1120 kilometers! "Mozi" hit another record
Economic Daily · China Economic Net reporter Shen Hui
1120 kilometers! Recently, the "Mozi" quantum scientific experimental satellite made new achievements: scientists used the "Mozi" as a source of quantum entanglement to distribute quantum entanglement to two distant places, achieving the first kilometer-level entanglement-based quantum in the world for the first time. Key distribution-laid an important foundation for quantum communication to practical applications.
Recently, scientists have used the "Mozi" for the first time in the world to realize the distribution of quantum keys based on entanglement in the thousands of kilometers. The results of this experiment not only increased the spatial distance of the previous groundless relay quantum secret communication by an order of magnitude, but more importantly, through physical principles, it was ensured that even in the extreme cases where the satellite is controlled by other parties, secure quantum communication can still be achieved. It has laid an important foundation for the application of quantum communication to reality.
The experiment was carried out by Pan Jianwei of the University of Science and Technology of China and colleagues, together with Artur Eckert of Oxford University, Wang Jianyu team of Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Microsatellite Innovation Research Institute, Optoelectronic Technology Institute and other related teams. The results have been published online recently in the international top academic journal "Nature" magazine.
Unconditionally secure quantum communication
Quantum is the most basic unit of matter, the basic carrier of energy, and cannot be divided and copied. For people, all well-known molecules such as molecules, atoms, electrons, and photons are collectively called quantum. Usually, photons are used to make quantum.
The so-called quantum key distribution is simply to generate and securely share a set of quantum keys between users in distant places to encrypt the transmitted binary information. Scientists have high hopes for this method of quantum communication because in addition to the sender and receiver of the information, if a third party attempts to copy or eavesdrop on the quantum key, the dual convenience of communication will be immediately noticed.
The realization of this new communication method is based on the characteristics of quantum superposition and non-replicability. According to classical physics, an object, such as a cat, has only two states, either alive or dead. But in the quantum world, this cat can be in a superimposed state of life and death. However, this superimposed state is extremely fragile, and once someone measures it, its state (life and death) will change immediately, instead of the original cat. In other words, if someone attempts to eavesdrop on the quantum key, it is necessary to measure the quantum state of the transmitted key in advance. However, the fragile superposition state causes the quantum itself to change its original state as soon as it is measured or copied, and thus be perceived by both parties to the communication .
"Quantum communication overcomes the inherent security risks of classic encryption technology because its security does not depend on computational complexity. This is a communication method that is unconditionally safe in principle. Once there is eavesdropping, it will be discovered." Pan Jianwei said.
In Pan Jianwei's view, quantum key distribution is like a person who wants to pass a secret to another person, and needs to pass the box containing the secret and a key to the receiver. The recipient can only obtain the secret by opening the box with this key. Without this key, others cannot open the box, and once this key has been moved by others, the sender will immediately find out-the original key will be invalidated, and then give a new key, until you are sure that the recipient can get it.
"The basic concept of quantum technology is to use the laws of quantum physics to manipulate microscopic substances, such as atoms, molecules, and electrons, so as to obtain functions that cannot be achieved by the macroscopic material world. Quantum cryptography is the key involved in this paper. In theory The communication after quantum encryption cannot be eavesdropped," said Yang Xiaohong, vice president of Natural Group.
Practical applications face challenges
Ideal is full, the reality is very skinny. In principle, the unconditionally safe quantum communication, from the laboratory to a wide range of applications, still faces two major challenges: one is the signal loss caused by long-distance transmission; the other is the security brought by the imperfection of real devices such as light sources and detectors. Sexual vulnerabilities. Therefore, it is not as simple as it sounds to realize long-distance quantum communication under realistic conditions.
Take signal loss as an example. Scientists generally use single-photon as a physical carrier to transmit the key, but because the single-photon signal cannot be amplified, plus the transmission channel-the absorption of single-photon by the optical fiber, as the transmission distance increases, the single-photon signal loss appears index increase. After more than 30 years of unremitting efforts, today, the international academic community has increased the distance of point-to-point fiber quantum key distribution to 500 kilometers in the laboratory.
How to realize quantum communication at a longer distance? Using trusted relay is an effective method. What is a trusted relay? Pan Jianwei believes that it can be understood as "relay running": a single photon runs from a ground to b ground in the fiber, but it can't run when running. At this time, a trusted node can be set to let the key "fall". , And then run forward by other photons. For example, using the "Mozi" as a relay, Chinese scientists achieved 7,600 kilometers of intercontinental communications on the free space channel.
"Theoretically, as long as no one climbs on the satellite to eavesdrop, then our communication is safe. However, the safety of these relay nodes still needs to be artificially guaranteed." Pan Jianwei gave an example: If people use satellites as the medium Following the node, the satellite has all the keys distributed by the user. As a result, there is another problem: if the satellite is manufactured by another country, there may be a risk of information leakage.
So, how to avoid this potential risk? In 2017, the "Mozi" achieved the free-space quantum entanglement distribution in the order of thousands of kilometers for the first time, successfully completing the established scientific goals. At this time, Pan Jianwei had a new idea: it took a lot of money to launch a satellite. Can you try to use "Mozi" as a source of quantum entanglement instead of a relay point for quantum keys to achieve entanglement-based Long-distance quantum key distribution?
The principle of quantum key distribution based on entanglement is that no matter how far apart two particles are in the entangled state, as long as the state of one of the particles is measured, the state of the other particle will be determined accordingly. "Using this feature, we can directly generate and securely share a set of quantum keys between users in two distant places to encrypt the transmitted binary information." Pan Jianwei said.
An important step towards secure communication
Running towards the peak of science, based on the preliminary experimental work and technical accumulation of the "Mozi" quantum satellite, the research team achieved a single-sided double and double-sided reception efficiency by upgrading the main optical and back optical paths of the ground telescope. Promote.
With technical support, the "Mozi" quantum satellite simultaneously established optical links with two ground stations, Urumqi Nanshan Station and Qinghai Delingha Station, at the speed of 2 pairs of photons per second over 1120 kilometers on the ground. Quantum entanglement is established between the two stations of, and the key is generated at a final code rate of 0.12 bits per second under a finite code length.
"In this experiment, the satellite as a source of quantum entanglement is only responsible for distributing quantum entanglement, and does not have any information on the quantum key; the key between users is directly generated through quantum entanglement, and no longer needs satellite transit." Pan Jianwei said Because the measurement of entangled particles is finally performed by the user, according to the characteristics of quantum entanglement, even if the source of the entanglement is provided by an untrusted other party, as long as the user finally detects quantum entanglement, a secure key can be generated. Therefore, the security problems caused by the imperfection of the source of quantum communication can be completely resolved, and finally the actual security of quantum communication is ensured.
In this regard, the reviewers of Nature magazine praised the work, "The experimental implementation of a long-distance entangled quantum key distribution protocol that does not rely on trusted relays is a milestone." Pan Jianwei admitted that when he just launched the "Mozi" satellite I dare not imagine that we can achieve today's results, and "lay eggs while walking" along the way, and finally have this important scientific breakthrough. "However, this scientific research achievement is currently just a demonstration of scientific principles, and there is still a long way to go before practical application." Pan Jianwei said.
Regarding the next step of development, he said that in combination with the latest development of quantum entangled source technology, 1 billion pairs of entangled photons can be generated on satellites in the future, and the final key code rate will be increased to tens of bits per second or a single transit 10,000 bits. By then, the dream of secure quantum communications will hopefully shine into reality.