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Last year I had the privilege to visit the Russian Quantum Centre in Moscow. I went there with basic knowledge of Quantum computing and technology, but being able to spend a few days there and with many of the world’s leading experts such as John Martinis (Google’s head of Quantum computing and University of Santa Barbara physics professor) Acronis Founder Serguei Beloussov I couldn’t help but learn and very quickly and including after hours in the evenings also having dinner with various global experts including world class researchers from RQC and a highlight being University of Calgary Professor Alexander Lvovsky explaining at length to me Quantum Supremacy and how Quantum computing will affect Bitcoin and Cryptocurrency, with the aid of diagrams on napkins.

I left with a wealth of knowledge (I learn fast) super inspired and fascinated about Quantum and now firmly believe that two countries will be at the forefront of Quantum technologies – Russia and China and testament to this is the level of contributions being made in academia and research from both countries. Indeed we are some years away from full Quantum computing, but progress is being made.

An example of this is successful intercontinental quantum key distribution experiments have recently been made by China’s Jian-Wei Group With two applications:
1) the transmission of images in a one-time pad configuration from China to Austria & from Austria to China;
2) a video-conference between the Austrian Academy of Sciences and the Chinese Academy of Sciences.

In short, an intercontinental video conference was held between the Chinese Academy of Sciences and the Austria Academy of Sciences. The video conference lasted for 75 minutes with a total data transmission of ~2
GB, which consumed ~70 kB of the quantum key generated between Austria and China.

Using Micius satellite as a trusted relay, they have demonstrated intercontinental quantum communication among multiple locations on Earth with a maximal separation of 7600 km.

This work already constitutes a simple prototype for a global quantum communications network. To increase the time and area coverage for a more efficient QKD network, they now plan to launch higher-orbit satellites and implement day-time operation using telecommunication wavelength photons and tighter spatial and spectral filtering.

One limitation of the current implementation of the QKD protocol is that you have to trust the satellite itself, which can be overcome in the future using entanglement-based systems. Other future developments will include multi-party connections from satellites to various ground stations in parallel, and the connection to large ground networks at first in China and Europe and then on a global scale.

Jian-Wei group, therefore, have successfully performed a decoy-state quantum key distribution between a low-Earth-orbit satellite and multiple ground stations located in Xinglong, Nanshan, and Graz, which established satellite-to-ground secure keys with ~kHz rate per passage of the satellite Micius over a ground station. The satellite thus establishes a secure key between itself and Xinglong, and another key between itself and, Graz.

Then, upon request from the ground command, Micius acted as a trusted relay. It performed 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 7600 km on Earth.

These keys are then used for intercontinental quantum-secured communication. This was, on the one hand, the transmission of images in a one-time pad configuration from China to Austria as well as from Austria to China. Also, a video conference was performed between the Austrian Academy of Sciences and the Chinese Academy of Sciences, which also included a 280 km optical ground connection between Xinglong and Beijing.

Their work creates a leap towards an efficient solution for an ultra long-distance global quantum network, laying the groundwork for a future quantum internet.

With the growth of internet use and electronic commerce, a secure global network for data protection is very necessary. A drawback of traditional public key cryptography is that it is not possible to guarantee information is theoretically secure. It has been shown throughout history that every advance of encryption has been defeated by advances in hacking. In particular, with the advent of Shor’s factoring algorithm. Most of the currently used cryptographic infrastructure will be defeated by quantum computers.

On the contrary, quantum key distribution (QKD) offers unconditional security ensured by the law of physics. QKD uses the fundamental unit of light, single photons, encoded in quantum superposition states which are sent to a distant location. By proper encoding and decoding, two distant parties share strings of random bits called secret keys.

However, due to photon loss in the channel, the secure QKD distance by direct transmission of the single photons in optical fibres or terrestrial free space was hitherto limited to a few hundred KMs. Unlike classical bits, the quantum signal in the QKD cannot be noiselessly amplified owing to the quantum no-cloning theorem, already contained at the core of Wiesner’s proposal of uncopiable quantum money, where the security of the QKD is rooted.

The main challenge for a practical QKD is to extend the communication range to long distances, ultimately on a global scale. A promising solution to this problem is exploiting satellite and space-based links. That way, one can conveniently connect two remote points on Earth with greatly reduced channel loss because most of the photons’ propagation path is in empty space with negligible loss and decoherence. Very recently, QKD from a low-Earth-orbit satellite, Micius, to the Xinglong ground station close to Beijing has been demonstrated with a satellite-to-ground-station distance of up to 1200 km.

In this Letter, we use the Micius satellite as a trusted relay to distribute secure keys between multiple distant locations in China and Europe.

In this work, QKD is performed in a downlink scenario—from the satellite to the ground. One of the payloads in the satellite is a space-qualified QKD transmitter, which uses weak coherent laser pulses to implement a decoy-state Bennett-Brassard 1984 (BB84) protocol that is immune to the photon-number-splitting attack. Eight tuneable fibre lasers, emitting light pulses with a wavelength of ~850 nm at a repetition rate of 100 MHz, are used to generate the signal, decoy and vacuum states. After being collected into single-mode fibres and collimated, the laser pulses enter a BB84-encoding module. It consists of a half-wave plate (HWP), two polarising beam splitters (PBSs), and one non-polarising beam splitter (BS). The photons emitted and sent to the ground station are randomly prepared in one of the four polarisation states: horizontal, vertical, linear 45°, and linear -45°. In the three ground stations, corresponding BB84-decoding setups are used, consisting of a BS, an HWP, two PBS and four single-photon detectors (see Supplemental Material).

For secure QKD, the average intensity per pulse sent over the channel has to be at the single-photon level. As the photons travel from the fast-moving satellite (~7.6 km/s) through the atmosphere to the ground station over typically ~1000 km, several effects contribute to the channel loss such as beam diffraction, pointing errors, atmospheric turbulence and absorption. As is typical for photonic communication, decoherence can be ignored. To obtain a high signal-to-noise ratio in the QKD protocol, one cannot increase the signal power but only reduce the channel attenuation and background noise. In order to optimise the link efficiency, you can combine a narrow transmitting beam divergence (~10 ?rad) with high-bandwidth acquisition, pointing, and tracking technique that ensures a typical tracking accuracy of ~1 ?rad. To reduce the background noise, the BB84-decoding setups in the optical ground stations are designed with a small field-of-view and employ low dark-count rate single-photon detectors.

The satellite flies along a sun-synchronised orbit which circles Earth every 94 minutes. Each night starting at around 0:50 AM local time, Micius passes over the three ground stations allowing for a downlink for a duration of ~300 s. Under reasonably good weather conditions, you can routinely obtain a sifted key rate of a ~3 kb/s at ~1000 km physical separation distance and ~9 kb/s at ~600 km distance (at the maximal elevation angle, note that this distance is greater than the satellite height due to the fact that the satellite does not fly directly over the ground stations), respectively. The observed quantum bit error rates are in the range of 1.0%-2.4%, which is caused by background noise and polarisation errors.

The satellite is equipped with an experimental control box (see Supplemental Material) that is able to exchange classical data with dedicated ground stations through radio frequency channels, with uplink and downlink bandwidth of 1 Mbps and 4 Mbps, respectively. This allows the ability to implement the full QKD protocol including sifting, error correction and privacy amplification (see Supplemental Material), to obtain the final keys between the satellite and the three ground stations.

Next, we rely on the satellite as a trusted relay to establish secure keys among the ground stations on Earth. Figure The new string can then be sent through a classical communications channel to Xinglong or Graz, who can decode the other’s original key by another exclusive OR (i.e. ?? = (?? ? ??) ? ??).

This process can be easily understood as Micius uses MX to encrypt MG and Xinglong decrypts the cypher text to recover MG, shared with Graz. Such a key is known only to the two communicating parties and the satellite, but not any fourth party. In this work, they established a 100 kB secure key between Xinglong and Graz. Similarly, secure keys between Nanshan and Xinglong, and between Nanshan and Graz can also be established.

For a real-world application of the space-to-ground integrated quantum network, they transmitted a picture of Micius (with a size of 5.34 kB) from Beijing to Vienna and a picture of Schrödinger (with a size of 4.9 kB) from Vienna to Beijing.

Well done to Jan-Wei group and those involved.

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