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

物理学家明白，光可以被认为是一种波，或者是一束被称为“光子”的微小粒子。光子是能存在的最小的光。使用单个光子，我们可以为密码学和计算编码信息。使用光子的量子密码学提供了数据安全的终极保障，线性光学量子计算为大规模并行数据处理提供了机会。然而，这些应用的进展受到单光子源当前性能的限制。这样的装置可以可靠地按需提供一个且只有一个光子。使用昏暗的传统光源代替真正的单光子源，总是有发射多个光子的可能性，危及量子密码的安全性，并破坏量子计算机的性能。真正的单光子源可以用半导体量子点来制造：具有原子性质的微小晶体，其本质意味着它们在光学或电激发下发射单个光子。人们正在探索不同的半导体材料，包括以砷化合物（“砷化物”）和氮化合物（“氮化物”）为基础的材料家族。其中，砷化物已经得到了相当广泛的研究，并且可以用于生产高效的单光子源，但有一个主要缺点：这些设备只能在非常低的温度下工作：通常是零下250度或更低。另一方面，氮化物已被用于证明室温下的单光子发射，这显然更便于实际应用。然而，这类材料的研究很少，而且与砷化物相比，目前的设备效率不高，光子发射率也很低。砷化物和氮化物之间的另一个区别是，前者发出红光或红外光，而后者目前在光谱的另一端最有用：绿色、蓝色和紫外线。（然而，氮化物确实有可能发射几乎任何颜色的光，这取决于所用材料的确切成分。）牛津大学和剑桥大学的一组研究人员最近发明了一种生长氮化物量子点的新方法，这可能有助于克服氮化物的一些缺点。通过改变生长量子点的衬底晶体的方向，我们已经证明光子发射的速率可以增加十倍或更多。此外，初步研究表明，这些更高效的量子点还能保持足够好的温度稳定性，从而可以设计出可以在片上冷却的设备，这将是实际应用的实际解决方案。在这个项目中，我们的目标是探索在这种新方向下生长的量子点的特性，并开发晶体生长技术，使它们能够被纳入实际设备中，然后我们将对其进行测试。我们希望基于我们对这些令人兴奋的氮化物材料的发现，开发出一种实用的量子技术。

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.