Quantum Communication

 development process

In 1993, CHBennett put forward the concept of quantum communication; in the same year, 6 scientists from different countries proposed a scheme to realize quantum teleportation by combining classic and quantum methods: transferring the unknown quantum state of a particle to another In one place, another particle is prepared to the quantum state, while the original particle remains in place. The basic idea is to divide the original information into two parts: classical information and quantum information, which are transmitted to the receiver through the classical channel and quantum channel respectively. Classical information is obtained by the sender performing some kind of measurement on the original substance, and quantum information is the rest of the information that the sender did not extract during the measurement; the receiver can prepare the original quantum state after obtaining these two kinds of information Fully reproduced. In this process, only the quantum state of the original substance is transmitted, not the original substance itself. The sender can even know nothing about this quantum state, while the receiver puts other particles in the quantum state of the original matter. In this scheme, the non-locality of entangled states plays an extremely important role. Quantum teleportation is not only important in the field of physics for people to understand and reveal the mysterious laws of nature, but also can use quantum state as an information carrier to complete the transmission of large-capacity information through the transmission of quantum state, achieving in principle undecipherable Quantum confidential communication.

 In 1997, Pan Jianwei, a young Chinese scholar studying in Austria, collaborated with Dutch scholar Bomiister and others to realize the remote transmission of unknown quantum states for the first time. This is the first time in the world that a quantum state has been successfully transferred from a photon in Earth A to a photon in Earth B. In the experiment, only the "state" of the quantum information is expressed, and the photon itself as an information carrier is not transmitted.

 In 2012, Chinese scientist Pan Jianwei and others successfully achieved the first 100 kilometers of free space quantum teleportation and entanglement distribution in the world, laying the technical foundation for launching the world’s first "quantum communication satellite". The international authoritative academic journal "Nature" magazine focused on this achievement on August 9. "Successful transmission of 100 kilometers on a high-loss ground means that the transmission distance in low-loss space can reach more than 1,000 kilometers, basically solving the problem of long-distance information transmission of quantum communication satellites." Research team member Peng Chengzhi said that Quantum The breakthrough in the core technology of communication satellites also shows that it is technically feasible to construct a global quantum communication network in the future. On August 9, the international authoritative academic journal "Nature" focused on this achievement, representing its general recognition in the international academic community. "Nature" magazine said that it is "promising to become a milestone of long-distance quantum communication", "towards a global quantum network", the European Physical Society website, the United States "Science News" magazine and other special reports.

 Fundamental contents

 The so-called quantum communication refers to a new type of communication method that uses the quantum entanglement effect to transmit information. It is a new interdisciplinary development developed in the past two decades and a new research field combining quantum theory and information theory.

 Optical quantum communication is mainly based on the theory of quantum entangled states, using quantum teleportation (transmission) to achieve information transfer. According to experimental verification, no matter how far apart two particles with entangled states are, as long as one changes, the other will also change instantaneously. The process of using this feature to realize optical quantum communication is as follows: a pair of particles with entangled states are constructed in advance. The two particles are placed on the two sides of the communication, and the particles with unknown quantum state and the sender's particles are jointly measured (an operation), and the receiver's particles instantaneously collapse (change), and the collapse (change) is in a certain state. This state is symmetrical with the state of the sender's particles after collapse (change), and then the joint measurement information is transmitted to the receiver through the classic channel, and the receiver performs unitary transformation on the collapsed particles according to the received information (equivalent to In the inverse transformation), you can get the same unknown quantum state as the sender.

  Compared with optical quantum communication, classic communication cannot be compared with its security and efficiency. Security-Quantum communication will never "leak". The key to quantum encryption is random. Even if it is intercepted by a stolen person, the correct key cannot be obtained, so the information cannot be cracked. Two particles with entangled states in one's hand, the quantum state of one of the particles changes, the quantum state of the other side will change immediately, and according to quantum theory, any macroscopic observation and interference will immediately change the quantum state, causing it Collapsed, so the information obtained by the stealers due to the interference has been destroyed, not the original information. Efficient, the unknown quantum state that is transmitted will be in an entangled state before being measured, that is, it represents multiple states at the same time, for example, a quantum state can represent two numbers 0 and 1 at the same time, 7 such quantum states can represent 128 simultaneously Status or 128 digits: 0~127. Such a transmission of optical quantum communication is equivalent to 128 times of the classic communication method. It is conceivable that if the transmission bandwidth is 64 bits or higher, then the difference in efficiency will be an astonishing power of 2 to N, and higher.

 Here is a further explanation of quantum entanglement. Quantum entanglement can be understood with "Schrodinger's cat": when you put a cat in a box containing poison, and then cover the box, after a while, ask if the cat is dead or alive? Quantum The answer to physics is: it is both dead and alive. Some people will say that you don’t know if you open the box and look at it. Yes, you will know if the cat is dead or alive when you open the box, but according to the interpretation of quantum physics: this dead or alive state is the result of human observation, that is, Human macro disturbances make cats dead or alive, not the true state when the box is covered. Similarly, microscopic particles have been in the "dead" and "live" states before being "disturbed" It can be said that it is both "0" and "1".

 Quantum communication has the characteristics of high efficiency and absolute security, and it is the research hotspot of international quantum physics and information science at this moment. To trace the origin of quantum communication, we have to start with the evidence of Einstein's "ghost", quantum entanglement.

 Because people have been skeptical about the interaction between entangled particles, physicists have been trying to verify whether this magical property is true for decades.

 In 1982, the French physicist Alain Aspect and his team successfully completed an experiment that confirmed the existence of the phenomenon of "quantum entanglement" of microscopic particles. This conclusion It has a major impact on the mainstream worldview of Western science. Since Descartes, Galileo, and Newton, the mainstream thinking of the Western scientific community believes that the components of the universe are independent of each other, and the interaction between them is limited by space and time (that is, localized). Quantum entanglement confirms the existence of Einstein's ghost-spooky action in a distance. It proves that between any two substances, no matter how far away, they may affect each other and are not affected by four-dimensional space-time. Constraint is nonlocal, and the universe has deep internal connections in the world.

 On the basis of quantum entanglement theory, in 1993, American scientist C.H. Bennett proposed the concept of quantum communication (Quantum Teleportation). Quantum communication is a communication method that carries information by quantum states. It uses the principle of quantum entanglement of elementary particles such as photons to implement a secure communication process. The introduction of the concept of quantum communication made Einstein's "spooky"-quantum entanglement benefit begin to really exert its true power.

 In 1993, after Bennett proposed the concept of quantum communication, six scientists from different countries, based on the theory of quantum entanglement, proposed the use of classical and quantum methods to achieve quantum teleportation, that is, the unknown quantum of a particle The state is transferred to another place, and another particle is prepared to the quantum state, and the original particle remains in place. This is the original basic scheme of quantum communication. Quantum teleportation is not only important in the field of physics for people to understand and reveal the mysterious laws of nature, but also can use quantum states as information carriers to complete the transmission of large-capacity information through the transmission of quantum states to achieve quantum indecipherable in principle. Confidential communication.

 In 1997, Chinese young scholar Pan Jianwei who studied in Austria cooperated with Dutch scholar Bomiister and others to realize the remote transmission of unknown quantum states for the first time. This is the first time in the world that a quantum state has been successfully transferred from a photon in Earth A to a photon in Earth B. In the experiment, only the "state" of the quantum information is expressed, and the photon itself as the information carrier is not transmitted.

 After more than 20 years of development, the discipline of quantum communication has gradually moved from theory to experiment and practical development. The main fields involved include: quantum cryptography communication, quantum remote teleportation, and quantum dense coding.

 Quantum communication systems are divided into two categories according to whether the information they transmit is classical or quantum. 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 transmission refers to a kind of "complete" information transmission that is separated from the real thing. From the perspective of physics, the process of invisible transmission can be imagined as follows: first extract all the information of the original, and then transmit the information to the receiving place, and the receiver selects the same basic unit that constitutes the original based on the information, and manufactures A perfect replica of the original. However, the uncertainty principle of quantum mechanics does not allow accurate extraction of all information of the original, and this replica cannot be perfect. So for a long time, teleportation was nothing but a fantasy.

 In 1993, six scientists from different countries proposed a scheme that uses classical and quantum methods to achieve quantum teleportation: to transfer the unrecognized quantum state of a particle to another place and prepare another particle to On the quantum state, the original particles remain in place. The basic idea is to divide the original information into two parts: classical information and quantum information, which are transmitted to the receiver through the classical channel and quantum channel respectively. Classic information is obtained by the sender taking a certain measurement of the original substance, and quantum information is the rest of the information that the sender did not extract in the measurement; after the receiver has obtained these two kinds of information, the original quantum state can be prepared completely replica. In this process, only the quantum state of the original substance is transmitted, not the original substance 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 non-locality of entangled states plays a crucial role. Quantum mechanics is a non-local theory, which has been confirmed by experimental results that violate Bell's inequality. Therefore, quantum mechanics exhibits many counter-intuitive effects. In quantum mechanics, two particle states can be prepared in such a way that the relationship between them cannot be classically explained. Such a state is called an entangled state. Quantum entanglement refers to the interaction between two or more quantum systems Non-local non-classical association. Quantum teleportation is not only extremely important for people to understand and reveal the mysterious laws of nature in the field of physics, but also can use quantum state as an information carrier to realize the transmission of large-capacity information through the transmission of quantum state, which is indecipherable in principle. Quantum confidential communication.

 In 1997, Pan Jianwei, a young Chinese scholar studying in Austria, and Dutch scholar Bomiister, etc., cooperated for the first time to realize the long-distance transmission of unknown quantum states. This is the first time in the world that a quantum state has been successfully transferred from a photon in Earth A to a photon in Earth B. What was transmitted in the experiment was just to express the "state" of quantum information, and the photon itself as an information carrier was not transmitted. Soon, Pan Jianwei and his collaborators have made new breakthroughs in the study of how to purify high-quality quantum entangled states. For teleportation of quantum states over long distances, it is often necessary to allow two distant places to share the largest quantum entangled state in advance. However, due to various inevitable environmental noises, the quality of quantum entangled states will become worse and worse as the transmission distance increases. Therefore, how to purify high-quality quantum entangled states is an important issue in quantum communication research at this moment.

 Many international research groups are working on this subject, and have proposed a series of theoretical solutions for the purification of quantum entangled states, but none of them can be achieved with existing technology. Later, Pan Jianwei and others discovered the theoretical solution of quantum entangled state purification that is experimentally feasible by using the existing technology, and in principle solved the fundamental problem of long-distance quantum communication nowadays. This research achievement has been highly evaluated by the international scientific community and is called "a leap in long-distance quantum communication research".