Understanding Quantum Non-Equilibrium Matter: Many-Body Localisation versus Glasses, Theory and Experiment

Many-body systems comprise everything from simple metals, over complicated organic molecules, all the way to living cells. While their physics can be extremely complex, this complexity is however often mostly irrelevant, as most such systems will - when left alone - thermalise, a process through which most information about their preparation history and their initial state is lost. Think of pouring milk into your coffee or tea: there are many different important individual details describing this process - how fast, how much milk, angle and position of milk jug, milk temperature, and so on. All of those parameters are needed to predict the intricate turbulent pattern seen when one starts stirring the tea. At long time, however, all this complexity is hidden and we see merely a homogeneous brownish liquid. This behaviour is typical of ergodic systems and central to statistical mechanics; it allows us to make concrete predictions about a system given only a handful of parameters such as total energy.

Physics knows, however, about exceptions to this behaviour. In particular, in recent years, the phenomenon of many-body localisation (MBL) has emerged as a new paradigm for the absence of thermalisation and non-ergodicity in interacting quantum systems. In these systems, all degrees of freedom become localised by an external disorder, and are therefore partially decoupled from each other and cease to thermalise. In particular, it could be shown that these systems keep a much better local memory of their initial conditions. This peculiar effect might for instance be exploited in the future to design better materials to host quantum bits with reduced decoherence - even if some information leaks into the local environment, it will remain local for much longer. While MBL is a novel quantum effect, in classical systems the paradigm of slow dynamics and non-ergodicity is the glass transition, whereby fluid systems - such as liquids, colloidal suspensions or even granular mixtures - cease to flow and fall out of equilibrium at low temperatures or high densities.

Here, we seek support for a new theory-experiment collaboration between Nottingham and Cambridge aimed at developing a fundamental understanding of central aspects of non-equilibrium quantum many-body systems. In particular, we propose to establish the connections between MBL and glasses, thereby unveiling new mechanisms for quantum slow relaxation and non-ergodicity, with potential implications for the design and control of novel quantum non-equilibrium materials and devices. Our team comprises researchers with ample experience in experimental and theoretical atomic physics, statistical physics and condensed matter, who have made central contributions to MBL, glasses, open quantum systems, and other topics directly related to this proposal. This joint project will allow us to work hand-in-hand such that new theoretical ideas can quickly be tested in the experiment, which directly feeds back into theoretical developments.

This proposed research will build a bridge between a recently discovered and yet largely unexplored state of matter - the many-body localised phase - and the physics of classical and quantum glasses. It is fundamental in nature and covers a broad and interdisciplinary spectrum and is expected to feed into the development of new theoretical and experimental concepts and techniques that will find their way quickly into the academic sector. Impact on society and economy will dominantly develop over a longer time span, once the newly established body of knowledge will contribute to the development of next-generation technologies, e.g. materials, devices, protocols or processes. In the following some possible impacts and measures for their realisation are outlined:

Impact by knowledge - Our research connects to priority areas such as the EPSRC Grand Challenges Emergence and Physics Far from Equilibrium and Quantum Physics for New Quantum Technologies. The proposal has a distinct interdisciplinary character as we are combining experimental and theoretical research teams from two UK institutions and cover a broad spectrum of expertise, including cold atomic physics, statistical physics, quantum optics as well as hard and soft condensed matter physics. This maximises synergies and acts as a facilitator for the creation of impact because it establishes cross-disciplinary ideas and thinking.

People - The research programme draws substantially from the complementary skills available at the participating UK institutions and aims at creating a novel and interdisciplinary body of knowledge by closely linking state-of-the-art experimental and theoretical research on many-body quantum systems. This offers excellent opportunities for all involved PDRAs to acquire a unique background in several physical fields, e.g. quantum optics, atomic physics as well as statistical mechanics and soft and hard condensed matter theory. It also means that the trained theoretician will be able to speak and understand the "language" of experimentalists and vice versa. This will significantly enhance their international competitiveness and their chances to secure prestigious fellowships as well as permanent academic positions, thereby impacting on the formation of the future generation of researchers.

Society and Economy - Fundamental research like the one proposed here is necessary in order to stay ahead of the curve by advancing current research priorities, but furthermore, by helping to shape what the next generation of research priorities will be. One current example is Quantum Technologies which aims at translating features unique to quantum systems into devices and applications (UK National Quantum Technologies Programme, EU flagship). Our research will generate impact by delivering progress in our general understanding of matter and its states, which will contribute to underpinning the next revolution of "complex quantum technologies". The basic research will thus lay the foundations for such applications, leading in the long run to novel applications and devices that are likely to impact on economic growth and society. The connections to other disciplines, in particular the broader condensed matter and materials science communities as well as researchers working on quantum technologies, will ensure that all results are efficiently propagated along the innovation pipeline.

Outreach - The project partners have a strong track record in engaging with the public and the creation of awareness for fundamental research. We will use the platform provided by this project to further strengthen these activities, directly using research results for the demonstration and promotion of fundamental research.