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Quantum cryptography hits the fast lane

15 May 2009

Toshiba unveils a high-speed photon detector that could pave the way to practical quantum encryption.

Scientists at Toshiba Research Europe in Cambridge, UK, are claiming a significant step forward in the race to make quantum cryptography a practical proposition for secure network communications.

Quantum cryptography, or quantum-key distribution (QKD), uses quantum-mechanical principles to distribute secure digital keys that can be exchanged across optical networks. The rules of quantum mechanics ensure that anyone intercepting the key is detected, which in turn ensures a highly secure key exchange.

The Cambridge team's breakthrough, based on some nifty re-engineering of the single-photon detector, is the demonstration of the fastest key rate (1 Mbit/s) to date for a QKD system (New Journal of Physics 11 045019).

"We have built an electronic circuit that has turned an old, slow avalanche-photodiode (APD)-based single-photon detector into a high-speed one at a negligible cost," Zhiliang Yuan, a researcher at Toshiba, told optics.org. "Single-photon detection speed has now improved by a factor of 100. As a result, record key rates for QKD have been achieved."

Yuan continued: "For ultimate security, key distribution rates must be as fast as message communication. Until now, key rates were only sufficient for the encryption of live speech. Now, with key rates at 1 Mbit/s, encryption of live video becomes possible."

What's more, high-speed key distribution allows many users access to the same network; previous key rates of 10 kbit/s over 20 km and 10 bit/s over 100 km were not sufficient for this purpose. The Toshiba team demonstrated a key rate of 1 Mbit/s over 20 km.

Toshiba's APD is based on a compact and robust semiconductor device, which helps to keep cost as well as maintenance to a minimum. By applying a self-differencing circuit to an InGaAs APD, the team has built a user-friendly QKD prototype.

"Our competitors often use inconvenient devices for high-speed single-photon detection, including superconducting nanowires and nonlinear up-conversion crystals," explained Yuan. "Such detectors are inconvenient to use, whereas Toshiba's system not only provides higher speed but also has a cost advantage."

Toshiba's APD format could also benefit other applications such as remote sensing, biological imaging, laser ranging and high-quality quantum random-number generation.

Yuan and colleagues expect governments and financial institutions to be among the first adopters of QKD technology. "As the cost of QKD keeps coming down with time, it will become affordable to a wide range of potential users, including home users. This could happen in the next 10 years," concluded Yuan.

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