Original title: Pan Jianwei’s team on Nature: how to realize 4600 km quantum communication network
Surging journalist Yu Hanqi
In 1989, when the first quantum key distribution (QKD) experiment was implemented in IBM laboratory, the line was only 32 cm, and because the equipment would make noise during operation, it was ridiculed that only the deaf could not crack quantum secure communication.
Now, under the series connection of Mozi quantum communication experimental satellite and Beijing Shanghai trunk line, China has realized 4600 km quantum secure communication network and provided services for more than 150 users.
At 0:00 Beijing time on January 7, pan Jianwei, academician of Chinese Academy of Sciences and professor of China University of science and technology, published an article in nature, the world’s top academic journal, explaining in detail how this “Lianliankan” is realized. The paper is entitled an integrated space to ground quantum communication network over 4600 kilometers.
This work shows that quantum technology is mature enough to be practical. The global quantum network can be realized by connecting more national quantum networks through terrestrial optical fibers and satellites.
Distance problem in quantum communication
The characteristic of quantum key is that it is encoded in the quantum state of photon. According to the quantum non cloning theorem, an unknown quantum state can not be accurately copied, and once it is measured, it will be destroyed. Therefore, once someone steals and tries to read the quantum key, it will be found.
There is also a disadvantage of non duplication, that is, the engineering can not be enhanced like the electrical signal. When photons are transmitted through long-distance optical fiber, loss will inevitably occur.
Coupled with the impact of environmental noise, under the current real world conditions, two ground users directly distribute quantum keys through optical fiber, and the longest distance can only reach about 100 km.
In the case of immature quantum repeater technology, 32 relay stations have been set up along the world’s first quantum secure communication trunk line “Beijing Shanghai trunk line” with a distance of 2000 km to “relay” and ensure the information security in the relay station by means of manual duty, network isolation and other means.
Of course, in addition to the available technologies, scientists are also exploring some more cutting-edge new technologies to solve the problem of distance. For example, two field quantum key distribution (tf-qkd). In this regard, the team of Wang Xiangbin and pan Jianwei of Jinan quantum technology research institute have cooperated to expand the double field quantum key distribution distance of real environment optical fiber from 300 km to 509 km.
On the other hand, high orbit satellites can be used as space-based relay stations. For long-distance or intercontinental users, due to the low attenuation level of quantum signal in free space and the negligible decoherence effect, satellite ground QKD has become the most attractive scheme.
In 2017, pan Jianwei’s team successfully distributed the quantum key to Hebei Xinglong ground station with the help of Mozi satellite, with the maximum distance of 1200 km and the average bit rate of 1.1 Kbps (1.1 kbps).
Although previous experiments have proved the feasibility of small-scale quantum man and key services, there are still several challenges to be overcome in the construction of practical large-scale quantum Wan.
The ground quantum communication optical fiber network has been providing services for more than 150 users. In this regard, pan Jianwei’s team demonstrated the core key technologies such as up conversion single photon detector, dense wavelength division multiplexing, efficient top and bottom transmission, real-time post-processing and monitoring, and the most important is to combat known quantum attacks.
Quantum key distribution device, control device and classical communication device. SPD is a single photon detector
In addition, they also increased the star ground QKD distance from 1200 km to 2000 km, with a corresponding coverage angle of 170 degrees, almost the whole sky. Remote users in Nanshan ground station can perform QKD with any node on the “Beijing Shanghai trunk line” without additional ground station or optical fiber link.
Satellite ground quantum communication network
Based on these breakthroughs, an integrated satellite ground quantum communication network is formed, which consists of a large-scale optical fiber network with more than 700 QKD links and two satellite ground free space QKD links.
The average bit rate of the network can reach 47.8.1kbps, which is more than 40 times higher than the previous Mozi experiment.
The optical QKD link is 2000 km long, while the satellite ground QKD link is 2600 km long. With the combination of the two phases, any user in the network can achieve quantum secure communication up to 4600 km long.
It can be seen from the schematic diagram that the satellite ground quantum communication network of the project includes four optical fiber quantum metropolitan area networks (red arrow) in Beijing, Jinan, Shanghai and Hefei, one “Beijing Shanghai trunk line” (Orange Line) and satellite ground links connecting Xinglong and Nanshan ground stations (blue box). Among them, the trunk line is 2000 km long, and the distance between the two satellite ground stations is 2600 km
At the same time, Xinglong ground station is also linked with Beijing quantum metropolitan area network through optical fiber.
The 1.2m Telescope at Nanshan ground station and the 1m telescope at Xinglong ground station
There are three kinds of nodes in each quantum man. The purple circle is the user node, the green circle is the all-optical switch, and the pink circle is the trusted node.
The paper introduces that the optical fiber network of quantum secure communication on the ground has been providing services for more than 150 users. So, how does quantum communication network architecture and management work? This paper takes the safe transmission from Beijing to Shanghai as an example.
Beijing users want to transmit information, the computer sends the command to the key management system to request the key, and looks for the classical path of classical information transmission to the router. The key management system checks whether the key is sufficient. If so, it sends the key to the computer; otherwise, it sends the command to the vector subsystem server to generate more keys. The quantum system server sends the command to the quantum control system, finds the best key generation path, and sends the key generation command. The key is generated in the quantum physical layer and stored in the quantum management system. After using the key to encode or decode the message, the information can be safely transmitted to the users in Shanghai.
Prospect of global quantum network
In conclusion, this work shows that quantum technology is mature enough to be practical. The global quantum network can be realized by connecting more national quantum networks through terrestrial optical fibers and satellites.
With the development of quantum signal manipulation technology, those new QKD schemes that are still in the laboratory stage will also be put into practice, such as device independent QKD, dual field QKD and so on.
Combining the device independent QKD with well calibrated devices, the QKD system can provide enough security in real conditions
Pan Jianwei’s team said that the “Beijing Shanghai trunk line” could be upgraded directly to adapt to these new schemes.
Looking forward to the future, with the expansion of quantum communication network to form more complex topology and complete loop, we can explore safe time-frequency transmission, basic test of quantum gravity and large-scale interferometry applications. Distributed quantum computing and quantum repeater may be realized in the near future.