NIST Quantum Key System Combines Speed, Distance

October 23, 2007 // Published as a news service by IHS

Experts said the new system can perform a theoretically unbreakable "one-time pad" encryption, transmission and decryption of a video signal in real time over a distance of at least 10 kilometers (km).

Key distribution - the problem of ensuring that both the sender and receiver of an encrypted message (and no one else) share the same long string of random digits (the so-called "key") used to encode and decode the message - has always been one of the most important challenges in cryptography.

Since the 1980s, it's been recognized that the unique properties of quantum mechanics - the fact that certain measurements cannot be made without altering the thing measured - offered the possibility of a system that could transmit as long a key as desired between two parties with no chance that it could be copied undetectably by a third party.

Since then, the race has been on to build a fast, practical and reliable QKD system. One important requirement for any candidate system is that it be compatible with existing fiber optic telecom networks that transmit at wavelengths of either 1550 or 1310 nanometers (nm) to reach the greatest distance.

Another requirement is a highly efficient photon detector that can detect single photons reliably without introducing significant amounts of "noise." One of the best low-noise detectors, a silicon-based avalanche photo diode (Si-APD), does not function at the telecom wavelengths.

Instead, it operates best at much shorter wavelengths - around 700 nm. To take advantage of the Si-APD, the NIST group designed a subsystem to "up-convert" single photons from a transmission wavelength of 1310 nm to 710 nm for high-efficiency detection.

The QKD system that incorporates this up-conversion technique, described recently in Optics Express, generates and transmits secure keys at a rate of over half a million bps over 10 km of optical fiber, fast enough to encrypt streaming digital video using one-time pad in real time.

The group also transmitted secure keys at rates near 10 kilobits per second at five times that distance. The same team last year set a speed record for transmission of secure keys over a kilometer of fiber. Experts said this work improves the distance by at least 10 times.

Advantages of the new system, according to the research team, include high speed, high efficiency, low noise and convenience of operation. The fact that it uses a 1310 nm transmission wavelength somewhat limits the propagation distance, but adds the advantage that the parallel "classical-quantum" communication, which is needed for a full QKD system, can be realized in a single fiber without significant interference.

Details of the NIST up-conversion QKD research are available at http://w3.antd.nist.gov/quin.shtml.

Source: National Institute of Standards and Technology (NIST).