The overall safety factor of the chain.
2) Improve the bit rate
The key conversion rate of quantum communication is the key index value to consider the software characteristics of the QKD system. A high code conversion rate can encrypt a large amount of statistical data, resulting in a more complicated data encryption management system, and can only reach The distribution of quantum keys that must be fast is of commercial use value. The coding rate will decrease exponentially with the increase of the spacing. Ultra-low loss fiber has a lower attenuation coefficient within the same transmission distance, so it can provide a higher coding rate under the same system setting. For example, for a distance of 150 km, the use of extremely low-loss optical fibers is about 3 dB lower than the outer chain attenuation coefficient of ordinary optical fibers, which significantly improves the system software key generation rate.
3) Promote the commercialization of co-fiber transmission of classical data signals and optical fibers
Application Scenario In the software of the quantum key distribution system of secondary electron technology, the quantum channel and the classical channel are transmitted separately from different optical fibers. This is because the signal strength of the quantum channel is much smaller than the compressive strength of the classical communication data signal. If the quantum channel and the classical communication are transmitted at the same time, the strong data signal of the classical channel will cause a series of discrete system effects, which will seriously damage the actual effect of the QKD system software transmission, such as Channel crosstalk, Raman scattering, spontaneous emission. If quantum communication and classical optical transmission system software can maintain common fiber transmission, it can greatly reduce the basic construction cost of quantum information security network communication, which is beneficial to the ease of use and marketing promotion of quantum information security communication.
At this stage, the European Philips European Laboratory, the University of Geneva in France, and the University of Madrid in Spain have all carried out relevant scientific research, and have maintained the test of gigabit optical communication, 10G wavelength division system software and QKD quantum channel multiplexing fiber. China, China Telecom Network and the National Shield of the University of Science and Technology have cooperated to conduct relevant scientific research, and have carried out experiments of 100M and 1000M optical communication and its wavelength division system software and QKD quantum channel using the same optical fiber. This experiment is the world's first commercial The quantum key distribution system software and the commercial 8 Tbps (90 × 150 Gbps) large-space aggregation wavelength division multiplexing system software shared a long-distance transmission experiment on the same fiber, and maintained a single-span transmission of about 150 km on the extremely low-loss fiber.
Therefore, after several scientific research organizations have tested and tested the ultra-low-loss optical fiber, the ultra-low-loss optical fiber has significant advantages in improving the safe communication distance, improving the security code rate and improving the safety factor of the system software. It will eventually promote the rapid development trend of quantum computing and quantum information security communication industry, and play a key role in the construction of Internet infrastructure in the period of quantum computing.
