Quantum crypto network debuts

By Eric Smalley, Technology Research News

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.

Because the Defense Advanced Research Projects Agency (DARPA) Quantum Network is switched, the organizations can share the costs of the fiber infrastructure, Elliott said. "BBN and Harvard can both talk to BU by sharing a single fiber, rather than each requiring its own fiber."

The network is resilient because any node in the network can act as a relay to connect two other nodes. Because there are multiple connections to and from any given node, "failure of a link or node does not mean that we have lost quantum cryptography," said Elliott.

Quantum cryptography schemes allow a pair of correspondents to securely exchange a one-time pad, or key that will unlock a scrambled message. The schemes call for transferring each bit of information using a single photon. The systems are potentially very secure because the quantum state of a particle cannot be observed without altering it. If the random string of bits that make up the key have been observed, it will be obvious to the sender and receiver and the key can be discarded.

The quantum network uses secure point-to-point connections between nodes and allows a given node to relay secure cryptographic keys between two other nodes. "The relays develop one-time pad keys with their nearest neighbors, and then use these one-time pads to protect end-to-end cryptographic material as it makes its way through the network, hop-by-hop," said Elliott.

The six nodes that make up the network are dubbed Alice, Anna, Ali, Bob, Boris and Baba. In its current setup, Alice-Anna can share keys through Bob and Boris, Bob-Boris can share keys through Alice and Anna, and Ali and Baba can share keys with all other nodes through Alice, said Elliott.

The network also contains a optical switch that can change the way the nodes are connected.

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.

The network's photon sources are currently heavily filtered lasers, which are extremely dim and sometimes emit more than one photon at a time. This makes them relatively inefficient and bits that are represented by more than one photon are not invulnerable to eavesdroppers. The Boston University researchers are working on more efficient photon sources that would emit entangled pairs of photons, one of which can be transmitted so two parties can share the pair. The sources would be brighter and would avoid the risk of multiple photons.

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.

The network is ready for practical applications today, said Elliott.

Elliott's research colleagues were Alex Colvin, Chris Lirakis, David Pearson, Oleksiy Pikalo, John Schlafer, Greg Troxel and Henry Yeh of BBN Technologies, Tai Tsun Wu, John Myers, Dionisios Margetis, F. Hadi Madjid and Margaret Owens of Harvard University, and Alexander Sergienko, Bahaa Saleh, Malvin Teich, Gregg Jaeger, Gianni Di Giuseppe and Hugues De Chatellus of Boston University. The research was funded by the Defense Advanced Research Projects Agency (DARPA).

Timeline:  Now
Funding:  Government
TRN Categories:  Quantum Computing and Communications; Cryptography and Security
Story Type:  News
Related Elements:  Technical paper, "Quantum Cryptography in Practice," posted on the arxiv physics archive at http://arxiv.org/abs/quant-ph/0307049


July 14/21, 2004

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