Understanding and engineering dissipation in nanoscale quantum devices

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The march of technological progress has given us devices that are ever smaller and more complex: today's smart phones for example are almost unrecognizable in their size and their range of functions from the models of 25 years ago. This progress has taken us to the point where devices must now be understood in terms of the quantum behaviour of their constituent particles, a new frontier in technology that furthermore will lead to completely new applications.However, building fully quantum mechanical models of devices is notoriously difficult: the amount of information needed to describe a quantum system scales exponentially with its size. The situation is even worse when one must consider how the environment interacts with the device, and yet this is a crucial consideration for real devices. However, we have recently developed a new quantum simulation technique with remarkable efficiency: by keeping just the most important information we are able to track the behaviour of a single particle even when it is interacting very strongly with all of the other particles in its environment. In this project, we will exploit this new technique to design, simulate, and optimize four types of nanoscale devices with various technological applications. The functioning of all these devices relies on similar physics, namely how the device interacts with the environment. As such, our new method is ideally suited to all these areas.First, we will model solid state single photon sources. These produce quanta of light - photons - one at a time, and underpin future ideas for secure communication and quantum computing. We will find how the coupling between the photons and the vibrations of the solid determines affects their performance. Understanding this will allow us to determine how devices, either machined as thin wires or membranes or drawn as nanometre patterns in a solid matrix, could create more effective photon sources.Second, solar panels need to first absorb light energy from the sun, and then to transport it to electrodes. We will investigate the quantum mechanics of this energy transport problem, in particular for solar cells made of organic materials. Here, vibrations are very strongly coupled to the excited electrons that transport the energy, and our new technique is ideal for studying how this process works and how it might be improved by informed selection of component organic molecules.Third, a new frontier in electronics will be enabled if we can build circuits using molecules. Electric current is then a consequence of how electrons can tunnel quantum mechanically from one molecule to the next; this depends both on electronic coupling between molecules and how the molecules vibrate. We will use our technique to build models of molecular junctions, and explore how strong electronic and vibrational coupling changes the quantum transport properties of these materials.Fourth, diamonds have recently been at the forefront of a whole new kind of imaging technology. In particular, single electrons in diamond have a tiny magnetic moment, a 'spin', whose motion depends on how strong the magnetic field is at the position of the electron. Remarkably, the spin of a single electron can be measured in diamond, and so magnetic imaging with nanometre accuracy is a possibility. The limit of how well these 'nano-magnetometers' can work is set by how well they can be isolated from their environment. In this project, we will first use our novel approach to understand the dynamics of a spin coupled to its environment, and then show how to isolate spins more effectively.The project will advance several different nanotechnologies, and at the same time we will develop a unique and freely available tool that can be applied to a huge variety of new systems in future.