It all began when Quantum Experiments at Space Scale (QUESS), which is a Chinese research project in the field of quantum physics, started facilitating quantum optics experiments over long distances.
As a proof-of-concept mission, QUESS allows the development of quantum encryption and quantum teleportation technology.
It was back in 2016, when Tiangong-2, China's second Space Laboratory module, brought a Space-Earth quantum key distribution and laser communications experiment to facilitate space-to-ground quantum communication.
This was then utilized by a satellite, nicknamed Micius or Mozi (Chinese: 墨子), to honor the ancient Chinese philosopher and scientist.
The satellite was developed by a cross-disciplinary multi-institutional team of scientists from the Chinese Academy of Sciences, led by Professor Jian-Wei Pan, who has spent more than 10 years developing it. The $100 million project demonstrates the secure satellite-to-ground exchange of cryptographic keys during the passage of the satellite Micius over a ground station.
Using Micius as a trusted relay, QUESS managed to create what is considered the ‘world’s most secure communication link’.
It was in 2017 that the Micius team managed to create the world's first quantum satellite transmission.
This involved designing the method for distributing the keys and a mechanism for preventing malicious attacks, such as blinding the telescopes with other light signals.
In 15 June 2020, a paper from Pan Jia-Wei’s lab published in Nature shows that the Micius satellite was able to send simultaneous streams of entangled photons to two ground stations to establish a direct link.
Separated by 1,120 kilometres, the two ground stations located in Delingha and Nanshan in China, used a finite secret-key rate of 0.12 bits per second.
Using the entangled photon pairs that were distributed via two bidirectional downlinks from the Micius satellite, the two parties could communicate using an unbreakable cryptographic protection.
Until this time, never has been done such kind of communication via satellite or at such great distances.
It should be noted that none of the communication went through Micius.
What the satellite did, was only providing the initial entangled photons as a convenient resource for the quantum cryptography for the two ground stations to use as an agreed protocol.
In other words, Micius only produced and distributed the encryption keys. After with that, both ground stations could talk, by having to trust on Micius as part of their communication systems.
This happened only during the relay, and before establishing a link with each other.
Quantum encryption uses the principle of entanglement to facilitate communication that is safe from eavesdropping, let alone decryption, by a third party.
This kind of secured private communications are fundamental for internet use. Using quantum cryptography, the link creates a secure network with global protection of data.
Traditional public key cryptography usually relies on the computational intractability of certain mathematical functions. In contrast, quantum key distribution (QKD) uses individual light quanta (single photons) in quantum superposition states to guarantee unconditional security between distant parties.
It's by producing pairs of entangled photons that QUESS allows its ground stations that are separated by many thousands of kilometres to establish secure quantum channels.
Micius, which is regarded as the world’s first quantum communications satellite, is operated by the Chinese Academy of Sciences.
Its ground stations are located in Xinglong (near Beijing), Nanshan (near Urumqi), and Graz (near Vienna), operated by The University of Vienna and the Austrian Academy of Sciences.
It was only within a year after the launch that QUESS managed to achieve three key milestones for a global-scale quantum internet satellite-to-ground decoy-state QKD with kHz rate over a distance of ~1200 km; satellite-based entanglement distribution to two locations on the Earth separated by ~1200 km and Bell test; and ground-to-satellite quantum teleportation.