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Fiber-based cryptography combats fraud

16 Jul 2007

A fiber-based quantum key distribution system could be a step closer to achieving secure, high-speed data transmission over distances up to 10 km.

In a bid to combat increasing fraudulent online activities researchers from the US have used wavelength conversion to create a high-speed, fiber-based quantum key distribution (QKD) system. (Optics Express 15 7247)

"The main motivation for our research is to develop unbreakably secure communication systems within current commercial telecommunication infrastructures," Xiao Tang, a researcher from the National Institute of Standards and Technology (NIST), told optics.org. "For current QKD systems, improvements in speed and distance tend to be mutually exclusive because of photon loss during transmission. Our latest work is aimed at achieving a system with both long transmission distance and high key speed for practical applications."

The team achieved a secure key rate of 500 kbit/s at 10 km and 9.1 kbit/s at 50 km in a 625 MHz QKD system. "The primary application is secure communication in real-time at high speed," explained Tang. "For example, an important teleconference can be held securely when the attendees are at different locations. Beyond that, there are several applications anywhere information needs to be securely communicated between two or more disparate locations."

Tang's QKD system uses single photons in different orientations to produce a continuous binary code, or key, to encrypt information. The rules of quantum mechanics ensure that anyone intercepting the key is detected, which in turn provides a highly secure key exchange.

For fiber-based QKD over transmission distances longer than 10km, the wavelength of the quantum signal must be in the low-loss telecommunication windows around 1310 nm or 1550 nm. The main problem has been finding a good single photon detector at these wavelengths.

"Wavelength conversion becomes a key technique to solve this problem" explained Tang. "We designed the periodically poled lithium niobate (PPLN) waveguide device and used them to convert photons at 1310 nm to 710 nm by applying a strong pump source at 1550 nm. Our scheme features very low noise because the pump wavelength (1550 nm) is longer than the signal wavelength (1310nm)."

The researchers still see room for improvement in terms of the speed and distance of data transmission and are also looking to develop LAN and MAN QKD networks. "NIST will transfer the technology developed here to US industries to help them become more competitive," concluded Tang. "The key problem is single photon detection with high speed, high efficiency and low noise at telecom wavelengths. For its commercialization in the near future, its relatively high cost could be a problem."

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