Less than a year after it's launch into orbit (August 16, 2016), the Chinese quantum network satellite has finally sent entangled photons down to Earth to two research stations about 1203km apart.

“The satellite is designed to establish ultra-secure quantum communications by transmitting uncrackable keys from space to the ground,” Xinhua, China’s state news agency.

“This is the first step towards worldwide secure quantum communications, and maybe even a quantum internet,” says Anton Zeilinger, an expert on quantum physics at the University of Vienna in Austria.

14 years of work

First they had to make sure the source of the entangled light survives the stresses of launch and that the entangled light would stay entangled after being beamed down to Earth through 10km of thick and turbulent air. After many groundbased test this was achieved and on 16. August 2016 the satellite Micius was launched to orbit the and pass over China at the exact same time and place each night.
Ground based stations in Delingha in Tibet and Lijiang and Nanshan in north-west China were locked on to the satellite to monitor and catch the entangled light.

The ground stations used adaptive optics — which is technology that can measure the turbulence of Earth’s atmosphere in real time and cancel out its blurring effects. They also used technology to filter out moonlight and light pollution from cities, to reduce the noise in the optical link to the satellite.

For every pass of the satellite over China, which happened at night for about 275 seconds, it had to establish two such downlinks simultaneously, either between Delingha and Lijiang (1203 kilometres apart) or Delingha and Nanshan (1120 kilometres apart).

If we were to use optical fibres to distribute pairs of entangled photon pairs on Earth over 1200 kilometres, the loss of signal strength with distance means that we could only transmit one pair per second.

The Chinese satellite smashed that barrier. “We have already improved the distribution efficiency by 12 orders of magnitude over former technologies,” says Pan.

What's the basic gist behind quantum entanglement?

In quantum physics particles change their state when observed. Otherwise they are in a superposition, they can be in both states if not observed.
Particles can also be entangled with each other which allows to predict the state of other particles through observing the state of one particle. This happens beyond space and time. The change is state is instantaneous and unaffected by the distance of the particles. This is also called quantum teleportation.

How is this used?

The satellite launched into space beams quantum entangled light back to two stations.
These two stations are then connected through quantum teleportation - by altering the light they can exchange information directly with each other. This connection is used to create "uncrackable" keys. In theory if a randomly generated key is long enough and is only used once it's impossible to crack it. The key cannot be eavesdropped on because quantum teleportation goes beyond time and space (it doesn't use time and space - it's undetectable). The key will only be know to the receiver and the sender.

Now if a message is encrypted using that key, only the receiver will be able to decrypt this message.

And that's how you create a secure quantum network.

If a third party would managed to catch the entangled light from the satellite beam, that would be immediately noticed through the two other parties.

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Sources:

https://qz.com/760804/chinas-new-quantum-satellite-will-try-to-teleport-data-outside-the-bounds-of-space-and-time-and-create-an-unbreakable-code/
https://www.newscientist.com/article/2134843-chinese-satellite-beats-distance-record-for-quantum-entanglement/
http://bgr.com/2017/06/17/quantum-entanglement-satellite-messaging/

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