Novel LED emits single photons

17 Jun 2002

The first practical electrically driven single-photon generator could lead the way to secure optical networks.

Courtesy of PhysicsWeb

Photon fix

Andrew Shields and colleagues at the UK's Toshiba Research Europe and the University of Cambridge have developed a new type of light emitting diode (LED) that emits single photons at 5 K.

By adding a layer of quantum dots to a conventional LED the team claims to have made a single-photon-emitting diode. This latest development looks set to provide secure quantum cryptography for optical telecoms as well take a key role in quantum computing and extremely sensitive optical experiments (Zhiliang Yuan et al 2001 Science to appear).

Quantum dots are nano-sized deposits of one semiconductor embedded in another semiconductor. The dot material has a smaller energy bandgap than its surrounding material, which means it can trap charge carriers.

To make the emitter, Shields and colleagues first deposited an array of indium arsenide quantum dots in a layer of undoped gallium arsenide. This composite was then sandwiched between layers of hole- and electron-doped gallium arsenide.

The team applied voltage pulses across the structure to force the holes and electrons from the doped GaAs layer into the undoped, quantum dot-filled GaAs layer. Because quantum dots have low potential energies, each quantum dot was able to capture a hole and an electron, which combined to form a single photon. A tiny aperture fixed on top of the device meant photons from just one of the quantum dots escaped.

Shields and co-workers designed their generator to emit infrared photons, which corresponds to the sensitive region of their single-photon counter. However they say that 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 above 5 K, the temperature at which the current device operates. Previously electrically-driven devices only worked at millikelvin temperatures while single-photon generators that were driven by laser were bulky and impractical.

"A single photon source is a building block for a wide range of applications in quantum information technology, of which secure optical communications is the most immediate," says Shields. "In the future we may see that quantum effects enable many new optical technologies, rather like the laser did a few decades ago."