Above image: A solar wing deployment test for the experimental quantum communication satellite at the Jiuquan Satellite Launch Center in Jiuquan, northwest China's Gansu Province, July 30 2016. Photo credit: Xinhua
According to a study published in the journal Science, Chinese scientists have broken the record for the greatest distance over which quantum entanglement has been achieved. They successfully demonstrated satellite-based entanglement distribution to receiver stations separated by a distance of over 1200 km. The results illustrate the feasibility of using quantum devices on satellites to build a future global quantum communication network.
Quantum entanglement is a phenomenon when pairs or groups of particles cannot be described independently of the others. The physical properties of the particles are correlated and the correlation persists regardless of the distance separating them.
Distribution of entangled particles over large distances could be used to establish unhackable communications via quantum cryptography, as quantum entanglement would ensure that the intrinsically random shared secret key for encrypting and decrypting messages, can only be deciphered only by authorised individuals or entities. Moreover, if we perform a measurement on one of two entangled objects, the entanglement correlation is broken. If this measurement is done by an eavesdropper then the end-users detect no entanglement, and the eavesdropper is revealed.
Another application could be perform a variant of quantum teleportation (a process by which quantum information can be transmitted from one location to another, with the help of classical communication and previously shared quantum entanglement between the sending and receiving location) protocol for remote preparation and control of quantum states.
Till now efforts to entangle quantum particles, such as photons, have been limited to distances of about 100 km. This is primarily because the entanglement is lost as they are transmitted along optical fibers, or through open space on land.
One way to overcome this issue is to break the line of transmission into smaller segments and use so-called quantum repeaters to repeatedly swap, purify and store quantum information along the optical fiber, while another approach is to make use of satellite-based technologies.
China launched the world’s first satellite dedicated to quantum experiments, Micius (named after a fifth century B.C. Chinese scientist) in August 2016, in order to explore the latter path for quantum entanglement. Pan Jianwei, a professor at the University of Science and Technology of China and Chief Scientist for Micius, and his colleagues used Micus to communicate with two ground stations 1,203 km apart, located in Delingha in northwest China's Qinghai Province and Lijiang in Yunnan Province in southwest China, separately. The distance between the orbiting satellite and the two ground stations varies from 500 km to 2,000 km.
Pan Jianwei explained to Xinhua that the team combined narrow-beam divergence with a high-bandwidth and high-precision acquiring, pointing, and tracking technique to optimise link efficiency, and establish entanglement between two single photons, separated at a distance of over 1,200 km apart.
In addition, compared with previous methods using the best performance and most common commercial telecommunication fibers, the effective link efficiency of the satellite-based approach is 12 and 17 orders of magnitude higher respectively.
New Scientist reported that the next step for the scientists is to operate during daytime. That means coping with far more light pollution from the sun, which can destroy the entanglement.