How to ensure that the secrets of heaven cannot be leaked? A brief discussion on quantum communication

With the rapid development of the times, the rapid transmission and widespread application of information have become an important part of people's lives and social development. However, information security issues have also arisen, especially in the fields of diplomacy, military, economy, etc., where the confidentiality of information is of vital importance. The emergence of quantum communication technology has brought new hope for solving this problem. It is based on the basic principles of quantum mechanics and precisely manipulates quantum states to achieve information transmission between two points. It is considered to be an unconditionally secure communication method.

The key to quantum communication

Quantum key distribution is a key link in quantum communication, and its core is to ensure the security of communication. In 1984, physicist Bennett and cryptographer Brassard proposed the "Quantum Key Distribution BB84" protocol based on the measurement principle of quantum mechanics. This protocol uses the polarization characteristics of photons to transmit keys. The polarization direction of photons corresponds to a specific binary code. The transmitter and receiver randomly select the measurement basis for measurement, thereby generating a set of binary quantum keys.

BB84 quantum key distribution protocol

Photons are the basic particles of light, and their polarization direction can be figuratively understood as the direction of the electric field vector in space. Common directions are horizontal and vertical, represented by H and V respectively. These two directions are perpendicular to each other and constitute a polarization basis. In addition, there are directions that are 45° to the horizontal and vertical directions, which are also often used in quantum key distribution protocols, represented by "+" (positive 45°) and "×" (negative 45°).

We can understand it this way. If a photon is compared to a coin, then the polarization direction of the photon is like the deflection angle of the coin. The BB84 protocol is like selecting four deflection angles of the photon and corresponding these deflection angles to specific binary codes. When distributing keys, the sender and the receiver need to randomly set two "holes", namely the measurement basis, such as "+" and "×", to allow "coins" (photons) with specific polarization directions to pass through. After the measurement is completed, the two parties exchange their respective measurement bases and measurement results through the classical communication channel. If the measurement results of both parties are consistent, it means that this part of the key is safe; if there is a certain probability of inconsistency, it means that there may be eavesdropping. This is because in the process of quantum key distribution, once there is an eavesdropper, his measurement behavior will inevitably interfere with the state of the photon. For example, when the eavesdropper measures the deflection angle of the "coin" (photon) and sends it to the receiver, since the measurement operation will change the deflection angle of the "coin" (photon), the receiver has a 1/4 probability of obtaining a different measurement result from the sender. Once such a discrepancy is discovered, the sender and receiver will stop key distribution. This indicates that the current key distribution process is not secure, so a secure environment needs to be selected again for operation until it is confirmed that there is no eavesdropping. This feature enables quantum key distribution to theoretically achieve unconditionally secure key sharing, fundamentally ensuring the security of the key.

Through the BB84 protocol, the sender and the receiver will eventually get an identical random key. Then they can use this key to encrypt real information. After the sender converts the information into binary plaintext through traditional cryptography, it can send the ciphertext to the receiver through a classical channel (ordinary network). After the receiver decrypts it with the same key, it will first get the original binary plaintext. After converting the binary plaintext into text through traditional cryptography, the receiver can successfully read the information.

Schematic diagram of BB84 quantum key distribution protocol

From the ground to space: the development and breakthrough of quantum communication

After scientists discovered the principle of quantum key distribution, what did they do to build a communication network? In terms of ground optical fiber network construction, the completion of the world's first quantum secure communication trunk line, the "Beijing-Shanghai Trunk Line", is an important milestone. However, ground optical fibers lose signals during transmission, which limits the safe distance of direct quantum communication transmission. In order to extend the transmission distance, multiple trusted relay stations need to be established, but this increases the risk of information being stolen. Therefore, how to extend the safe distance of direct transmission of optical quantum communication has become one of the difficulties in research.

A quantum communication network connecting the sky and the earth (diagram)

With the development of aerospace technology, scientists have turned their attention to outer space. The near-vacuum environment of outer space reduces the loss of optical signals. Using quantum satellites as transit stations can also connect metropolitan quantum communication networks in multiple cities, expanding the distance of quantum communication. my country's Tiangong-2 space laboratory carried out the "Special Project on Quantum Key Distribution". Its payload, the "Quantum Key Distribution Experimental Space Terminal", established a quantum channel through cooperation with the ground terminal, and conducted an air-to-ground quantum key distribution experiment, laying the foundation for the construction of a practical, space-to-earth integrated wide-area quantum secure communication network in the future. In addition, the "Mozi" quantum science experimental satellite also played an important role in the quantum key distribution experiment. It was connected to ground stations such as the "Beijing-Shanghai Trunk Line" to build a quantum communication network connecting the sky and the earth, achieving a key generation speed of several thousand bits per second, effectively protecting information security.

Quantum key distribution experimental space terminal (schematic diagram)

The application prospects of quantum communication technology are very broad. In daily life, when we conduct online banking transactions, browse videos online, order takeout, etc., quantum encryption technology may be silently guarding our information security behind the scenes to prevent personal privacy leakage and data theft. In the field of aerospace, quantum communication is indispensable for highly confidential data transmission. It can ensure the safe transmission of important information and avoid serious consequences caused by information leakage. With the continuous advancement of technology, quantum communication will play an important role in more fields, escort information security, and promote the development of society in a safer and more efficient direction.

As a revolutionary technology, quantum communication has made continuous breakthroughs from the ground to space with the unique advantages of quantum key distribution, showing great potential and application prospects in the field of information security, and is gradually turning "the secrets of heaven cannot be leaked" into reality.

Some information comes from the Chinese Academy of Sciences, Xinhuanet, the official website of the State Cryptography Administration, etc.:

(Scientific review: Lu Chaoyang, professor at the University of Science and Technology of China, executive director of the Shanghai Research Institute;

Peng Chengzhi, Distinguished Professor and Doctoral Supervisor at the University of Science and Technology of China, and Fellow of the Optical Society of America)

Editor-in-charge: Wang Lei