Quantum communication has advanced at speed, with long-distance transmission and security emerging as promising new areas of information science. China is well on the way to establishing itself among the powers in the field.

 With the rapid advancement of information technology, microelectronics-based information technology will soon reach its physical limits. Quantum communication based on quantum science, by contrast, is driving the future of information science and technology and is expected to bring about another technical revolution. Quantum communication, more generally quantum information science and technologies, provides a new way of transferring information using a mix of quantum mechanics, quantum systems and quantum mechanical measurements. Subfields include quantum cryptography, quantum teleportation and quantum dense coding. The focus now moves from theory to the experience of quantum information science for practical applications. Driven by the benefits of high efficiency and absolute limits, quantum information science is emerging as a central research topic in the field of quantum physics and information science.

 Scientific revolutions in human history all began with a change in perception; taking objectivity in the way of seeing nature. Scientists made important findings through objective observation. For example, discoveries made in the field of electromagnetics in the early 19th century led to the second scientific revolution. Similarly, the exploits of scientists in the 20th century have driven the rapid advancement of quantum communication, of which two of the most important fundamentals are the uncertainty principle and the no-cloning theorem.

 In the micro world, some physical quantities are not continuous but a sudden change in the smallest unit. This smallest, fundamental unit is called a quantum, such as an atom, electron and photon (light quantum). The quantum theory was built up through over a century of research.

 In 1927, renowned German physicist Heisenberg came up with the uncertainty principle in quantum mechanics. He argued that there is a fundamental limit to the precision with which certain pairs of physical quantities, for example, position and momentum, angular momentum, time and energy, can be measured. The principle states that the more precisely one quantity is measured, the less precisely the other is known, and the minimum range of error in position (x) times the minimum range of error in momentum (p) is at a minimum, about equal to the Planck constant.

 The no-cloning theorem is another underlying principle of quantum mechanics. It states that it's impossible to create an identical copy of an arbitrary unknown quantum. Given a coin that's spinning in the quantum world, we should measure it first in order to make its identical copy, but doing so will change its motional state. In other words, we cannot measure an arbitrary quantum state without changing its state and it means that the copy will not be identical to the original.

 There is also very interesting physical phenomenon called quantum entanglement. If a pair of particles are generated in an entangled way, then as the behavior of one particle changes, the behavior of both particles changes almost simultaneously, no matter how far apart they may be. In short, a pair of entangled particles remain connected, know and affect each other's state, even when separated. And that's what is called quantum entanglement.

 Quantum entanglement or what Albert Einstein called 'spooky action at a distance' was, however, not accepted. It was shunned by all. Einstein said that the relative independence of objects far apart in space, A and B: external influence on A has no direct influence on B in this principle of local action (the principle of locality). But, another great physicist Niels Bohr was opposed to Einstein about quantum mechanics and philosophy of science over a series of public disputes; it is called the Bohr-Einstein Debates. The Copenhagen School led by Bohr argued that a pair of particles entangled influence each other regardless of the distance between them – the principle of quantum nonlocality – and that there is a deep link between particles in the universe in darkness. The debate between the two physicists lasted years and made a great contribution to the development of science.

 Irish physicist John Stewart Bell, in support of Bohr, presented Bell's inequality in 1962 and proved the violation of local realism in quantum mechanics. Over the years, experiments of Bell's inequality have been conducted and it has been found that local theory is inconsistent with the predictions of quantum mechanics and that quantum effects can be transmitted at the speed of light. Bell's theorem is taken as the fundamental explanation of quantum physics by mainstream physics textbooks. Moreover, French physicist Alain Aspect and his research team successfully proved the reality of quantum entanglement in 1982.

 The Internet is now an inextricable part of our everyday lives. But, because a bulk of sensitive information is transferred through the Internet, we must keep our personal information secure to prevent identity theft. Encryption provides protection against identity theft, but some of the widely used encryption using mathematical methods have an inherent security risk. That's where scientists came up with the idea of quantum cryptography. They made quantum cryptography, which uses quantum mechanical properties based on the uncertainty principle and the no-cloning theorem to produce and distribute a secret key, in contrast to traditional public key cryptography. If there is an attempt of eavesdropping on the key or copying messages, it must be a kind of test. As we all know, it's impossible to measure and copy data encoded in a quantum state and it's also secure against tapping according to the no-cloning theorem. In theory, quantum communication is very secure and safe.

 In quantum communication, a secret key made through quantum key distribution using the property of quantum state, entanglement. Two users first stock up entangled quantum states before they can be connected. Entangled quantum states react correspondingly to each other. When one user performs an action on the states, the states at the other side reacts accordingly. Both sides come to a certain point through such a detection and produce a secret key for secure communication.

 Two distant peers can obtain a secure secret key through either BB84 protocol or by using the entanglement of quantum states. Quantum communication, however, requires four processes – quantum transmission, data selection, calibration and security enhancement – due to environmental noise and the presence of eavesdroppers.

 A typical quantum communication system works as follows: A secret message requires a secret key encoded in quantum states. The message is sent through a communication channel to the receiver and he receives it by measuring the quantum state and uses the secret key to decrypt the message.

 In the 1990s, scientists in China and around the world conducted research aiming to put quantum communication in practice. In 1993, American IBM researchers came up with the theory of quantum communication and worked together with the National Science Foundation (NSF) and the Defense Advanced Research Projects Agency (DARPA) on the project. Quantum computing and communication research began in Europe in 1999 with the help of international organizations and over 12 research projects are being undertaken. The Japanese Ministry of Posts and Telecommunications designated quantum communication as its 21st century strategic project. China as well jumped into the field of quantum optics in the 1980s and, in recent years, the quantum research team at the University of Science and Technology of China (USTC) have made great advances in the field of quantum communication.

 On October 5, 2002, the University of Munich jointly with the UK Defense Science and Technology Laboratory succeed in transmitting a key encrypted into photons from Zugspitze on the border between Germany and Austria to Karwendel-Bahn (nearly 23.4km away) using a laser.

 In 2003, Korean, Chinese and Canadian scientists presented decoy-state quantum key distribution, which completely solved the limitation in the secure transmission rate or the maximum channel length in practical quantum key distribution system.

 On June 3, 2004, BBN Technologies built a quantum key distribution network with six nodes. In summer of 2006, the quantum research team at the University of Science and Technology of China, Los Alamos National Laboratory and joint research team of the University of Munich and the University of Vienna successfully implemented decoy-state quantum key distribution and demonstrated it over 100km. This heralded the application of quantum key distribution.

 At the end of 2008, the quantum research team at the University of Science and Technology in China developed a quantum communication system using optical fiber based on decoy-state quantum key distribution and built the world's first photonic telephone network composed of three nodes in Hefei, China, which made it one of two research teams in the world to have successful applied very secure quantum key distribution.

 In September 2009, China built the world's first all-pass quantum communication network over three nodes of its photonic telephone network for secure real-time voice communication system using a quantum network. This proved that China came near to put its metropolitan quantum network technology into practical use, putting it on a par with advanced countries.

 In September 2010, the quantum communication research team of researchers from the University of Science and Technology of China, Shanghai Institute of Applied Physics and Institute of Optics and Electronics under Chinese Academy of Sciences conducted a quantum key distribution experiment using a hot air balloon as part of satellite simulation test on the border of a lake in Qinghai and it proved to a success. Through the success of this experiment, China laid the foundation for the implementation of a quantum communication network beyond satellite-ground quantum key distribution.

 On August 16, 2016, China launched the world's first satellite for the purpose of quantum communication, developed by the quantum communication research team of researchers from the University of Science and Technology of China, Shanghai Institute of Applied Physics, Institute of Optics and Electronics and Shanghai Engineering Center for Microsatellites under the Chinese Academy of Sciences. This satellite, named Micius, is designed to help demonstrate the feasibility of quantum communication between Earth and space.

 The study of quantum communication system is currently based around the use of two media : optical fiber and free space.

 Information highway, i.e., fiber-optic network and the Internet for high-speed transfer of information are already in wide use, along with the development of related technologies. This has derived terms favorable to the study of quantum communication over optical fiber. In fact, optical fiber based quantum communication will be put into real practical application for security purposes over the next few years. But, it is not easy to implement ground-based long distance quantum communication because of the Earth's curvature and the transmission capability of optical fiber for ground-based long distance quantum communication. For example, the transmission of entangled photons is limited to about 100km in optical fiber. Photons, however, can travel far more in free space and it makes ground-to-satellite quantum communication more attractive.

 In order to implement ground-to-satellite quantum communication in the universe, a communication route must first be made using an ATP system for quantum key distribution backed up by technologies such as optical positioning, measurement and tracking. Once the route is set through quantum key distribution, the communication system loads information using the polarization of a single photon and a secret key can then be shared between distance locations over a communication network that's very secure and unable to eavesdrop. This is how ground-to-satellite quantum communication works using the secret key. This not only transfers data from ground to satellite, but also enables arbitrary peer-to-peer quantum communication.

 Chinese Academy of Sciences undertook a Knowledge Innovation Program (the study and experiment of quantum mechanics and measurement) in 2008 to gain the lead in the race for free-space quantum communication. And under Pan Jianwei's guidance, the quantum communication research team has made great advances in recent years. Their achievements include the development of a sample of ATP system in satellites for quantum communication, advanced ground-to-satellite quantum system ideas, the implementation of core technologies for ground-to-satellite quantum communication, the world's first quantum key distribution experiment using a hot air balloon and quantum transfer over 100km. The research team has presented dozens of papers during the project and many have been published in the world's renowned academic magazines, including Nature, Proceedings of the National Academy of Sciences of the United States of America (PNAS) and Physical Review Letters. But, what's arguably more important is that China has brought up groups of young talented researchers in the field of space quantum communication for its long-term growth in the field of quantum communication and strengthened its presence in the world. In addition, the team did something that no-one in the West has done and that's launch its very first quantum communication satellite, Micius, on August 2016.

 Many countries are working on implementing their own quantum communication system on the satellite platform and so, too, is China. The Chinese Academy of Sciences initiated the ground-to-satellite communication plan in 2008 and finally achieved ground-to-satellite communication through Micius Satellite and the Tiangong 2 space station in 2016. These achievements represent the culmination of decade of efforts that have put China safely among the world powers in the field of quantum communication.