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Editor’s note: This article is from the WeChat public account AI frontline (ID: ai-front), 36 氪 released with authorization.Finishing | Yu Ying Recently, a Chinese research team published a new paper in the British “Nature” magazine. They successfully achieved quantum entanglement between two quantum memories connected by 50 km long optical fiber, laying a foundation for building a quantum relay-based quantum network.basis.The research was carried out in collaboration with the University of Science and Technology of China, the Jinan Institute of Quantum Technology, and the Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences.50 km communication distance: enough to connect two cities. In the experiment, the research team combined several new technologies to successfully achieve two-node quantum entanglement between two quantum memories connected by 50 km fiber. This distance is enough to connect the twoCities.The team said that the quantum communication network development route widely used in academia is to achieve wide-area coverage through satellite-based free space channels, while using fiber optic networks to achieve urban and inter-city ground coverage.However, when photons are transmitted between nodes on an optical fiber, they are limited by the inherent attenuation of the optical fiber. At present, the farthest point-to-point ground safety communication distance is only on the order of one hundred kilometers.One of the principals of the experimental scheme schematic research, Professor Pan Jianwei of the University of Science and Technology of China, said that to extend the distance of quantum communication, one method is to change point-to-point transmission to segment transmission, and use quantum relay technology to cascade, that is, the entire communicationThe line is divided into several sections, each of which has a small loss, and these sections are connected through a quantum repeater, which makes it possible to build a full quantum network.However, limited by technical bottlenecks such as low entanglement of light and atoms, the farthest optical fiber quantum relay was only on the order of kilometers.An important step to realize quantum relay To achieve the connection between long-distance quantum memories, the team has overcome many technical challenges.First, to enhance the coupling between the single photon and the atomic ensemble by using the ring cavity enhancement technology, and to optimize the transmission efficiency of the optical path, the brightness of the previous entanglement of light and atoms was increased by an order of magnitude.Second, because the optical wavelength corresponding to the atomic memory loss in the fiber is about 3.5dB / km, the optical signal will be attenuated to one billionth of a billion in 50 kilometers of fiber, making quantum communication impossible, and the team independently developed periodic polarization.The lithium niobate waveguide converts the light wavelength of the memory from near-infrared to the communication band through a non-linear difference frequency process. After 50 kilometers of optical fiber attenuation is only over one percent, the efficiency is improved by 16 orders of magnitude.Finally, in order to achieve long-range single-photon interference, the research team designed and implemented a dual phase-locking scheme, which successfully controlled the optical path difference caused by the transmission of 50 kilometers of optical fiber to about 50nm.Through the combination of multiple technologies, the research team finally achieved two-node entanglement via 50 km of fiber transmission and demonstrated two-node entanglement via 22 km of external field fiber.This work was highly praised by the reviewers of “Nature” “The result is very outstanding. It has taken an important step towards the realization of quantum relay. Extending these operations to urban distances is a major advance in this field. Quantum communicationThe development of so-called quantum communication is, from a conceptual point of view, a technology that uses the information transfer function of a quantum medium to communicate, mainly including technologies such as quantum key distribution and quantum teleportation. Quantum Cryptography uses the properties of quantum mechanicsDeveloped cryptosystem. Unlike traditional cryptosystems, its security depends on quantum mechanical properties (not measurable, non-clonable, etc.) rather than mathematical complexity theory. At present, quantum key distribution is the most mature quantum researched.Cryptography. In China, in 2004, Guo Guangcan’s team completed the quantum key distribution via Beijing Wangjing—Hebei Xianghe—Tianjin Baodi, with a distance of 125km. In 2008, Pan Jianwei’s team built a 3-node quantum based on commercial fiber and decoy phase codingCommunication network, with a distance of up to 20km between nodes, real-time networkCall and talk with 3 parties. In 2009, Guo Guangcan’s team built the world’s first “quantum government network”. In September of the same year, the University of Science and Technology of China built the world’s first 5-node fully-connected quantum communication network to realize real-time voice quantum cryptographyCommunication.In May 2011, Wang Jianyu’s team developed a “star-ground quantum communication system” compatible with classic laser communications, enabling simultaneous quantum communication and classic laser communications between satellites and ground.On February 17, 2012, the Hefei City Quantum Communication Experimental Demonstration Network was completed and entered the trial operation phase. It has 46 nodes and a fiber length of 1700km. It connects 40 sets of “quantum phones” through 6 access exchanges and centralized control stations.Users and 16 “Quantum Video” users.In May 2013, the Chinese Academy of Sciences successfully achieved the first full-scale ground test of satellite-ground quantum key distribution in the world.In November of the same year, the Jinan Quantum Confidential Communication Test Network was completed, including three centralized control stations and 50 user nodes.In December 2015, the Chinese Academy of Sciences, the University of Science and Technology of China, and the National Science and Technology University of China’s National Shield Quantum jointly initiated the establishment of the “China Quantum Communication Industry Alliance” in Beijing.In the same year, Academician Pan Jianwei and Professor Lu Chaoyang from the National Laboratory for Microscale Physical Sciences of Hefei University of Science and Technology of China and Professor Lu Chaoyang completed the “Invisible Teleportation of Multiple Free Metrics” by the European Physical Society News website “Physics World” as the annual breakthrough.In 2007, Pan Jianwei’s group achieved the first 100 kilometer-scale decoy quantum key distribution in the world, successfully solving the security loopholes caused by non-ideal single photon sources, but in theory, there are still security risks.In 2013, Pan Jianwei’s group developed the interference technology of an independent laser light source, and jointly developed a conversion detector with Stanford University in the United States. Combining with the theoretical analysis of Professor Ma Xiongfeng of Tsinghua University, it achieved the world’s first quantum key distribution independent of measurement devices and successfully resolvedThe single-photon detection system is vulnerable to hacking in real environment, and it greatly improves the security of the actual quantum key distribution system.At present, Hefei, Jinan, Urumqi, Wuhu and other cities have successfully established urban experimental networks.Among them, the Hefei Metropolitan Quantum Communication Experimental Demonstration Network has successfully developed a series of products such as quantum communication network terminal equipment, quantum communication low-light detection core devices, and quantum communication network control equipment, which have achieved miniaturization of core devices and high integration of terminal products.And other important goals.On August 16, 2016, China launched the first Mozi quantum satellite, which indicates that China has constructed the first prototype of a world-wide integrated quantum communication network in the world, and will realize global quantum confidential communication for the future.The network has taken a new step.In February 2020, the academician Guo Guangcan’s team made important progress in the experimental research of quantum communication. The laboratory Li Chuanfeng, Liu Biheng and others collaborated with the theoretical physicist of the University of Hong Kong to realize the coherent superposition of the causal order of quantum channels for the first time.The sequential causal structure has advantages over standard quantum Shannon theory in transmitting both classical and quantum information. The results were published on January 24 in the internationally renowned academic journal of physics, Physical Review Letters.This work is the first international experimental verification of quantum communication beyond the standard quantum Shannon theory using coherent superposition of quantum channel causality, and it is an important step in the second quantization of Shannon theory.Information transmission experimental device diagram of non-sequential causal structure channel. In the past, the significance of quantum communication has attracted great attention from scientific research institutions in various countries. IBM, NIST, Battelle, NTT, Toshiba, Siemens and other companies and institutions have been paying close attention to their development and investment.Related research.The British government released a five-year quantum information technology project in 2013, investing 270 million pounds in the transformation of research results in quantum communication and quantum computing, and promoting the formation of new applications and industries.Many foreign entities have been established to specialize in the transformation and commercial promotion of quantum communication technology achievements.For example, MagiQ company in the United States and idQuantique company established by the University of Geneva, Switzerland, etc., can provide commercial devices, systems and solutions for QKD quantum communication.SeQureNet, established by the French Institute of Telecommunications, develops continuous variable quantum key distribution products.The Los Alamos National Laboratory in the United States established Qubittek to focus on the field of smart grid secure communications.Representative institutions that carry out quantum communication-related research in China include the University of Science and Technology of China, the Institute of Microsystems and the Institute of Technical Physics of the Chinese Academy of Sciences, Tsinghua University, Shanxi University, and Nanjing University.With the relevant research teams of the University of Science and Technology of China as the core, industrial entities such as the National Shield Quantum of Anhui, Quantum of Anhui, and Quantum of Shandong have been established to transform the research results of quantum communications into applied technologies and products.The continuous special investment and policy support in the field have provided a strong impetus for its development.Reference link: https://www.nature.com/articles/s41586-020-1976-7http://www.cas.cn/syky/202002/t20200213_4734442.shtml.

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