Experiments using modern laser technologies and new light sources look at quantum systems undergoing dynamic change to understand molecular function and answer fundamental questions relevant to chemistry, materials and quantum technologies. Typical questions are: How can molecules be engineered for maximum efficiency during energy harvesting, UV protection or photocatalysis? What happens when strong and rapidly changing laser fields act on electrons in atoms and molecules? How fast do qubits lose information due to interactions with the environment? Will an array of interacting qubits in future quantum computers remain stable over long time-scales? Interpreting time-resolved experiments that aim to answer these questions requires Quantum Dynamics (QD) simulations, the theory of quantum motion. QD is on the cusp of being able to make quantitative predictions about large molecular systems, solving the time-dependent Schrödinger equation in a way that will help unravel the complicated signals from state-of-the-art experiments and provide mechanistic details of quantum processes. However, important methodological challenges remain, such as computational expense and accurate prediction of experimental observables, requiring a concerted team-effort. Addressing these will greatly benefit the wider experimental and computational QD communities. In this programme grant we will develop transformative new QD simulation strategies that will uniquely deliver impact and insight for real-world applications across a range of technological and biological domains. The key to our vision is the development, dissemination, and wide adaptation of powerful new universal software for QD simulations, building on our collective work on QD methods exploiting trajectory-guided basis functions. Present capability is, however, held back by the typically fragmented approach to academic software development. This lack of unification makes it difficult to use ideas from one group to improve the methods of another group, and even the simple comparison of QD simulation methods is non-trivial. Here, we will combine a wide range of existing methods into a unified code suitable for use by both computational and experimental researchers to model fundamental photo-excited molecular behaviour and interpret state-of-the-art experiments. Importantly we will develop and implement new mathematical and numerical ideas within this software suite, with the explicit objective of pushing the system-size and time-scale limits beyond what is currently accessible within "standard" QD simulations. Our unified code will lead to powerful and reliable QD methods, simultaneously enabling easy adoption by non-specialists; for the first time, scientists developing and using QD simulations will be able to access, develop and deploy a common software framework, removing many of the inter- and intra-community barriers that exist within the current niche software set-ups across the QD domain. The transformative impact of method development and code integration is powerfully illustrated by electronic structure and classical molecular dynamics packages, used routinely by thousands of researchers around the world and recognised by several Nobel Prizes in the last few decades. Our programme grant aims to deliver a similar step-change by improving accessibility for QD simulations. Success in our programme grant would be the demonstrated increase in adoption of advanced QD simulations across a broad range of end-user communities (e.g. spectroscopy, materials scientists, molecular designers). Furthermore, by supporting a large yet integrated cohort of early-career researchers, this programme grant will provide an enormous acceleration to developments in QD, positioning the UK as a global leader in this domain as we move from the era of classical computation and simulation into the quantum era of the coming decades.

The theoretical description of matter in strong laser fields is a rather challenging task. This is due to the fact that the external laser field is comparable to the atomic binding forces, and the usual theoretical methods considered in optical physics, such as perturbation theory with the laser field, are not applicable. In particular, it is very difficult to apply analytical or semi-analytical methods to such a physical framework. There exists, however, one such method, namely the Strong-Field Approximation. This method has served to establish the main paradigms in strong-field laser physics, and has been employed in over 500 publications in this field of research. In particular, it is very powerful for studying quantum interference effects in detail. This approximation suffers, however, from severe drawbacks, which are particularly critical for molecules and systems involving more than one electron. Such systems can not be described by such an approximation in a satisfactory way, and indicate that new, radical ideas are necessary in order to develop the theory further. In this project, we intend to bring ideas and methods from quantum-field theory and mathematical physics to strong-field laser physics to develop a new semi-analytical approach which replaces such an approximation. As a testing ground, we will use such a theory to describe molecules in strong laser fields, and, simultaneously, make a rigorous assessment of the limitations of the Strong-Field Approximation. Such systems have been chosen not only due to their critical behavior, but also due to the fact that, nowadays, there exists pioneering experiments in Britain, at the Imperial College, involving molecules, which will pave the way towards dynamic measurements of matter with a never-imagined precision. This will not only be important for the specific physical systems above, but will revolutionalize a whole area of research.

In this project, we will develop new software for the accurate description of atoms and molecular systems in intense, ultra-short light fields with arbitrary polarisation. This involves generalising two world-leading suites of codes: The R-matrix with time-dependence codes (RMT) for ultra-fast atomic dynamics and the UKRmol+ suite for electron/positron scattering and photoionisation processes in molecules. By making these codes available to the wider community, in a form that can be easily used and efficiently run, we will help build the software infrastructure in the UK. Significant development in laser technology over the last couple of decades has led to the birth of attosecond science: lasers are now available that can produce extremely short pulses (around 0.1 femtosecond or 10(-16) s in duration) to image and control the motion of electrons in atoms and molecules. This development has, for example, enabled scientists to 'see' how charge is transferred in a molecule after it is ionised, a process that has biological importance (for example, in photosynthesis). Light can be treated as an electromagnetic wave; the direction in which the electric field oscillates defines the polarisation of the light. This polarisation, in turn, determines how the light interacts with matter. Until very recently intense, ultra-short light pulses were linearly polarised. However, it has recently become possible to generate laser pulses with different types of polarisation. New scientific research areas and new opportunities have become available via these latest technological developments. With control over the polarisation of light pulses, one can control the electron dynamics and even fine-tune it: In simple terms, using light pulses which oscillate in more than one-dimension gives an additional control parameter in experiments, and this is the underlying mechanism in so-called multidimensional spectroscopy. This field is becoming increasingly interesting, as experiments begin to probe the interface of the quantum and classical worlds. In addition, light pulses with elliptical polarisation will enable the detailed study of electron dynamics in chiral molecules. (Chiral molecules are those that cannot be superimposed to their mirror images, like human hands). These molecules are immensely interesting: a lot of biologically important molecules, like the amino acids and sugars that are building blocks of living organisms are 'homochiral': only one variant is present in life (but never its mirror image). New computer codes, which can handle general atomic and molecular systems in arbitrarily polarised light are needed to complement experimental advances, to assist in their theoretical interpretation and also to guide them. At present, the RMT codes can model atoms in a linearly polarised light field. Expanding them to treat the effect of arbitrarily polarised light is a substantial task: It requires lifting symmetry restrictions which have limited the size of previous calculations, and consequently a significant improvement in the codes' efficiency to account for the much larger-scale calculations will be necessary. In addition, we will massively expand the impact of the method by developing an equivalent method to treat molecules in a time-dependent fashion. The data needed to study the effect of the laser pulses on molecules will be generated by the UKRmol+ suite. This, in turn, requires the overhauling of these codes so they can produce sufficiently accurate input in an efficient way. The computational development within this project will be strongly connected to the CCPQ community, which involves research groups across the UK developing scientific software for use in atomic and molecular physics and computational chemistry. Through CCPQ we will not only share the suites of codes, but also the expertise and software development skills gained.

Magnetisation switching between two stable bit states (1 and 0) is the key principle of modern-day storage technology. With the explosion in the number of "always connected" devices, and the consumer desire for multimedia and social media content, the volume of data being stored and processed globally has risen at an unprecedented rate, as evidenced by the number of new data centres being built (e.g. Facebook's new data centre in Singapore). The vast quantities of data being generated globally is leading to the emergence of new markets with companies exploiting and trading data in diverse ways - an EU estimate values the digital economy in Europe will be worth 739bn euros by 2020[1]. This growing demand for data storage poses several big questions: where is this volume of data going to be stored? How can the growing demand for data storage and processing be made compatible with the political and social imperative for energy responsibility and, ideally, carbon neutrality? It is estimated that 20% of the world's electricity demand will be used to power data centres by 2025[2], a figure that will undoubtedly grow, and therefore any technology that reduces the energy requirements of data processing and storage is of great national and international importance. This proposal concerns research into reducing the energy use involved in data storage and processing. Magnetic hard disk drives still form most of the data storage at the server (and hence cloud) level due to their low cost per bit. However, the process of writing information in disk drives uses a relatively large amount of energy due to the magnetic field needed to toggle bits between the two states. Studies in ultrafast magnetization dynamics using femtosecond (1 femtosecond is one millionth of a billionth of a second) laser pulses have demonstrated that low-energy switching is possible, using orders of magnitude less energy. Switching in these studies occurs within two picoseconds (one picosecond is a thousandth of a billionth of a second) opening up the possibility of writing up to 10^12 (a million million) bits per second, one thousand times faster than conventional recording methods, an extremely attractive avenue to realise much faster and more responsive devices that requires research investment. However, the use of strong laser pulses often results in a large amount of heating and can excite a lot of non-linear dynamics. One possible solution to this is to use light at frequencies that are in the THz range with high intensities. Historically, it has been very difficult to generate such light pulses, but recent experimental developments have made this possible and the area of THz science in general has attracted significant attention over the past decade and more recently to control magnetism. Initial studies have shown that significantly lower amounts of energy are required to switch the magnetisation state than in conventional recording, which could revolutionise the way we store and process information. This proposal is aimed at developing these ideas with the goal of understanding the underlying physical processes and how we can engineer efficient, low energy control of magnetism. The work will be carried out alongside world-leading experimental groups to provide important validation and comparisons with theoretical work. 1 - https://ec.europa.eu/digital-single-market/en/news/final-results-european-data-market-study-measuring-size-and-trends-eu-data-economy 2 - https://data-economy.com/data-centres-world-will-consume-1-5-earths-power-2025/

The R-matrix methodology is a UK success story. The method was originally developed to provide rigorous treatment of electron collisions with ions and atoms. It was then expanded to treat electron molecule collisions and matter in intense laser fields. Further extension to treat ultracold chemistry forms part of this proposal. The development of the software based on R-matrix methodology has received extensive support from CCPQ (and its predecessors), EPSRC and eCSE. The resulting code set is at the forefront of international atomic, molecular and optical (AMO) physics and is used by researchers world-wide. R-matrix studies have ridden successive waves of computer development and as a result are making an increasing contribution to science and technology in many areas. UK AMOR is a new High End Computing consortium which will work in the general area of AMO physics. Problems studied using ARCHER will include: a) The interaction of atoms and molecules with light including intense light sources. R-matrix with time-dependence (RMT) is the leading code in this area, allowing calculations at the intersection of atomic and strong-field physics. Ongoing extensions to molecules, and atoms in arbitrarily polarised laser pulses will further establish the code on the world stage. This will provide key support for exciting experimental work being performed on these physical processes. b) Electron collisions with atoms, ions and molecules using UK codes which are widely used internationally. Calculations will focus on applications ranging from fusion plasmas to radiation damage in biological systems. For fusion we will focus on high accuracy calculations on atoms and ions, and key molecules important for fusion experiments. We will also perform high accuracy electron-molecule collisions calculations to study: 1) large systems such as molecular clusters and biomolecules where results are important for studies radiation of tracks and DNA damage. 2) processes of applied relevance for extended energy ranges, 3) processes of applied where improved models will provide more accurate scattering data 4) benchmark problems with full uncertainty quantification. These studies are only possible using the new UKRMol(+) code and ARCHER. c) Ultracold chemistry: this a new area of study. Codes will be developed to treat ultraslow collisions for reactive systems over deep potential wells. Such systems are characterised by complex resonance structures whose study offers unique opportunities for chemical control and insights into this fundamental process. The methodology will also be applicable to a variety of related low-energy processes such as radiative association. Calculations will be performed on systems accessible to planned state-of-the-art experiment.