Using Micius for a quantum-safe intercontinental video conference between China and Austria.

The first quantum-safe video conference was held between President Bai Chunli of the Chinese Academy of Sciences in Beijing and President Anton Zeilinger of the Austria Academy of Sciences in Vienna on Sept 29, the first real-world demonstration of intercontinental quantum communication.

Message sending from Vienna to Beijing through a space-ground integrated quantum network (Image by Pan Jianwei's team)

Private and secure communications are fundamental human needs. 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 perceived computational intractability of certain mathematical functions. By contrast, quantum key distribution (QKD) uses individual light quanta (a single photon) 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 channel loss of fibers or terrestrial free space. A promising solution to this problem is exploiting satellite and space-based links, 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.

Illustration of the experimental set-up (Image by Pan Jianwei's team)

A cross-disciplinary multi-institutional team of scientists from the Chinese Academy of Sciences, led by Professor Pan Jianwei, spent more than ten years in developing a sophisticated satellite, named Micius, dedicated to quantum science experiments, which was successfully launched on Aug 16 2016 from Jiuquan, China to an orbit with an altitude of ~500 km. The satellite is equipped with three payloads: a decoy-state QKD transmitter, an entangled-photon source, and a quantum teleportation receiver and analyzer. Five ground stations have been built in China to cooperate with the Micius satellite. They are located in Xinglong (near Beijing, 40°23'45.12''N, 117°34'38.85''E, altitude 890m), in Nanshan (near Urumqi, 43°28'31.66''N, 87°10'36.07''E, altitude 2028m), in Delingha (37°22'44.43''N, 97°43'37.01"E, altitude 3153m), in Lijiang (26°41'38.15''N, 100°1'45.55''E, altitude 3233m), and in Ngari in Tibet (32°19'30.07''N, 80°1'34.18''E, altitude 5047m).

Within a year after the launch, three key milestones that will be central to a global-scale quantum internet were achieved: satellite-to-ground decoy-state QKD with kHz rate over a distance of ~1200 km (Liao et al. 2017, Nature 549, 43); satellite-based entanglement distribution to two locations on the Earth separated by ~1200 km and Bell test (YIN et al. 2017, Science 356, 1140), and ground-to-satellite quantum teleportation (REN et al. 2017, Nature 549, 70). The effective link efficiencies in the satellite-based QKD were measured to be ~20 orders of magnitudes larger than direct transmission through optical fibers at the same length at 1200 km.

The satellite-based QKD has now been combined with metropolitan quantum networks in which fibers are used to efficiently and conveniently connect many users inside a city with 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.

The largest fiber-based quantum communication backbone was recently built in China by Professor Pan's team, linking Beijing to Shanghai (going through Jinan and Hefei, and 32 trusted relays) with a fiber length of 2,000 km. The backbone uses decoy-state protocol QKD and achieves an all-pass secure key rate of 20 kbps. It is on trial for real-world applications by government, banks, securities and insurance companies.

This is the world's longest and most sophisticated quantum link, and it will serve as the backbone connecting quantum networks in four cities: Beijing, Shanghai, Jinan in Shandong province and Hefei in Anhui province, said Pan Jianwei, China's leading quantum physicist.

During the link's launch in Beijing, Bai Chunli, the president of the Chinese Academy of Sciences, made video phone calls using the quantum link to scientists and government officials in Jinan, Hefei and Shanghai, congratulating them on their hard work.

Establishment of a reliable space-to-ground link for quantum state transfer (Image by Pan Jianwei's team)

The Micius satellite can be further exploited as a trusted relay to conveniently connect any two points on the earth for high-security key exchanges. Earlier this year the Chinese team implemented satellite-to-ground QKD in Xinglong. After that, the secure keys were stored in the satellite for 2 hours until it reached Nanshan station near Urumqi, ~2500 km from Beijing. By performing another QKD between the satellite and Nanshan station, and using one-time-pad encoding, secure keys between Xinglong and Nanshan were then established. In tests of the robustness and versatility of the Micius, QKD from the satellite to Graz ground station near Vienna was also carried out successfully this June as collaboration between Professor Pan and Professor Anton Zeilinger's group. Upon request, similar future experiments will be planned between China and Singapore, Italy, Germany, and Russia.

Performance of satellite-to-ground QKD performance during one orbit. (Image by Pan Jianwei's team)

Source: english.cas.cn