22 October 2008—Vienna has been the backdrop of some major milestones in the new science of quantum cryptography, and on 8 October, the city made its mark again. Scientists there have booted up the world’s largest, most complex quantum-information network, in which transmitted data is encoded as the quantum properties of photons, theoretically making the information impervious to eavesdropping.
Built at a cost of 11.4 million euros, the network spans approximately 200 kilometers, connecting six locations in Vienna and the neighboring town of St. Poelten, and has eight intermediary links that range between 6 and 82 km. The new network demonstrated a first for the technology—interoperability among several different quantum-cryptography schemes. The project, which took about six months longer to complete than planned, was so complex that some wags are calling the Viennese network ”the mother of all quantum networks.”
Since 1984, when the concept was put forward by Charles Bennett of IBM and Gilles Brassard of the University of Montreal, quantum cryptography has seemed likely to become the ultimate unbreakable code. A fundamental tenet of quantum mechanics is that measuring a quantum system changes the nature of that quantum system. This makes data encoded as the quantum characteristics of particles—such as the polarization of photons—ideal for secure communication between two parties. According to convention in cryptography, the communicating parties are called Alice and Bob, while the eavesdropper is called Eve. Let’s say that Alice and Bob are using the polarization of photons for an encrypted exchange over a quantum channel. If Eve eavesdrops along the route, she can’t help but change the polarization of some of the photons, so Bob will always detect her.
Quantum communication devices do not communicate a message but are used instead to securely generate and distribute cryptographic keys—data needed to encode or decode a message. There are at least three companies marketing quantum-key distribution devices today.
Each of the individual technologies used to build the Viennese network—including quantum cryptography through optical fiber and quantum communication through the air— had been demonstrated before, but no one had been able to tie the various techniques together.
Project manager Christian Monyk, of the Austrian Research Centers, says that if a worldwide quantum-information network is to exist someday, it must be able to deal with various technology preferences that different users choose, just as a telecommunications network does today.
”This is something new in quantum cryptography,” said Nicolas Gisin of the University of Geneva, who worked on quantum-key distribution over the network. ”The network aspect was the most important aspect. This is not a trivial network, as every quantum system can speak to the other ones.”
Gisin says that the various quantum-cryptography techniques have different strengths and weaknesses, so it became necessary to establish a certain minimum standard. This was done by setting specifications for error correction, bit rates, and response times and by developing software to ensure that the standards were met.
Conceptually, networks are built in layers, with the physical layer—the quantum-cryptography links, in this case—as the bottom layer and other parts, such as the functions that route messages, atop that layer. The hardest part of the project, according to Norbert Lütkenhaus of the University of Waterloo, in Canada, a theorist who worked on the security of the network, was adding layers atop the quantum cryptography while maintaining the security level of the cryptography.
In the network’s first demonstration, its designers showed that if one quantum link broke down, messages were securely rerouted without disruption using other nodes and links, much like what happens in an ordinary data network today.
The European Union�funded project, called SECOQC, which stands for ”secure communication based on quantum cryptography,” was the culmination of four and a half years of effort. The project involved more than a dozen groups of researchers and industrial partners, including Siemens, which provided the standard commercially available optical fiber for the network, and ID Quantique, one of the producers of quantum-key distribution machines.
The next step, says Monyk, is ”to try to install a virtual private quantum network in a network belonging to a telecom operator.”
About the Author
Saswato R. Das is a science writer based in New York City. In the September 2008 issue, he reported on plans for a satellite-based quantum-cryptography system.