Understanding and engineering dissipation in nanoscale quantum devices

Lead Research Organisation: University of St Andrews
Department Name: Physics and Astronomy

Abstract

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.

Planned Impact

Our work will first have an impact on researchers in academia and industry working on the four specific technologies we have highlighted, but in the longer term the tools we will develop have the potential for even more wide-ranging impact.

Single photon sources are a fundamental resource for many proposed quantum technologies. For example, they are important for secure quantum communication, and our work will be of benefit for developers and users of this technology. There is a large effort in this area through the UK Quantum Technology Hub for Quantum Communication. Many companies have interest in this technology, including HP, NEC, and ID Quantique. Other photonic quantum technologies, such as photonic quantum computers, also rely on good single photon sources and are being pursued internationally (e.g. in the CQC2T ARC Centre of Excellence, Australia and the Psi Quantum spin-out).

Solar energy is the most readily available and plentiful potential energy source, yet its take up has been somewhat inhibited by high embedded cost and complex fabrication techniques. Efficient exciton transport underpins organic molecular architectures for photovoltaic cells, a technology that promises to develop a cheap and flexible solution to power generation and which can also be used to make mobile local power sources, required for the 'internet of things'. Our work promises a new approach to designing appropriate materials for this. Academia and industry are heavily invested in the area (e.g. companies G24Power, Ossila, Merck, Cambridge Display Technology).

The use of molecular electronics promises to deliver smaller scale circuits that has the potential to extend Moore's Law. However, at this scale fully quantum mechanical models of (strongly coupled) molecular junctions are vital: the lack of accurate models is one reason why progress in this area has perhaps been slower than initially hoped. This kind of model is what we will deliver. The reliance of modern society on devices of ever decreasing size means that our work could potentially be felt across a very wide user base: Current interest from companies in molecular electronics includes labs at IBM.

Magnetic imaging with NV centres, has great potential for example in medical imaging, where it can be used to detect the weak magnetic signals coming from brain or heart (see e.g. Budker and Romalis, Nat. Phys. 3 227). Our full account of environmental effects and how this might lead to more sensitive devices will benefit developers of such devices, and ultimately the patients who might be treated. As well as interest in academia, there is also industry work here from Lockheed Martin and Element Six.

Beyond these four specific areas of application, the tools we will develop as part of this proposal will be coded in a very general way and so enable fully quantum mechanical simulations of any Hilbert space of small (~10) dimension, strongly coupled to most kinds of environment. Our publicly-available code could then be applied to many other kinds of system underpinning further applications. For example, understanding decoherence in solid state quantum computers based on silicon or superconducting qubits, atom-photon bound states for controlled quantum gates, heat transport in nanojunctions, or fundamental studies of spatio-temporal correlations. To ensure these benefits are maximised, we will continue to release up-to-date code with all our latest functionality, so that other groups might benefit directly from it (see doi.org/10.5281/zenodo.1322407).

An obvious way in which our work will impact society is through training of individuals in a range of skills. Well-trained quantum theorists are important for a plethora of emerging industries, and are increasingly required in UK academia through the Hubs initiatives. Our track record of producing excellent graduates and post-doctoral scientists means that we are well placed to make such impact.

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