Micius Satellite Enables Intercontinental Quantum Communication
Staff posted on January 19, 2018 |
Images and video transmitted between Beijing and Vienna using quantum key distribution.
Illustration of the three cooperating ground stations (Graz, Nanshan, and Xinglong). Listed are all paths used for key generation and the corresponding final key length. (Image courtesy of University of Science and Technology of China.)
Illustration of the three cooperating ground stations (Graz, Nanshan, and Xinglong). Listed are all paths used for key generation and the corresponding final key length. (Image courtesy of University of Science and Technology of China.)
A cross-disciplinary, multi-institutional team of researchers from the Chinese Academy of Sciences, led by professor Jian-Wei Pan, has spent more than ten years developing a sophisticated satellite, Micius, dedicated to quantum science experiments, which was launched on August 2016 and orbits at an altitude of ~500 km (310 mi).

China has built five ground stations to cooperate with the Micius satellite, located in Xinglong (near Beijing), Nanshan (near Urumqi), Delingha, Lijiang and Ngari in Tibet.

Within a year of the launch, the University of Science and Technology of China is reporting that three key milestones for a global-scale quantum internet have been achieved:

  1. Satellite-to-ground decoy-state quantum key distribution (QKD) with kHz rate over a distance of ~1,200 km (745 mi).
  2. Satellite-based entanglement distribution to two locations on the Earth separated by a distance of ~1,200 km.
  3. Ground-to-satellite quantum teleportation.

The effective link efficiencies in the satellite-based QKD were measured to be approximately 20 orders of magnitude larger than direct transmission through optical fibers the same length. According to the researchers, these three experiments are the first steps towards a global space-based quantum internet. That explains the origin of this engineering project: First Particle Successfully Quantum Teleported into Space. What’s next?


Quantum Communication

Private and secure communication is a fundamental human need. In particular, with the exponential growth of Internet use and e-commerce, it is of paramount importance to establish a secure network with global protection of data.

Traditional public key cryptography usually relies on the computational intractability of certain mathematical functions. In contrast, QKD uses individual light quanta (single photons) in quantum superposition states to guarantee unconditional security between distant parties. Previously, the quantum communication distance had been limited to a few hundred kilometers, due to the optical channel losses of fibers or terrestrial free space.

A promising solution to this problem exploits satellite and space-based link, which can conveniently connect two remote points on the Earth with greatly reduced channel loss because most of the photons' propagation path is in empty space with negligible loss and decoherence.

One-time-pad file transfer. (Image courtesy of University of Science and Technology of China.)
One-time-pad file transfer. (Image courtesy of University of Science and Technology of China.)
The satellite-based QKD has now been combined with metropolitan quantum networks, in which fibers are used to connect numerous users inside a city over a distance scale of ~100 km. For example, the Xinglong station has now been connected to the metropolitan multi-node quantum network in Beijing via optical fibers.

Recently, Professor Pan’s team also built the largest fiber-based quantum communication backbone in China, linking Beijing to Shanghai (going through Jinan and Hefei, and 32 trustful relays) with a fiber length of 2,000 km (1,243 mi). The backbone is being tested for real-world applications by government, banks, securities and insurance companies.

To further demonstrate the Micius satellite as a robust platform for quantum key distribution with different ground stations on Earth, the team performed QKD from the Micius satellite to Garz ground station near Vienna this past June in collaboration with Professor Anton Zeilinger of Austrian Academy of Sciences.

The satellite establishes a secure key between itself and, say, Xinglong, and another key between itself and, say, Graz. Upon request from the ground command stations, Micius acts as a trusted relay. It performs bitwise exclusive OR operations between the two keys and relays the result to one of the ground stations. That way, a secret key is created between China and Europe at locations separated by 7,600 km on Earth. This work points towards an efficient solution for an ultra-long-distance global quantum network.

A photography of a quantum-secure intercontinental video conference held between Chinese Academy of Sciences and Austrian Academy of Sciences on 29 September, providing a real-world demonstration of quantum communication. (Image courtesy of Chinese Academy of Sciences.)
A photography of a quantum-secure intercontinental video conference held between Chinese Academy of Sciences and Austrian Academy of Sciences on 29 September, providing a real-world demonstration of quantum communication. (Image courtesy of Chinese Academy of Sciences.)
A picture of Micius (with a size of 5.34 kB) was transmitted from Beijing to Vienna, and a picture of Schrödinger (with a size of 4.9 kB) from Vienna to Beijing, using approximately 80 kbit secure quantum key for one-time-pad encoding.

An intercontinental videoconference was also held between the Chinese Academy of Sciences and the Austria Academy of Sciences, employing the Advanced Encryption Standard (AES)-128 protocol that refreshed the 128-bit seed keys every second. The videoconference lasted for 75 min with a total data transmission of ~2 GB, which included ?560 kbit of the quantum key exchanged between Austria and China.

For more on the future of quantum communications, read about A Major Leap Toward Global Quantum Internet.

Source: University of Science and Technology of China

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