Hooked on photonics

BBN researcher builds hack-proof quantum cryptography network that uses photons to generate secure keys.

CAMBRIDGE, MASS. - Chip Elliott is every hacker's worst nightmare.

Elliott, principal scientist at BBN Technologies , leads a team building the world's first continuously operating quantum cryptography network, a 12-mile snoop-proof glass loop under the streets of Boston and Cambridge.


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Quantum cryptography uses single photons of light to distribute keys to encrypt and decrypt messages. Because quantum particles are changed by any observation or measurement, even the simplest attempt at snooping on the network interrupts the flow of data and alerts administrators.

While the technology is still in the pilot stage, Elliott envisions a day when quantum cryptography will safeguard all types of sensitive traffic. "It's not going to overnight replace everything we have," he says. But it will be used to augment current technologies.

Defense funding

BBN's research is funded by the Pentagon's Defense Advanced Research Projects Agency , so it's likely the government would be first in line to roll out the super-secure technology. Elliott predicts financial firms will deploy quantum cryptography within a few years and estimates that businesses in general will deploy within five years. The technology also could move to the consumer market - for example, in a fiber-to-the-home scenario to protect the network between a home and service provider.

"People think of quantum cryptography as a distant possibility, but [the network] is up and running today underneath Cambridge," Elliott says. The team of nine researchers from BBN, four from Boston University and two from Harvard University, have put together "a set of high-speed, full-featured quantum cryptography systems and has woven them together into an extremely secure network," he says.

The system is essentially two networks - one for quantum key distribution and one that carries the encrypted traffic. And although it's probably the world's most secure network, it's not protecting any real secrets, at least not yet. For this pilot phase, BBN encrypts normal Internet traffic such as Web pages, Webcam feeds and e-mail.

The network has 10 nodes. Eight are at BBN's offices in Cambridge, one is at Harvard in Cambridge, and another is across the Charles River at BU's Photonics Center.

In keeping with the traditional naming convention that IT security professionals use (see story ), the nodes are named Alice, Bob, Ali, Baba, Amanda, Brian, Anna, Boris, Alex and Barb.

Inside the BBN labs

Elliott works out of an unassuming lab in a two-story brick building on BBN's campus. A labyrinth of blue corridors leads to the two-room lab tucked away in the basement.

A mass of cords and wires snake from all varieties of electronics on a table. BBN built much of the optics and electronics that are housed on server racks, and there are several Windows and Unix machines. Pink neon wire is strung high above, and a server rack is embellished with glowing blue plastic cylinders - all props obtained from a comic book.

The only hints that this isn't your run-of-the-mill network are a large pink rectangular box that contains a coupler and phase shifter, and a door marked "Danger." Behind it lies a laser about the size of a cement block. Despite the warning, the laser is low-powered enough that it's safe to enter without protective goggles, unless someone needs to open the laser source to make an adjustment inside.

How it works

The two oldest nodes, Alice and Bob, have been running about a year and use phase-modulated cryptography.

A laser is used in phase-modulated cryptography to separate individual photons and send them to a modulator. The modulator pumps them out to other nodes over fiber-optic cable. The photons are encoded by sending them out at different intervals: A long gap indicates one bit of information, and a shorter one a different bit.

BBN's super-secure networkOn the receiving end, another device accepts the photons and recognizes how they're modulated. If the sequence matches what was originally sent, the keys are stored and used to unscramble data sent through conventional means, such as over the Internet.

Some of the other nodes use an entanglement quantum key-distribution system, which essentially splits one photon into conjoined twins. If you manipulate one, the other is affected. BBN also is using free-space quantum cryptography to send keys in the air rather than over fiber.

Next steps

As Elliott approaches the first anniversary of the network, he says, "It's a miracle we ever assembled the fiber for this.'' It took BBN researcher Henry Yeh more than two and half years dealing with multiple carriers just to piece together the fiber optics.

Looking ahead, Elliott has several initiatives on his plate.

He wants to keep adding nodes to the network.The network currently runs at up to 5M bit/sec, and Elliott wants to boost speeds into the hundreds of megabits per second. The limiting factor is the detector, which senses the passage of the photon. None are commercially available that run at those speeds, so he wants to build his own.

No one has ever built a quantum cryptography eavesdropping mechanism, and Elliott wants to create the first one, which he's calling Eve. "Building Bob is hard enough. Eve is a lot harder,'' he says.

Another challenge is the distance limitation - with current technology quantum cryptography works at a distance of up to 50 miles. But Elliott believes the technical hurdles ultimately will be dealt with. "Someday it will be possible to do it across continents or under the ocean, but not right now," he says.

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