Is it safe to use quantum satellite positioning to check the weather?

Traditional communication relies on electromagnetic waves, which propagate in the air and anyone can get information. In order to solve this problem, quantum communication, a new interdisciplinary subject, has been developed in recent twenty years, which is a new research field combining quantum theory with information theory. Quantum communication has the characteristics of high efficiency and absolute security. The encryption level is higher and it is more difficult to be stolen.

Quantum communication Quantum communication refers to a new communication mode that uses quantum entanglement effect to transmit information. Quantum communication is a new interdisciplinary subject developed in recent twenty years, and it is a new research field combining quantum theory with information theory. Quantum communication mainly involves: quantum cryptographic communication, quantum teleportation and quantum dense coding. This subject has gradually moved from theory to experiment and developed into practice. Efficient and secure information transmission has attracted more and more attention. Based on the basic principle of quantum mechanics, quantum communication has the characteristics of high efficiency and absolute security, so it has become the research hotspot of quantum physics and information science in the world.

The basic components of quantum communication system include quantum state generator, quantum channel and quantum measuring device. According to whether the information it transmits is classical or quantum, it can be divided into two categories. The former is mainly used for the transmission of quantum keys, while the latter can be used for the distribution of quantum teleportation and quantum entanglement. The so-called invisible communication refers to a kind of "complete" information communication that is divorced from the real thing. From a physical point of view, we can imagine the process of invisible transmission: first extract all the information of the original, and then send it to the receiving place. Based on this information, the receiver selects the same basic unit as the original and makes a perfect copy of the original. However, the uncertainty principle of quantum mechanics does not allow all the information of the original to be accurately extracted, and this replica cannot be perfect. So for a long time, teleport was just an illusion.

1993, scientists from six different countries put forward a scheme to realize quantum teleportation by combining classical and quantum methods: transfer the unknown quantum state of a particle to another place, prepare another particle on the quantum state, and leave the original particle in its original place. The basic idea is to divide the original information into classical information and quantum information, and transmit them to the receiver through classical channel and quantum channel respectively. Classical information is obtained by the sender's measurement of the original object, while quantum information is the remaining information that the sender did not extract during the measurement. After receiving these two kinds of information, the receiver can prepare a complete copy of the original quantum state. In this process, only the quantum state of the original is transmitted, not the original itself. The sender can even know nothing about this quantum state, while the receiver puts other particles in the original quantum state.

In this scheme, the nonlocality of entangled States plays a crucial role. Quantum mechanics is a nonlocal theory, which has been confirmed by the experimental results that violate Bell's inequality. So quantum mechanics shows many counter-intuitive effects. In quantum mechanics, two particle states can be prepared in this way, and the relationship between them cannot be explained classically. Such a state is called an entangled state, and quantum entanglement refers to the nonlocal nonclassical correlation between two or more quantum systems. Quantum teleportation is not only of great significance for people to understand and reveal the mysterious natural laws in the field of physics, but also can be used as an information carrier to complete the transmission of large-capacity information through the transmission of quantum States, which is an unbreakable quantum secure communication in principle. 1997, Pan Jianwei, a young China scholar studying in Austria, cooperated with Boister, a Dutch scholar, to realize the long-distance transmission of unknown quantum states for the first time. This is the first time in the world to successfully transfer the quantum state from the photon at A to the photon at B. In the experiment, only the "state" of quantum information is transmitted, but the photon itself as the information carrier is not transmitted. In order to teleport a quantum state over a long distance, it is often necessary to let two distant places have the largest quantum entangled state in advance. However, due to various unavoidable environmental noises, the quality of quantum entangled States will get worse and worse with the increase of transmission distance. Therefore, how to purify high-quality quantum entangled States is an important topic in quantum communication research.

Many international research groups have been studying this topic and put forward a series of theoretical schemes for the purification of quantum entangled States, but none of them can be realized by the existing technology. Pan Jianwei et al. discovered a theoretical scheme of purifying quantum entangled states by using the existing technology, which is feasible in experiment, and solved the fundamental problem in long-distance quantum communication in principle. This research achievement is highly praised by the international scientific community and is called "a leap in the research of long-distance quantum communication".