Qscale Quantum technologies for extending the range of quantum communications

General audience

The QScale project is devoted to the development of advanced quantum communication technologies, specifically of quantum repeater architectures. Here is a non-specialist intended description of the project.

Quantum information Vs classical information


Classical information theory (our current description of information, with applications in computer science, data transfer technology, and so on …) relies on the fact that information is being processed in a world obeying the laws of classical physics.
However, classical physics is only a (very good) approximation to physical reality, and we know that in the microscopic world, phenomena must be described by a richer physics, namely quantum physics. Although quantum physics has been known for almost a century, scientist realized only about twenty-five years ago that it can have applications in information processing.
This was the beginning of quantum information science.

Indeed, scientists have shown theoretically that using quantum physics instead of classical physics can lead to the accomplishment of practically useful and otherwise impossible achievements in information technology. For example, quantum cryptography allows one to teleport a secret message. The quantum teleportation process guarantees that it is impossible to spy on the message.

Quantum teleportation and the secure sending of secret messages


 Sending secret messages if of course a problem of great importance in all the fields where confidentiality is required. Quantum teleportation achieves the ultimate data transfer security. It comes in different versions, but this is a quite general outline of how it works :

 Imagine two persons, usually nicknamed Alice and Bob, located at different places, who wish to communicate secret messages with one another. They want to be sure that no-one can intercept and spy on the message.

In a first time, Alice and Bob will create a quantum link, called ‘entanglement’. Photons, i.e. particles of light, are used to propagate entanglement. They are used because they travel extremely fast through the air or through optical fibres.

 In a second time, by openly communicating, they can check the quality of the secret link without altering this quality. If the quality has been found good, then the quantum link is safe, and it can be used to perform teleportation. If not, they discard the link and will look for the spy. In any case, they can be sure of the privacy of their quantum link before they use it to actually transfer data.

However, there is still a major difficulty to overcome in order to be able to use quantum cryptography at long distances. Indeed, if the distance to travel is too long, then the photons used to create the quantum link will be lost during their propagation, retarding and eventually impeaching the creation of the required entanglement.
Amplifying the quantum message is also no good, because it would degrade the quantum link quality in a similar fashion to a spying attempt. There is therefore a need of innovative solutions in order to be able to transmit quantum entanglement at long distances. This is the challenge addressed by the QScale project.

Long distance quantum communications need quantum repeaters


A cloud of cold atoms, a single ion trap and a rare earth doped crystal : different physical systems that can be used as quantum memories for light : the first step towards quantum repeaters and long distance quantum networksThe concept of quantum repeaters was introduced in the late nineties, and would provide a solution if one were able to successfully construct and operate such devices. The key element is a quantum memory : a device able to record and store the quantum state of light without degrading the quality of the entanglement it carries.

The goal of the QScale project is to develop such quantum repeaters, the first step towards a future quantum networks.
Such networks will offer many new capabilities, from the efficient simulation of collective phenomena to a future “quantum internet”.

Some general readings


Jeff Kimble, the quantum internet (Nature) : http://www.nature.com/nature/journa...
Julien Laurat : Mémoires quantiques, stocker l’insaisissable (for French readers) :
http://www.pourlascience.fr/ewb_pag...
The quantum information wiki (for more advanced readers) :
http://www.quantiki.org/
Textbook on quantum information (for specialized readers) :
Quantum Computation & Quantum Information (ISBN : 9788175960923), Michael Nielsen & Isaac L. Chuang

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