Chinese scientists claim to have taken an important step towards the creation of "unhackable" communications, transmitting a quantum signal 1,200km from China's communications satellite Micius to Earth, much further than the 100km distance achieved before.
The experiment, the results of which are published in Science, demonstrates "satellite-based distribution of entangled photon pairs to two locations separated by 1203 kilometers on Earth, through two satellite-to-ground downlinks," the scientists write.
The experiment is significant because it could pave the way for a highly secure global communications system.
Quantum entanglement is the phenomenon by which the quantum states of two subatomic particles are connected. Changing the state of one by observing it or otherwise interfering will instantaneously change the state of the other, even if they are at opposite sides of the universe. This means information can be transmitted securely because if anyone changes or even observes the data, this will be obvious to the receiving party.
However, controlling the states of entangled particles in the real world is a uniquely challenging task. Transmitting entangled particles through fibres or through the open air tends to break the entanglement as the particles interact with the medium. However they are much more stable in the near-vacuum conditions of space.
In the current experiment, a laser beam projected from the satellite was split into two polarised states. One of these beams was used to transmit the entangled photons, while the other served to receive the photons. The scientists found that photons retained their entangled states during their journey to Earth.
"The obtained effective link efficiency is orders of magnitude higher than that of the direct bidirectional transmission of the two photons through telecommunication fibres," Juan Yin, lead author and physicist at the Science and Technology University of China in Shanghai, said.
Of course, conveying real-world communication data between two points is a very different matter from transmitting a pair of entagled photons, at least using current methods, and the way that quantum signals will be relayed between satellites remains to be worked out. Nevertheless, the leap in the distance over which such transmissions are now possible surely brings the era of ultra-secure communications a step closer.
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