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Quantum dots detect single photons

17 Jun 2002

A single-photon detector based on quantum dots has been developed for the first time by researchers at Toshiba's European laboratories in the UK.

Conventionally single photons are detected by multiplying a photo-generated electron using an avalanche process. The Toshiba researchers, in collaboration with Cambridge University, developed a device for detection of single photons based on a GaAs/AlGaAs modulation doped field effect transistor (MODFET) which does not rely on avalanche processes.

The conducting channel of the transistor is closely spaced from a layer of quantum dots. If the separation of the quantum dots and the channel is just several nanometers, the resistance of the transistor is sensitive to a change in the occupancy of a single quantum dot by just a single electron.

By avoiding the avalanche process the device should be less prone to noise. It is also anticipated to have a fast time response because it is based on a transistor.

Project leader, Andrew Shields says the main motivation for the research was the application for quantum cryptography for secure data transfer. He said: "Quantum effect semiconductor devices are the ideal technology for detecting single photons. Although there is now tremendous interest, the field of quantum information technology is still relatively young."

By using single photons, two parties communicating via an optical fibre can detect if their communication has been intercepted or altered en route. Security is guaranteed because a quantum state cannot be measured without altering its properties in a detectable way.

"In the CLEO paper we show data taken at liquid He temperature, 4K. We have, however, since its submission, demonstrated single photon detection at 77K, which allows the device to be cooled in a dewar of liquid nitrogen. However, there are still improvements to be made to the device, so we hope to achieve much higher operating temperatures."

Story courtesy of Opto and Laser Europe magazine.

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