Quantum communication: You don’t understand my “magic”

For more than 2,000 years, passwords have been the main means for people to protect information. However, since someone created the password, someone else has tried every means to crack it. When various information security incidents occur one after another, people can't help but ask: Is there a communication method that can never be deciphered and the information is absolutely safe?

The answer is quantum communication, which is the only communication method that has been rigorously proven to be unconditionally secure so far.

Quantum is the smallest unit in the microscopic physical world. It sounds mysterious, but it is around us all the time. For example, common light is composed of a large number of light quanta. The concept of quantum is often used in physics, referring to an indivisible basic individual, for example, "quantum of light" (photon) is the unit of light. In modern scientific cognition, almost everything known is measurable, that is, the state of matter is determined; in the quantum world, some substances can be in multiple possible states at the same time.

Let's imagine that in the microscopic world, a quantum is like a dice, constantly rotating, with all sides facing up. Only when it is observed or measured will it stop and randomly present an upward side. However, this measurement itself means interference, which means that the original state is destroyed and changed. In other words, once a quantum is measured, it is no longer the original quantum.

Based on this principle, scientists use quantum state materials as the password for quantum communication, which is different from the mathematical calculations commonly used in existing electronic communications to generate keys. This seemingly small change has completely changed the security of the key. Quantum keys store digital keys in special quantum information and transmit them in quantum communication lines. When quantum information is eavesdropped, it will produce irreversible traces. Scientists use the non-replicability of quantum to achieve the purpose of confidential communication.

We can imagine that if an eavesdropper wants to eavesdrop on information at a certain place, or copy the information content, this is a measurement behavior. This measurement means the destruction of the entire system for the quantum system, and the result is that the measured information will all disappear. For example, once someone tries to open a letter, the quantum key will cause the letter to self-destruct and let the user know.

To pass on this key, scientists also used another feature of quantum theory: quantum entanglement. Entanglement is a quantum correlation phenomenon that Einstein called "spooky action at a distance": two microscopic quanta with a common source, even if they are separated by a solar system, as long as one quantum changes, the other quantum will immediately change accordingly, just like a pair of twins with "telepathy" with each other. The transmission speed between the two quanta is at least 10,000 times the speed of light, and the delay is almost zero, thus achieving the fastest communication.

Quantum communication is more secure

Schematic diagram of quantum communication

Unlike modern communication methods, as long as the entangled quantum is created and controlled, this absolutely safe, superluminal quantum communication without any electromagnetic radiation pollution can be realized. Moreover, quantum communication can also achieve long-distance communication, and scientific experiments have confirmed that it can even be used for interstellar communication.

Since American scientists proposed the concept of quantum communication in 1993, many countries have invested a lot of manpower and material resources in theoretical and experimental research on quantum communication. In this international competition, although my country is a latecomer, it has a high starting point and rapid progress, and has already led the world in many aspects of applied research.

In the field of quantum communication, my country has taken an international leading position. In 2016, my country successfully launched the world's first quantum science experimental satellite "Micius". The reason why my country wants to launch a quantum satellite is that the quantum communication technology that has been successfully developed by people now has a maximum distance of only 300 kilometers. It requires a large number of relay stations to build a long-distance quantum communication network. Through the quantum satellite in the sky, it is easy to realize a long-distance and even global quantum communication network.

In 2017, my country built the world's first commercial quantum secure communication line, the "Beijing-Shanghai Trunk Line", which is more than 2,000 kilometers long. From the earliest Anhui Wuhu and Hefei metropolitan area networks to the world's first commercial quantum secure communication trunk line "Beijing-Shanghai Trunk Line", if compared with transportation infrastructure such as railways and highways, the domestic quantum secure communication network has experienced the development from "subway network" to "high-speed rail network".

It can be expected that perhaps in the near future, people will be able to completely bid farewell to the threat of information leakage.

The world's earliest coded intelligence

In 405 BC, at the end of the Peloponnesian War between Athens and Sparta, the Spartan army captured an Athenian messenger who had a belt covered with chaotic Greek letters. When the belt was spirally wound around a wooden stick of a specific diameter, the letters magically formed a readable text. This was the world's earliest coded intelligence.