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1120 km! Recently, the "Mozi" quantum science experimental satellite re-emergence: scientists use "Mozi" as a quantum entanglement source, distribute quantum entanglement from the distant two places, and realize thousand kilometers-based entangled quantum Key distribution – lays an important foundation for quantum communication to reality. Recently, scientists use the "Mozi" in the international first time to achieve a thousand kilometers of entangled quantum key distribution.

The experimental results not only increase the spatial distance of the previous ground without relay quantum confidential communication, but more importantly, the physical principle ensures safe quantum communication even if the satellite is controlled by others, it is still possible to achieve safe quantum communication. It lays an important foundation for quantum communication. The experiment was completed by Pan Jianwei, China University of Science and Technology Pan Jianwei and his colleagues, and Oxford University Attori Ext, the Chinese Academy of Technology, China Academy of Sciences, Wang Jianyu team, Micro Satellite Innovation Research Institute, Optoelectronic Technology Research Institute and other related teams. This results have been published in the International Journal of Japanese Journals recently.

The quantum is the most basic unit constituting the substance, which is the basic carrier of energy and cannot be split and replicated. For people, all micro particles such as molecules, atoms, electrons, photons, etc. are well known, are collectively referred to as quantum.

Typically, photons are used to prepare quantum.

The so-called quantum key distribution, simply, that is, a set of quantum keys are generated between users in the remote two places, encrypted for the transferred binary information. This quantum communication method is highly hoped by scientists because, in addition to the sending, receiver, if a third party tries to copy or eavesher the quantum key, the communication between the communication will immediately perceive.

The implementation of this new communication method is based on quantum superposition and unproductive characteristics. According to classic physics, one object, such as a cat, there are only two kinds, or it is either, or die. However, in the quantum world, this cat can be in another day and die. However, this superimposed state is extremely fragile. Once some people go to measure, their state (alive and dead) will change, no longer the original cat. In other words, if someone tries to eavesdrop the quantum key, you need to measure the quantum state of the transfer key in advance. However, the fragile superimposed state causes the quantum to change the original state immediately, thus being perceived by communication. .

"Quantum communication overcomes the internal security hazards in classic encryption technology, because its safety does not depend on computational complexity, which is a communication method in principle unconditionally safe, once there is eavesdropping inevitable." Pan Jianwei said.

In Pan Jianwei, the quantum key is distributed, it is like a person who wants to pass secrets to another person, and it is necessary to transfer the secret box with a key to the recipient.

The receiver only opens the box in this key to take the secret. Without this key, others can’t open the box, and once this key is moved by others, the sender will immediately discover that the original key will be invalid, give a new key until the recipient himself will be. "Quantum technology, its basic concept is to use quantum physical law to manipulate microscopic substances, such as atoms, molecules and electrons, thereby obtaining a function of macro material world that cannot be realized, wherein the quantum password is intended to be the key involved in this paper.

In theory, communication with quantum encryption is unable to be eavesdropped.

"Natural Group Vice President Yang Xiaobo said.

Ideal is full, the reality is very skinny. In principle, there is unconditionally safe quantum communication, from the laboratory to wide application, still facing two major challenges: First, the signal loss brought by long-distance transmission; the second is the safety of real devices, such as light sources, detectors, etc. Sexual vulnerability.

Therefore, it is necessary to achieve a long-distance quantum communication under realistic conditions. It is not as simple as it sounds. Take signal loss as an example.

Scientists typically use single photons as a physical carrier to transmit keys, but because the single optical sub-signal cannot be enlarged, then the transmission channel-optical fiber is absorbed by the single photon, and the transmission distance is elongated, the single photon signal loss is index increase.

Through more than 30 years of persistent efforts, today, the international academic community will increase the distance from the point-to-point fiber quantum key in the laboratory to 500 kilometers. How to achieve a more far-distance quantum communication? Using trusted relay is an effective way to use.

What is credible? Pan Jianwei believes that it can be understood as "relay run": Single light is running to B in the fiber, can run, there is no strength, then set a trusted node to let the key "landed" , Then run forward by other photons.

For example, using the "Mozi" as a relay, Chinese scientists achieve 76,00 kilometers of Intercontinental communications in the free space channel. "In theory, as long as no one goes to the satellite, then our communication is safe. However, the security of these relay nodes still needs to be artificially guaranteed." Pan Jianwei got an example: if people use satellite as a Contrary to the node, the satellite masters all the keys distributed by the user.

As a result, another question: If this satellite is manufactured by other countries, there may be the risk of information disclosure. So how do I avoid this possible potential risk? In 2017, the "Mozi" first realized a thousand kilometers of free space quantum entangled distribution, successfully completed the established scientific goals. At this time, Pan Jianwei gave birth to a new idea: launching a satellite spent a lot of expenses, can you try to use the "Mozi" as a quantum entanglement source, rather than the quantum key’s relay point, realization based on entangled Long-distance quantum key distribution? The principle of the entangled quantum key distribution is that the difference between the two particles in the entangled state is much different, and as long as the state of one of the particles is measured, the state of the other particles will be determined accordingly. "Using this feature, we can generate directly between users in the remote land and securely share a set of quantum keys to encrypt the binary information transmitted.

"Pan Jianwei said. Running towards scientific peaks, based on the pre-test work and technology accumulation of" Mozi "quantum satellite, the research team upgraded the main optical path of the ground telescope, achieving single side double, bilateral four The increase in reception efficiency.

With the support of the technology, "Mozi" quantum satellite transit, at the same time, it established a light link with two ground stations of Nanshan Station and Qinghai Deal, and Qinghend, with the speed of 2 to the ground in the ground. More than 1120 km The quantum entanglement is established between the two stations, thereby generating a key at a finite code length at the final yard rate of each second.

"In this experiment, the satellite as a quantum entanglement source, only responsible for distributing quantum entanglement, no information on the quantum key; the user’s key is directly generated by quantum entanglement, no longer need satellite transfer." Pan Jianwei said Since the measurement of the entangled particles is finally made by the user, according to the quantum entanglement characteristics, even if the entanglement is provided, the security key can be generated as long as the user finally detects quantum entanglement.

Therefore, the safety issues that the quantum communication source is not perfectly resolved and ultimately ensure the real safety of quantum communications.

In this regard, the "Natural" magazine reviewer praised the work, "The experimental implementation of the long-distance entangled quantum key distribution agreement that does not rely on the trusted relay is a milestone." Pan Jianwei frankly, just launched the "Mozi" satellite I don’t dare to fantasize today’s results, all the way to go to the side of the egg, and finally have this important scientific research breakthrough.

"However, this scientific research is currently just a scientific principle demonstration, and there is still a long way to go at the actual application." Pan Jianwei said. Regarding the development of the next step, he said that combined with the latest quantum entanglement source technology, the future satellite can generate 1 billion parallel photons per second, the final key is increased to several tens of bits per second or single transit Wanbit. At that time, the dream of safe quantum communications will be expected to take reality.

(Reporter Shen Hui) (Original title: One 2,000 km! "Mozi No." re-creation