Toshiba Research Europe Ltd., Cambridge Research Laboratory

Quantum Information Group, Single Photon Detection

We have also pioneered the detection of single photons using quantum dots. The quantum dot single photon detector proposed and demonstrated by Toshiba, shown in the illustration, is based upon a transistor structure in which the conducting channel 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. This attribute allows the device to act as a detector of single photons, since absorption of a photon creates carriers in the semiconductor, which after capture by a dot, produce a detectable change in the resistance of the channel of the transistor.

The quantum dot single photon detector is expected to have several advantages over conventional types of single photon detector based on avalanche processes, such as the photomultiplier tube or the avalanche photodiode. Photomultiplier tubes are vacuum tube devices that, while excellent for many applications, are expensive, fragile, bulky and relatively inefficient. On the other hand, avalanche photodiodes are very prone to dark count noise, especially when operated at the high frequencies used in optical communications. This issue is especially important for quantum communication, since the noise levels of avalanche photodiodes are recognized as limiting the transmission distance and bit rate. Several other features of conventional detectors, including the requirement for high bias voltages, cryogenic cooling and extreme sensitivity to temperature changes and excess bias also make them inconvenient to use.

The quantum dot single photon detector works on an entirely different principle to conventional avalanche devices and can therefore, overcome many of their problems. In particular, by avoiding the avalanche process and its associated problems, it is less prone to noise. As it is based upon a transistor, the building block of today's high-speed electronic circuitry, it is predicted to have a fast time response. Another advantage is that the quantum dot detector works at low operating voltages (<5V) and is more robust.

Quantum cryptography is a very attractive emerging application for single photon devices. However, single photon detectors find many other uses in science and technology. In medical imaging, for instance, single photons are detected in PET and CT scanners and, more recently, for laser optical imaging. Lifetime fluorescence measurements using single photon counting is also used in the diagnosis of some medical conditions. It is also widely used in analytical chemistry for determining the chemical recipe of samples. Another application is in laser ranging, tracking and imaging and for industrial scanning and process control. Finally, single photon detection is also widely used in scientific research in the fields of particle physics, astrophysics and materials science

Further Reading

	Type	Title and author(s)	Source
1	Technical	Low-noise photon counting with a radio-frequency quantum-dot field-effect transistor by B. E. Kardynał et al	Applied Physics Letters 84, 419 (2004)
2	Technical	Single photon detection with a quantum dot transistor by A J Shields et al	Japanese Journal of Applied Physics, 40, p2058-64 (2001)
3	Technical	Detection of single photons using a field effect transistor gated by a layer of quantum dots by A J Shields et al	Applied Physics Letters, 76, 3673, 2000
4	Intermediate	Quantum dots detect single photons by A.J. Shields and M. Pepper	Photonik, February 2001
5	Intermediate	Quantum dots spot single photons by David Mowbray	Physics World (July 2000)
6	Intermediate	To catch one photon by Gilbert Chin	Science (7 July 2000)
7	Easy	Unhackable cryptography using light quanta (in Japanese)	Nikkei (3 June 2000)