Gallium-nitride quantum dots can emit single photons at room temperature, according to new experimental observations made by researchers in Japan. The findings prove once and for all that such gallium-nitride quantum dots, which are wide-band-gap semiconductors, can be employed as room-temperature single-photon sources. The structures might prove to be ideal for on-chip communications in quantum-information processors and could also be a source of "flying" qubits for the quantum computing of the future.

Quantum dots (QDs) made from gallium-nitride materials could have many potential applications thanks to their unique properties, which include the fact that they are very stable (both chemically and at high temperatures) and have a large breakdown voltage. They can also emit photons over a wide range of wavelengths from the ultraviolet to infrared parts of the electromagnetic spectrum.

Unfortunately, until now, no-one had ever seen single-photon emission from these materials at room temperature because sample quality was poor. "Although researchers have observed room-temperature single-photon emission from other nanostructures, such as the colour centres in diamond, this is the first time that the photons have been seen emanating from a structure in which the quantum emitter has been fabricated at a pre-defined location," says team member Mark Holmes of the University of Tokyo. "Indeed, previous studies relied on structures that had formed at random locations on a substrate."

Growing quantum dots

The Tokyo team, led by QD pioneer Yasuhiko Arakawa, fabricated its devices in a clean room using a process called "selective-area metal-organic" chemical vapour deposition. The researchers grew the quantum dots on sapphire substrates covered with a 25 nm layer of aluminium nitride. This process included sputtering a 25 nm deep silicon-dioxide layer onto the substrate surface, and then creating arrays of apertures (25 nm in diameter) using electron-beam lithography and reactive-ion etching. These apertures then house gallium nitride nanowires and quantum dots, which were grown separately. The researchers' process allows them to control where each QD ends up on the substrate, since the in-plane position of every dot depends on where each nanowire is located in the first place and its distance from the substrate is defined by the nanowire height (which is about 700 nm). Arakawa and colleagues then measured the light-emission properties of their quantum dots by exciting them with a pulsed laser beam and detecting the light that came out.

Single check

In theory, a single-photon quantum emitter should emit a single photon per excitation light pulse. To test this, the researchers split the light emitted into two paths and, using two separate detectors, measured the time elapsed between the light pulses recorded at each detector. "For a pure single-photon source, we should not see photons at both detectors simultaneously – something that we verified in an experiment for the first time for this kind of site controlled gallium nitride quantum dot," says Holmes.

More importantly, the observations hold up even when the quantum dots are at room temperature. Holmes explains that this is because the team is using small gallium-nitride dots the positions of which the researchers can control accurately. Such dots are less contaminated spectrally, which means that they can still be detected at such high temperatures.

Single-photon emitters are often touted as being ideal for quantum-cryptography applications, but the researchers believe that the devices they have made will be more suited to on-chip communications for quantum-information processors. "We are now busy looking at ways to measure the operating speeds of our devices," says Holmes. "We are also trying to make them work by injecting current into them rather than exciting them with a laser."

The current work is detailed in Nano Letters.