The first practical, electrically driven, single-photon generator could play a key role in the emerging technologies of quantum cryptography and quantum computing.
Researchers at Cambridge and from Toshiba added a layer of "quantum dots" to a conventional light-emitting diode to create their device, which could also be used for very sensitive optical experiments.
Quantum dots are nano-sized deposits of one semiconductor embedded in another. The dot material has an energy bandgap that is smaller than that of the surrounding material. This allows it to trap charge carriers.
Scientists deposited an array of indium arsenide quantum dots in a layer of undoped gallium arsenide, which they sandwiched between layers of hole- and electron-doped gallium arsenide. They then applied voltage pulses across the structure.
These pulses force positive holes from the hole-doped layer and electrons from the electron-doped layer into the undoped layer, which contains the quantum dots. Owing to its low potential energy, each of the quantum dots can capture a hole and an electron, which combine to produce a single photon. A tiny aperture on top of the device allows photons from one of the quantum dots to escape.
The generator was designed to emit infrared photons, which corresponded to the sensitive region of the single-photon counter. The quantum dots can be tuned to emit light at a range of wavelengths, including 1.3 µm - the wavelength used in fibre-optic communications. Dots that generate photons with longer wavelengths are also efficient at greater than 5 K - the temperature at which the current device operates.
Until now, single-photon generators have been driven by lasers, which makes them both bulky and impractical. Previous devices that were electrically driven only worked at milliKelvin temperatures.
Reference
Zhiliang Yuan et al. (in press) Science.