Roland Piquepaille's Technology Trends
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mercredi 14 juillet 2004

Quantum cryptography has reached another big step towards potentially perfect secure communications. BBN Technologies, Harvard University and Boston University researchers have built a six-node quantum cryptography network in Cambridge, Massachusetts. The network, which is operational since December 2002, "operates continuously to provide a way to exchange secure keys between BBN and Harvard, which is about 10 kilometers away," providing a totally secure virtual private network. In "Quantum crypto network debuts," Technology Research News (TRN) reports that this network uses existing Internet protocols including the secure Internet Protocol (IPsec). In other words, this technology is ready for practical applications as soon as today.

Here is the introduction of the TRN article.

Quantum cryptography has the potential to guarantee perfectly secure communications, but until now all of the prototype systems have been point-to-point links rather than networks that share connections.
BBN Technologies, Harvard University and Boston University researchers have built a six-node quantum cryptography network that operates continuously to provide a way to exchange secure keys between BBN and Harvard, which is about 10 kilometers away. "Any node in the network can act as a relay," said Chip Elliott, a principal scientist at BBN Technologies. The researchers will soon move one of the network nodes across town to link Boston University into the network, said Elliott.

Elliott and his colleagues have published their research work, which is available online on the arxiv physics archive. Here is a link to the abstract of this paper, "Quantum Cryptography in Practice."

BBN, Harvard, and Boston University are building the DARPA Quantum Network, the world's first network that delivers end-to-end network security via high-speed Quantum Key Distribution, and testing that Network against sophisticated eavesdropping attacks. The first network link has been up and steadily operational in our laboratory since December 2002. It provides a Virtual Private Network between private enclaves, with user traffic protected by a weak-coherent implementation of quantum cryptography. This prototype is suitable for deployment in metro-size areas via standard telecom (dark) fiber. In this paper, we introduce quantum cryptography, discuss its relation to modern secure networks, and describe its unusual physical layer, its specialized quantum cryptographic protocol suite (quite interesting in its own right), and our extensions to IPsec to integrate it with quantum cryptography.

The three images below are extracted from the full paper (PDF format, 12 pages, 212 KB) (Credit: BBN Technologies). This paper contains many more diagrams and is a must-read if you're in the field of quantum cryptography.

Quantum Key Distribution Here is a diagram of the quantum key distribution.
BBN's Alice These two pictures show the physical layers of the weak-coherent link in the laboratory. Here is Alice.
BBN's Bob And this is Bob.

Let's return to the TRN article, for more details about the network.

Four of the nodes are connected via fiber-optic cable and two nodes use wireless optics. The network is limited to metropolitan areas and will require the development of quantum repeaters to span greater distances. Because the quantum properties of photons are lost if they are observed, they cannot be copied, but making copies of light signals is the way signals are boosted along ordinary telecommunications lines.
Quantum repeaters, which are under development at several research labs around the world, would instead transfer the quantum state of one photon to another through interactions with atoms or through the strange quantum phenomenon of entanglement, which allows traits of two or more particles to be linked regardless of the distance between them.

Finally, the big news is that this network is using existing Internet protocols, meaning that practical implementations are possible today.

The quantum cryptography network works with Internet protocols including the secure Internet Protocol (IPsec) and creates a type of virtual private network, which provides secure communications over unsecured networks like the Internet at large. The idea is that even if an eavesdropper is able to listen in on a line, he would be unable to learn much about the communications traversing it.

Sources: Eric Smalley, Technology Research News, July 14/21, 2004; BBN Technologies, via the arxiv physics archive

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