Redrawing the boundaries: new approaches to many-body open quantum systems

I propose a new perspective and approach to understanding the interactions and interplay between quantum systems and their surrounding environments. By redrawing the boundary between what we term the system and what we term the environment, I shall provide a unified framework that will permit a breakthrough in the analysis of larger and more complex systems than presently possible. This will result in important new insights into both fundamental and applied physics across numerous settings.

The behaviour of nanoscale systems comprising up to several thousand atoms is dominated by quantum physics, which can lead to surprisingly strong collective features. As an analogy, consider the cooperative behaviour of a crowd at a sports ground who sing in unison, allowing songs to be discerned despite the many voices. Likewise, in nanoscale systems quantum correlations can be shared across numerous constituent atoms, enabling them to behave as single entities in many situations. One of the most dynamic and exciting areas of scientific research over the past decade has been the quest to understand, control, and exploit these correlations for technological applications. Further progress in the field - which could lead to the next technological revolution - requires the development of an unprecedented level of understanding of the intricate quantum nature of matter. This is a formidable challenge, but also a central reason to engage in this fascinating area of research.

A primary obstacle to exploiting quantum features of nanoscale systems arises due to the fact that no physical system can ever be completely isolated from the influence of its surroundings. The forces exerted by this large, fluctuating, and uncontrolled environment give rise to unwanted random variations in the system's properties, known as noise. Returning to our analogy, this is akin to each crowd member randomly singing a song with no regard to the songs of others, with the result (which would literally be noise!) that a listener would perceive no underlying structure.

In the quantum realm noisy processes are particularly harmful. As well as obscuring the information we learn as we probe a system, interactions with the environment can destroy the very nature of the quantum state itself. From a quantum technology point of view, noise thus seems to render our system to be completely useless.

This is the conventional view, at least. However, one of the exciting aims of my research programme is to give a viable alternative perspective on the role of noise in quantum processes. Imagine a crowd in which groups are singing different songs. Depending on how these groups form, compete, and evolve we may still perceive some structure beneath the din. In fact, by developing a new and unified understanding of the interactions between quantum systems and their environments I shall show that noisy processes can actually be harnessed to drive systems into exotic, robust, and useful quantum states.

Indeed, a series of groundbreaking experiments have suggested that interplay between quantum effects and noise may unexpectedly exist in the natural light-harvesting networks of bacteria, algae, and plants. These systems are thus currently the subject of intense exploration and debate, motivated by the remarkable possibility that quantum physics may play an important role in the basic processes of life. Moreover, by understanding whether this helps natural systems to achieve robust and efficient solar-energy conversion, I aim to develop new design principles for quantum technologies that draw on solar light as a clean, sustainable, and efficient energy source.

By tackling a core issue in quantum physics and a primary obstacle to exploiting quantum processes in the laboratory, my research will impact across a broad range of fields and technologies, paving the way to future applications of far-reaching social and economic importance as well.

This programme will develop new theoretical methods to understand the behaviour of many-body quantum systems subject to strong environmental influences. This understanding will be used to devise schemes that - even under such seemingly adverse conditions - can robustly prepare and manipulate quantum systems by harnessing, rather than mitigating, the resulting processes.

A number of areas will significantly benefit from this research, including:

Quantum technology - All quantum systems are open, meaning that they evolve not in isolation but under the influences of their surroundings. The study of open quantum systems is thus vital in understanding how to prepare and control quantum states, as well as being key to interpreting experimental measurements. The promised technological applications of quantum information science rely on environmental influences and the resulting loss of quantum coherence being well understood, and in many cases strongly suppressed. Furthermore, the mechanisms by which quantum systems lose coherence to their surroundings can become more complex and subtle as they are scaled up beyond small numbers of qubits. By focussing on many-body open quantum systems, my research programme addresses precisely these issues, and as such should be of major benefit to the recently-established National Network of Quantum Technology Hubs, their industrial partners, and the extensive global community of quantum technology researchers that exists both within and outside academia.

Solar energy conversion - The experimental generation and observation of dynamically evolving coherences in natural photosynthetic complexes has initiated a recent resurgence in the study of quantum effects in biological systems. The question of whether these effects play a role in the natural function of such systems remains open, as do the potential implications for solar energy conversion devices (which would have both significant societal and economic impact). Further progress requires accurate and flexible dynamical modelling of pigment-protein complexes under both laboratory and natural conditions, with the insights gained applied to the design of novel next generation solar cells, leading to impact across both communities.

Thermodynamics - Open quantum systems theory provides a natural framework in which to explore the thermodynamics of quantum systems, and the advances I propose will allow strong couplings and system-environment correlation effects to be incorporated within a unified approach. Furthermore, the thermodynamics of systems at the quantum scale is of direct relevance to existing nanoscale devices, and will also be a crucial consideration in the development of future quantum and solar energy conversion technologies. The continuing miniaturisation of components is now leading to a situation where accounting for quantum fluctuation effects is becoming increasingly important, with timely applications, for example, to the magnetic data storage industry.

Chemical physics - System-environment models are of fundamental importance in studying the charge and energy transfer dynamics of systems such as biomolecular aggregates and molecular junctions. Strong coupling between electronic and, for example, vibrational degrees of freedom routinely leads to the breakdown of weak-coupling approaches, while standard methods to go beyond such treatments often rely on semiclassical approximations valid only in restricted parameter regimes. The impact of the unified methods I propose to develop will be in bridging this gap.

An important aspect of this Fellowship programme is to encourage public awareness and understanding of cutting-edge quantum science, and support for further investment in both fundamental and applied research. Here, the impact will be on local communities within the North West, the wider public within the UK, and also abroad. The project will also deliver high-level and varied skills training to the PDRA.