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Oct. 25, 2004 – Thirty-six years ago, BBN Technologies helped to invent the Internet, inadvertently opening the door to today’s world of computer viruses, hacking and identity theft.
Now BBN is back again, creating another technology that may well shut that door for good.
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Earlier this summer Cambridge, Mass.-based BBN launched the first network employing quantum-key distribution, exploiting the finicky nature of photons to create theoretically unbreakable encryption. The six-node network connects BBN, Harvard University and Boston University (BU), and researchers say it quickly is demonstrating that the strange world of quantum mechanics has useful applications here in the human one.
“At this point, the technology clearly works,” said Chip Elliott, a principal scientist at BBN and leader of the project. “Now you shift to the value proposition: How much does it cost, and what does it do for me?”
Preliminary results suggest quantum-key distribution (QKD) might do a lot to secure Internet data. Internet communication works by transmitting data as packets of electrons, encrypted with some numeric formula. Factoring the product of two large prime numbers, for example, is a standard encryption technique. But if hackers can factor that product, the code is cracked.
QKD sends each bit of data across the network as a single photon. As such, the photons are so delicate that their travels cannot be monitored at all; the very act of observation disrupts their motion. So any attempts at hacking should, in theory, be detected immediately.
The Defense Advanced Research Projects Agency (DARPA, the same agency that paid BBN in the 1960s to design the Internet) has had an interest in quantum cryptography for several years. Through a conference DARPA hosted in 2000 to discuss quantum technologies, Elliott met Harvard quantum theorists John Myers and Tai Wu. Next aboard was the BU Photonics lab, and the group won funding from DARPA to develop a quantum-encrypted network.
Physically, the network looks bland. Two nodes, named Alice and Bob, reside in a specially designed room in the basement of BBN. A laser next to each node spits out photons. The photons run through interferometers covered in pink boxes, which calibrate the particles to a precise wavelength.
Then the photons travel through an optical switch that directs them to their destination: either across the room to the other node, or through fiber laid under the streets to Anna (Harvard’s node) or Boris (BU’s node). The receiving node catches the photon and runs it through various software protocols designed by BBN, converting it into the original message.
The technical challenge is the precise control and modulation of single photons, Elliott says. The interferometers must be synchronized perfectly, so that the faint photons travel at precisely the same wavelength and the nodes can find them.
“To make a single photon is not too hard, but to detect them is extremely difficult,” Elliott said.
Myers adapts quantum theory to the real-world effects the network often generates. For example, lasers sometimes fire more than one photon at a time. His job is to deduce what consequences may come from that.
“One of the big challenges for me is figuring out how the other stuff affects behavior,” he said. “When you build these things, you find the components don’t always work the way they should.”
One obstacle is QKD’s effective range. Photons can only travel about 50 or 60 miles before they run out of steam; to rejuvenate them through an amplifier for further travel (a common procedure for standard Internet communication) risks ruining their delicate state.
Elliott’s brigade wants to tackle that challenge with photon entanglement, another branch of quantum mechanics that says one photon can affect the behavior of another photon far away. They plan to launch photon-entanglement tests on the network later this fall.
“That’s the only serious limit, the distance,” said BU physicist Gregg Jaeger. “It’s a very big thing.”
While QKD would involve only minor adjustments to a company’s network hardware, the software differs considerably. Elliott estimates that even basic QKD systems would approach $100,000. Plus, top speeds so far only rival dial-up modems, and issues such as authenticating the identity of other users haunt QKD as they do every other encryption system.
Myers concedes that much work remains to be done there. “We’ve taken one link of the chain and made it much stronger,” he said.