UK breakthrough promises secure comms

The privacy of information sent on optical communication networks could be greatly improved thanks to ground-breaking development work by Cambridge-based Toshiba Research.

The company has developed a very sensitive photo-detector capable of detecting the faintest possible optical signals and able to respond to individual photons, the smallest indivisible units, or quantas of light.

At the centre of the device are tiny disks of a semiconductor, or quantum dots, each only nanometers in diameter and a few nanometers high. Restricted to such short length scales, the electrons inside the dots display 'quantum' properties not seen in larger pieces of semiconductor.

In accordance with quantum theory, photon incident upon the device liberates an electron trapped within one of the dots. Until now, it has been difficult to detect the tiny charge associated with this single electron.

However, Toshiba Research has shown that this can be achieved by integrating the quantum dots inside a transistor structure, which was prepared in collaboration with the University of Cambridge.

Dr Andrew Shields, the project's leader, said: "The fact that a transistor containing quantum dots can detect individual photons is significant, because such a structure is likely to have several important advantages over conventional detectors."

The device has applications in the emerging field of quantum communication where data is transmitted along a conventional optical fibre encoded at the single photon level. By using single photons, the two parties involved in a communication are able to do the seemingly impossible - and detect if their communication has been intercepted or altered en route by a hacker or eavesdropper.

This security is guaranteed by fundamental laws of quantum mechanics, which mean that the quantum state of a photon cannot be measured without altering its properties in a detectable way.

Quantum communication can be used to allow two users of an open network, such as the internet, to exchange secret cryptographic key that can subsequently be used to encrypt data sent between them.