11 April 2012—In the quest to build networks that can carry information reliably and with absolute security over long distances, scientists are now a big step closer, according to research in this week’s issue of Nature.
Stephan Ritter and his colleagues at the Max Planck Institute of Quantum Optics, in Garching, Germany, have constructed an elementary quantum network with two nodes. They say it is a proof of concept that could one day be extended to create large-scale quantum information networks that guarantee the security of message transmission by encoding data using the quantum state of photons.
“There is this vision of a quantum Internet enabling worldwide exchange of quantum information, similar to what is nowadays possible with classical information,” says Ritter, putting the work into context.
Quantum information networks are a subject of scientific fascination because they are not susceptible to eavesdropping. First described in 1984 by Charles Bennett of IBM and Gilles Brassard of the University of Montreal, quantum cryptography—which relies on the transfer of information encoded in the quantum states of photons—is considered by many to be the ultimate unbreakable code.
A fundamental rule of quantum mechanics says that when a quantum system is measured, the nature of the quantum system changes. Because of that, data encoded in quantum properties of particles—such as the polarization of photons—facilitate secure communication between two parties. Any eavesdropper necessarily changes the quantum properties in noticeable ways.
Rudimentary quantum information networks have been built before. There are at least three companies that market quantum information devices. In 2008, the city of Vienna installed a quantum information network that was sponsored by the European Union. But in those networks, some nodes could only send data, and others could only receive it.
Ritter and colleagues built a universal node capable of receiving quantum information, storing it, and transmitting it. They built the nodes in the elementary quantum information network using single rubidium atoms that were more or less permanently trapped in optical cavities. (Optical cavities are atom traps built at cryogenic temperatures, where lasers are used to confine the atoms.) Single photons served as information carriers, with the quantum information from the rubidium atoms encoded in the polarization of the photons.
“The single atoms are excellent quantum memories, and the single photons are ideal for transmission,” says Ritter.
The two nodes were 21 meters apart, although the optical-fiber link was even longer at 60 meters.
The work was challenging, says Ritter, because quantum information is extremely fragile. “In order to prevent alteration, or even the loss of the information, it is necessary to have perfect control over all quantum network components,” he says.
He says his colleague Gerhard Rempe has spent most of the past decade “continuously developing and improving single-atom cavity systems into a reversible interface between light and matter, which finally rendered this first elementary demonstration of a quantum network possible.”
Raymond Laflamme, executive director of the Institute for Quantum Computing at the University of Waterloo, in Canada, says nodes will need higher accuracy to become practical. But he calls the work “an important step towards moving quantum information science towards quantum engineering….What was a dream 10 years ago now is becoming reality.”
Other experts in the field echo this sentiment. Alain Aspect, professor at the École Polytechnique in Palaiseau, France, says that getting the nodes to work together seamlessly in a quantum network was “a tour de force.”
Ritter says his team has a strategy for improving every aspect of its proof-of-concept approach. “One obvious route is to scale up the system and go beyond two-network nodes, realizing more complicated network architectures,” he says. Another “long-term goal is to build a ‘quantum repeater’ based on single-atom-cavity systems that would enable quantum communication over very long distances.”
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