Quantum crypto boosted to 2 GHz

June 15/22, 2005

Quantum cryptography makes it possible to tell for sure whether a secret encryption key has been compromised. The trouble is, today's quantum cryptography systems are generally quite slow.

Researchers from Heriot-Watt University in Scotland, Milan Polytechnic in Italy, and University College Cork in Ireland have upped the speed of their quantum key exchange system to 2 gigahertz over several kilometers of optical telecommunications fiber.

Quantum cryptography methods use the properties of photons to represent the 1s and 0s of computer information. Given a single photon representing a single bit of data, it is impossible for an eavesdropper to examine the data and not be detected.

The method could eventually be used as a way to send secure encryption keys over networks within metropolitan areas, according to the researchers. The system works with 850-nanometer-wavelength light, which limits it to distances under 20 kilometers.

Key to speeding things up is a pulse pick-off linear network -- a device that improves the performance of the silicon single photon avalanche diodes that detect the photons on the receiving end. This type of diode generates an avalanche of electrons when a photon hits it, making it easier to electrically detect photons.

The pulse pick-off linear network detects the beginning of an electron avalanche. Being able to detect an avalanche at the start reduces the amount of noise at the point of detection, which reduces the system's overall error rate from 18 percent to 7 percent at a speed of 2 gigahertz. Quantum cryptography systems must operate at error rates below 10 percent to guarantee security.

It will be at least five years before the method is ready for practical use, according to the researchers. The work appeared in the April 18, 2005 issue of Optics Express (Quantum Key Distribution System Clocked at 2 GHz).

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