Scientists have reported an important speed breakthrough which brings closer the day when quantum encryption becomes a usable part of communications security.

The National Institute of Standards and Technology (NIST), a U.S. government agency, has reported that it has managed to shift quantum-encrypted information at a "raw" throughput of 4 million bits per second across a 1 km-long fiber link.

This is at least twice NIST's previous record, which has been rising since the agency announced it had broken the 1 million bits per second barrier in May 2004.

At such transfer rates, it becomes practical to use quantum key distribution (QKD) cryptography to secure a video stream.

"This is all part of our effort to build a prototype high-speed quantum network in our lab," said NIST's Xiao Tang. "When it is completed, we will be able to view QKD-secured video signals sent by two cameras at different locations. Such a system becomes a QKD-secured surveillance network."

Despite sounding to non-initiates like a branch of mathematical magic, the benefit of quantum key distribution in communications is really the 100 percent certainty it offers that data has not been intercepted or tampered with as it moves between two points.

Anyone attempting to decrypt a photonic data stream encoded using such a system will -- as predicted by Heisenberg's oft-quoted uncertainty principle -- inevitably alter the state of the photons in a way that can be detected. This introduces a degree of verifiability impossible in conventional electromechanical encryption systems.

The agency reckons its test-bed demonstration will pave the way for the technology's use in specific applications such as military data transfer, or the transmission of sensitive financial and health-care information.

Despite the upbeat atmosphere, there is a way to go for QKD. The trade-off is still speed relative to distance, which at 1 km for the stated throughput is still too short a distance to have many practical applications. As NIST admits, other labs have managed further distances but at the cost of cutting data rates.

In 2003, a collaboration between a number of Japanese companies achieved distances of up to 100 km in tests.

In the U.K., the Cambridge Research Laboratory has demonstrated a working commercial system running at 25 kilobits per second over a distance of 122 km.