China launched a quantum satellite called Micius from the Gobi desert last August. It is all part of a push towards a new kind of internet that would be far more secure than the one we use now. The experimental Micius, with its delicate optical equipment, continues to circle the Earth, transmitting to two mountain-top Earth bases separated by 1,200km.
The optics onboard are paramount. They’re needed to distribute to the ground stations the particles, or photons, of light that can encode the “keys” to secret messages.
“I think we have started a worldwide quantum space race,” says lead researcher Jian-Wei Pan, who is based in Hefei in China’s Anhui Province.
A successful quantum communication network will rely on the ability to distribute entangled photons over large distances between receiver stations. So far, free-space demonstrations have been limited to line-of-sight links across cities or between mountaintops. Scattering and coherence decay have limited the link separations to around 100 km. Yin et al. used the Micius satellite, which was launched last year and is equipped with a specialized quantum optical payload. They successfully demonstrated the satellite-based entanglement distribution to receiver stations separated by more than 1200 km. The results illustrate the possibility of a future global quantum communication network.
Long-distance entanglement distribution is essential for both foundational tests of quantum physics and scalable quantum networks. Owing to channel loss, however, the previously achieved distance was limited to ~100 kilometers. Here we demonstrate satellite-based distribution of entangled photon pairs to two locations separated by 1203 kilometers on Earth, through two satellite-to-ground downlinks with a summed length varying from 1600 to 2400 kilometers. We observed a survival of two-photon entanglement and a violation of Bell inequality by 2.37 ± 0.09 under strict Einstein locality conditions. The obtained effective link efficiency is orders of magnitude higher than that of the direct bidirectional transmission of the two photons through telecommunication fibers.
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