Cryptography breaks 100 km barrier

Jun 9, 2003


Researchers in the UK have broken the distance record for quantum cryptography, the optical technique that enables ‘unhackable’ communication along an optical fibre. Andrew Shields and colleagues from Toshiba Research Europe, UK, revealed their record-breaking link, which reaches over 100 km, at the Conference on Lasers and Electro-Optics (CLEO) in Baltimore, US.

"As far as we are aware, this is the first demonstration of quantum cryptography over fibres longer than 100 km," said Shields. "The technique could be deployed in a wide range of commercial situations in less than three years."

Communication with quantum cryptography is inherently secure because it takes advantage of the physical properties of single photons. In the technique, each transmitted bit of a cryptographic key is encoded upon a single photon. The sender and recipient each have a key to decode the photon stream, but any attempt to hack into the link and capture the key is doomed to failure as it alters the quantum state of the intercepted photons. These changes are easily detectable, revealing the presence of the hacker. In practice, attenuation in the optical fibre and noise in the detection unit limits the distance over which quantum cryptography works.

The Toshiba team was able to improve the link distance by minimizing errors due to noise in the avalanche photodiode that detects the single photons. In the future the system could be extended further by using a GaAs/AlGaAs modulation doped field effect transistor (MODFET). This device, which is under development at Toshiba, does not rely on avalanche processes and is therefore less prone to noise than conventional devices (see related story).

The previous transmission record of 87 km was set by researchers from the Japanese company Mitsubishi Electric in November last year. They also developed a novel kind of detector, which had a low dark-count probability, to extend the link distance.

Banks and government organizations are expected to be the first users of quantum cryptography systems when they become commercially available.