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  • Quantum Mechanics Keeps Secrets Safe

Photonics Spectra
Nov 2004
Hank Hogan

If you want to keep a secret, it helps to whisper. That's what researchers at NuCrypt LLC have demonstrated in a technique for quantum encryption that uses off-the-shelf components, runs at up to 650 Mb/s, is compatible with the existing telecommunications infrastructure and is highly resistant to eavesdropping. The approach could be deployed within a few years to protect data in transit between two points separated by tens or hundreds of kilometers.

Rather than using single photons, the method employs a laser that's a factor of 100 to 1000 times weaker than those used in classical telecom networks. Prem R. Kumar's group at Northwestern University in Evanston, Ill., performed the original research, and NuCrypt was formed to commercialize the technique.

Data is encrypted by altering either the polarization or the phase of a photon stream. The phase, for example, might be changed to some value between 0° and 360° in any one of 4000 or so equal steps. The sender and receiver share a short secret key, which they extend into a longer running key using a prearranged algorithm. The longer key determines the phase encoding of each bit. Thanks to the running key, the receiver can decipher the phase of the incoming beam into a string of ones and zeros.

An eavesdropper, on the other hand, wouldn't have that crucial bit of knowledge and so would be forced to attempt a precise measurement. That effort, however, runs up against quantum mechanical limitations. By lowering the power by several orders of magnitude, the researchers produced a beam with some 10,000 to 100,000 photons in every nanosecond time slice, or bit. That photon density is low enough that the particlelike nature of light and the resulting inherent noise make the measurements required to eavesdrop technically very difficult.

Although commercial quantum key distribution products are available from such companies as MagiQ Technologies Inc. of New York and id Quantique SA of Geneva, these devices depend on the single-photon method. The resulting limits on range and data rates are one reason why these products provide only quantum key distribution and do so only in metropolitan settings.

The NuCrypt technique, in contrast, can work over much greater distances and at data rates compatible with the OC-12 standard of 622 Mb/s. Research continues, with the aim of achieving even higher data rates, such as those of OC-48, or 2.48 Gb/s.

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