1000 years ago, if we were in Guangdong and wanted to send a letter to our family in Beijing, it would take a year or a half for a round trip.
100 years ago, we could send a telegram. It might take a few days to use it, and we charge by character.
Ten years ago, we could make a call and it only took a few seconds to connect.
Now, we can directly video call, the delay can be controlled at the millisecond level.
▲ 5G further reduces communication delay. Picture from: Apple's official website
The carrier of information transmission has gradually developed from manpower to radio waves, telephone lines and optical fibers. The information delay is getting smaller and smaller, but the maximum speed of light is 299792458 m/s. If the distance is far enough, there will still be a noticeable delay.
Is there a faster way to disseminate information?
In theory, it is quantum communication. But the current technology shows that it is difficult to achieve, so scientists have set their sights on "quantum encryption."
The University of Science and Technology of China announced on January 7 that the Chinese scientific research team has successfully realized the satellite-to-earth quantum key distribution spanning 4,600 kilometers, marking that China has built the prototype of the "space-earth integrated wide-area quantum communication network" , which has been in the UK Published in "Nature" magazine.
What is quantum?
The theoretical quantum communication is realized by using several mysterious properties of quantum:
- Quantum entanglement
- Not cloned
The theory proves that even if two entangled quanta are separated by a cosmic distance, one quantum changes, the other will change simultaneously, breaking the "speed of light cage" and realizing real real-time communication.
Please note that we have added an attribute before quantum communication, called "theoretically". This is because quantum communication still has difficulties to be overcome. Many people think that quantum communication is impossible to achieve. "Domain Quantum Communication Network" actually uses the indivisible and non-clonal nature of quantum to improve the confidentiality of communication . It is not perfect quantum communication in the true sense .
How to realize the 4600 km transmission distance?
The paper shows that the "space-earth integrated wide-area quantum communication network" is mainly composed of three parts.
- A quantum satellite "Mozi"
- More than 700 optical fiber quantum key distribution links
- Two ground test stations
▲The World Wide Quantum Communication Network . Picture from: University of Science and Technology of China
The communication network has deployed a total of 4 key queue managers in Beijing, Jinan, Hefei and Shanghai respectively, followed by a backbone optical fiber link over 2,000 kilometers and two ground-to-satellite links connecting Xinglong and Nanshan (2600 Kilometers)" connection.
At work, the "Mozi" satellite orbits in a sun-synchronous orbit at a height of about 500 kilometers. It is connected to ground stations in Xinglong and Nanshan for quantum key distribution. Finally, after an efficient BB84 protocol post-processing program, the final security key is extracted. (BB84 protocol is the world's first quantum cryptographic protocol, a quantum key distribution scheme jointly developed by quantum physicists Charles Bennett and Gil Brasard in 1984)
▲ Schematic diagram of quantum communication satellite and ground station experiment. Picture from: University of Science and Technology of China
Currently, researchers can obtain a total key size of approximately 36Mbit per week, and the key generated by "Mozi Number" can support approximately 6000 users.
What are the difficulties in this attempt?
Quantum is really too small. One breath, the light from an electric bulb, and a simple action can drive hundreds of millions of quanta, and there is uncertainty in a single quanta. It is not so easy to "control" the quantum completely. Therefore, there are many technical difficulties in the distribution of satellite-to-earth quantum keys that span 4,600 kilometers.
1. Single photon preparation
Pan Jianwei, Academician Pan Jianwei, who published a paper on "The World-Earth Integration Wide-area Quantum Communication Network" said that a light bulb of about 15 watts can emit ten trillion light quanta per second, and the preparation of a single light quantum is like One of them was captured at the instant when these ten trillion light quanta were emitted.
▲ The movie "Ant-Man" uses artistic techniques to show the scene of the microcosm. Picture from: "Ant-Man"
2. Single photon detection
A single photon is already the smallest unit of light energy, and the energy is very weak. It is necessary to develop a very precise and efficient single-photon detection technology. Only with the ability to prepare and detect a single light quantum can secure quantum communication be realized.
▲ The "Mozi" quantum science experimental satellite and Xinglong Quantum Communication Ground Station established a space-to-earth link taken on November 26, 2016. Picture from: University of Science and Technology of China
3. Satellite-to-earth communication is like dropping coins at a height of 10,000 meters
For example, Wang Jianyu, the executive deputy chief designer of the "Mozi" satellite, said, "To launch one photon to the ground station from an altitude of 1,000 kilometers, it is like a plane flying at a high speed at an altitude of 10,000 meters and two rotations to the ground at the same time. Throw coins into the slender money slot of the money box. The money slot of the money box is slender, which is equivalent to the polarization of light. It has a direction. Coins must be oriented correctly to be thrown in, and it is not Throw one, you have to throw both sides accurately."
4. Ground detection is far more difficult than clairvoyance
According to calculations, there are about 1017 photons when a match is struck, and the detection capability of the satellite ground station is equivalent to that a person on the earth will find someone struck a match on the moon. In addition, it is necessary to figure out which photon is detected on the ground within 100 million photons per second. These photons are all lined up. To detect one, you must know which is the first, so that it can be used to do Key.
Note: The source of the above technical difficulties is CCTV News.
On confidentiality, quantum encryption is the most reliable
Someone may have questions after reading it. The information still depends on satellite and optical fiber transmission. What does it have to do with quantum communication?
Let me talk about sending WeChat first. When we use mobile phones to send WeChat, all information must be encrypted with algorithms before being transmitted, otherwise the information will be easily intercepted and viewed.
However, all encryption algorithms can be cracked theoretically, and there is still a risk of information leakage. For example, commercial secrets, military information, etc., in order to ensure higher security, encryption algorithms are more complex than general information transmission.
▲ Diagram of RSA encryption and decryption process. Picture from: Baidu Baike
Is there a foolproof encryption method? Scientists thought that quantum keys could be used to encrypt information.
Quantum key distribution uses the "single quantum unclonable theorem" to achieve absolute security of key distribution. The "Unclonable Theorem" means that the process of completely copying any unknown quantum state is impossible, because the premise of copying is measurement, and measurement generally changes the state of the quantum.
▲ Schematic diagram of quantum entanglement, two entangled quanta, one of the quantum state changes, the other will also change simultaneously. Picture from: ScienceNews
According to the University of Science and Technology of China, the quantum key, as an information carrier, is inseparable from a single photon and its quantum state cannot be cloned, thereby ensuring that the encrypted content cannot be deciphered. Quantum key distribution is the only communication method that has been strictly proven to be unconditionally secure in principle.
To make an inaccurate analogy, the information encrypted by the quantum key is like a special envelope. As long as the person intercepting the envelope wants to open it, the letter inside will automatically disappear, and the interception becomes meaningless.
This time, the "Achievement of an Integrated World-Earth-wide Quantum Communication Network spanning 4,600 kilometers" in the "Nature" magazine, in popular terms, is to add quantum keys to traditional communication methods.
It is reported that quantum secure communication technology has provided information security for important national conferences and events such as the military parade to commemorate the 70th anniversary of the victory of the War of Resistance against Japan and the 19th National Congress of the Communist Party of China . In addition, the banking regulatory information reporting, RMB cross-border receipt and payment information management system, and online banking data remote disaster recovery system have all applied quantum secure communication technology.
The future of quantum communication
▲ Picture from: ScienceNews
In 2016, US cyber security technology provider Symantec revealed that a hacker group called "Strider" has carried out cyber attacks on China, Russia and other countries in the past five years. The hacker group has advanced technical methods. Symantec suspects that the team has the support of a national team.
In fact, this is only the tip of the iceberg of hacker attacks. Attacks are happening all the time and cannot be prevented. The "Heaven-Earth Integrated Wide-area Quantum Communication Network" and the 2000-kilometer Quantum Secure Communication Trunk were born under this background. The "Great Wall" of information security.
Many people have seen the tremendous role played by the quantum communication network, and should not help but ask: Will the current communication network based on optical fiber be replaced soon?
Academician Pan Jianwei said in an interview with Science and Technology Daily that quantum encrypted communication is not intended to subvert or replace existing communication methods, but can greatly improve the security of existing systems .
It seems that true quantum communication still has a long way to go.
#Welcome to follow Aifaner's official WeChat account: Aifaner (WeChat ID: ifanr), more exciting content will be provided to you as soon as possible.