Published: Sun, June 18, 2017
Science | By Tyler Owen

Lijiang observatory helps unravel mysteries of quantum communication

Lijiang observatory helps unravel mysteries of quantum communication

"By developing an ultra-bright space-borne two-photon entanglement source and high-precision APT, the team established entanglement between two single photons separated by 1203 km, with an average two-photon count rate of 1.1Hz and state fidelity of 0.869±0.085". If you separate these particles spatially, then they remain entangled with these properties until you observe one of them - then the other particle assumes its corresponding value. The feat showed that the particles of light can retain a unusual type of interconnectedness, known as quantum entanglement, even when flung to opposite ends of a country, researchers from China report in the June 16 Science. To confirm that the particles were entangled, and that the weird qualities of quantum mechanics held, the researchers used the photon pairs to perform a Bell test (SN: 9/19/15, p. 12), which analyzes correlations between the two particles. Any attempt to eavesdrop would disturb the particles and be discovered.

Despite its apparent violation of the rules of cause-and-effect, the effect has practical consequences because entangled photons can be used, in principle, to communicate the key to an encrypted message that no eavesdropper would be able to steal. The project was led by Jian-Wei Pan, a physicist at the University of Science and Technology of China in Shanghai.

But, photons can travel more smoothly through space.

So, technologically, this is a huge step as Jürgen Volz, a physicist at the Vienna Center for Quantum Science and Technology told the LA Times. "So how can you do similar experiments at thousand-kilometers distance scale and with the optical elements vibrating and moving at a speed of 8 kilometers per second [5 miles per second]?"

Launched in August 2016 by QUESS (Quantum Experiments at Space Scale program), China's Micius satellite was created to function as a global quantum communication network, as reported by Economic Times. The experimental Micius, with its delicate optical equipment, continues to circle the Earth, transmitting to two mountain-top Earth bases separated by 1,200km.

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Only the lowest 6 miles (10 km) of Earth's atmosphere are thick enough to cause significant interference with the photons, the scientists said. "We have done something that was absolutely impossible without the satellite". Launched in 2016, the one-of-a-kind satellite is laying the groundwork for a space-based network of quantum communication.

Because any change in one particle will be apparent in the other instantly, quantum entanglement has been considered a potentially powerful tool for sending information securely.

Thomas Jennewein, who is at the University of Waterloo in Canada and part of the Candanian mission, says that his group and others around the world are now racing to catch up with the Chinese effort. "When I proposed the entangled-based quantum key distribution back in 1991 when I was a student in Oxford, I did not expect it to be elevated to such heights!" Their work is the first time that quantum features of laser signals have been measured so carefully from so far away, they say. The distance between the orbiting satellite and the ground stations varies from 500 to 2,000 kilometers, said Pan. This required focusing the photons through special receivers, with the ground stations using adaptive optics.

"When I had the idea of doing this in 2003, many people thought it was a insane idea", Pan told the BBC World Service "Because it was very challenging already doing the sophisticated quantum optics experiments in a lab - so how can you do a similar experiment at a thousand-kilometre distance and with optical elements moving at a speed of 8km/s?"

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