Non-polar nitride quantum dots for application in single photon sources

Physicists understand that light can be thought of as either a wave, or a stream of tiny particles called "photons". A photon is the smallest amount of light which can exist. Using single photons, we can encode information for cryptography and computing. Quantum cryptography using photons offers the ultimate in data security, and linear optical quantum computation provides the opportunity for massively parallel data processing. However, progress towards these applications is limited by the current performance of single photon sources. Such a device can reliably provide one - and only one - photon on demand. Using a dim conventional light source in place of a true single photon source always risks the possibility of emission of multiple photons, compromising the security of quantum cryptography and corrupting the performance of quantum computers.

True single photon sources can be made using semiconductor quantum dots: tiny crystals with atom-like properties, whose very nature means that they emit a single photon upon optical or electrical excitation. Different semiconductor materials are being explored, including families of materials based on compounds of arsenic (the "arsenides") and on compounds of nitrogen (the "nitrides"). Of the two, the arsenides have been fairly widely studied, and can be used to produce efficient single photon sources, but with one major disadvantage: these devices only operate at very low temperatures: typically, 250 degrees below zero, or lower. The nitrides, on the other hand, have been used to demonstrate single photon emission at room temperature, which would obviously be much more convenient for real-world applications. However, this family of materials has been studied much less, and current devices are not very efficient and have a low rate of photon emission compared to the arsenides. Another difference between the arsenides and the nitrides is that whilst the former give red or infra-red light, the latter are currently most useful at the other end of the colour spectrum: in the green, blue and ultra-violet. (However, the nitrides do have potential for emission of almost any colour of light depending on the exact composition of the material used.)

A team of researchers at Oxford and Cambridge Universities have recently invented a new way to grow nitride quantum dots which may help to overcome some of the disadvantages of the nitrides. By changing the orientation of the substrate crystal on which the quantum dots are grown, we have shown that the rate of photon emission could be increased by a factor of ten or more. Furthermore, initial studies suggest that these more efficient quantum dots also retain sufficiently good temperature stability that devices could be designed which can operate with on-chip cooling, which would be a practical solution for real applications. In this project, we aim to explore the properties of quantum dots grown in this new orientation, and develop the crystal growth techniques which allow them to be incorporated into practical devices, which we will then test. We hope to develop a practical quantum technology based on the discoveries we have made about these exciting nitride materials.

The aim of this project is to develop non-polar nitride quantum dots for application in single photon sources. An inherent feature of such devices is that a single photon state cannot be amplified, so the devices need to be bright with a high repetition frequency from the outset. Such developments in quantum information science form the basis of a disruptive technology, with structures such as these having the potential to achieve efficient single photon emission at room temperature - or at least at temperatures accessible by on-chip Peltier cooling. Such apparently esoteric devices might be initially commercialised for use in the laboratory context, providing a compact and convenient single photon source for use in experiments concerning quantum key distribution and non-linear quantum computation. In the longer term, we would envisage this technology being transferred into the wider world, where it would be used in free-space quantum cryptography. Such quantum cryptographic applications might be short-range (involving communication between a mobile electronic device and an ATM for example), or long range (involving communication between satellites outside the earth's atmosphere). In the long-range situation, using blue or UV wavelengths, conveniently accessible using nitride materials, would give the lowest beam divergence for the currently available, weight-limited telescopes, minimizing losses from the quantum key distribution system. Looking beyond quantum cryptography, blue-emitting single photon sources have wider applications in high resolution quantum imaging and improved quantum sensing, which are of particular interest to our industrial partners at Toshiba.
In order to achieve economic impact based on our work towards devices for these applications, we plan to (1) exploit pre-existing contacts with relevant industries including Toshiba and Plessey Semiconductor Ltd., (2) utilise know-how within the Cambridge GaN Centre concerning patenting and spin-out companies to develop new companies or licence IP if appropriate, and (3) safeguard public expectations of such nanotechnologies by pursuing opportunities for two-way engagement with the public. Further societal impact will then arise if these new technologies are commercialised, since they could provide opportunities for improved security of data transfer to the general populace. Paradigms already exist for the use of single photon sources in "Quantum ATMs" which any mobile phone user could utilise in order to totally securely download a supply of secret "keys" (binary numbers used in encoding) which would then be used in securing their internet transactions, or other cashless payments. This could allow all of us to protect ourselves more effectively against internet fraud.
Furthermore, we anticipate that our work on nitride crystal growth for single photon sources will also generate knowledge relevant to other societally important projects such as the development of white light emitting diodes (LEDs) for low energy solid state lighting, the development of ultraviolet LEDs for water purification and the development of high efficiency power electronics. (Work on defect reduction, doping of non-polar materials and the impact of growth parameters on quantum dot array formation will be particularly relevant in this context.) Impact in these sectors will be ensured by strong communications links both within the Cambridge Centre for GaN which has a diverse portfolio of nitride research and also by our close interaction with the UK Nitride Consortium, which provides regular fora for the discussion of nitride research results within the UK academic and industrial community.
Lastly, we anticipate that students and post-doctoral research associates involved in the project will receive a broad training including vital technical knowledge and transferrable skills and will contribute to the people pipeline for the emerging quantum technologies industry and for other sectors.