CCPQ: Quantum Dynamics in Atomic, Molecular and Optical Physics

The dynamics of quantum particles is the basis to describing the material world. Collisions between nuclei provides basic chemical reactivity, while the movements of electrons around nuclei provides the fine mechanistic details. To understand these motions we need to solve the time-dependent Schroedinger equation - a non-trivial problem for more than 3 particles that requires a huge computational effort.

State-of-the art experiments using attosecond or femtosecond pulses of radiation allow us to follow the motion of these particles, but without computer simulations the results are difficult to understand. This field of research is presently undergoing a huge expansion, due to the provision of new light sources such as free electron lasers (FELs), and software needs to be developed to keep up to the new capabilities. CCPQ has two community codes (R-matrix suite, MCTDH wavepacket dynamics) to treat these processes. The results give a deep inside into the fundamental reactivity of molecules, where quantum mechanical behaviour must be considered.

The interactions of anti-matter particles are also a topic of much interest, primarily due to the use of positrons in medical imaging, but also as a field of fundamental science in experiments such as the ALPHA project. Here, anti-matter particles are collided with normal matter and the different decay channels investigated. CCPQ is developing a code in collaboration with experimentalists to help understand the behaviour of these exotic sounding, but useful, particles.

Going from few bodies to many-bodies introduces some of the most fascinating phenomena in physics, such as superfluidity, superconductivity and ferroelectricity. However, to directly simulate them also introduces an exponentially scaling overhead in computation effort with the system size. While usually the preserve of condensed matter systems such strongly-correlated physics, where particles behaviour collectively, are now accessible in controlled ways with cold-atoms trapped in optical lattices. This has opened up previously inaccessible coherent dynamics in many-body systems to experimental scrutiny, such as examining what happens if the interaction and kinetic energies of particles are quenched across a quantum phase transition. The advances of this unique perspective are now reciprocating back to condensed matter problems where interaction of THz radiation on femtosecond timescales is also revealing correlated coherent electrons motion in solid-state systems. This topic of strongly-correlated many-body dynamics is the final strand of CCPQ development - embodied by the TNT project which introduces new ways of compressing many-body states to overcome the exponential barrier. It will support not only the emerging quantum technology of cold-atom quantum simulation, but also may eventually aid in designing and controlling real materials where optical pulses can switch properties such as superconductivity or ferroelectricity with great technological potential.

CCPQ supports the development of these world leading community codes by providing a forum for the exchange of ideas, by providing networking opportunities for researchers to help disseminate the codes, and by supporting training workshops for users of the codes. It also provides direct support in the form of computer experts at the Daresbury laboratory who help optimise the codes for use on large high performance computers (HPC).

The profile of CCPQ is of direct benefit to UK science as a whole, helping to keep it at the forefront of research into the fundamental properties of matter. The CCPQ codes impact directly in the fields of research addressed in the Academic Beneficiaries. Outside academia, the direct beneficiaries are industry requiring data to help develop technologies. This is exemplified by the success of Quantemol Ltd., and Quantemol will remain a close project partner of CCPQ in the proposed work. Accurate data on the interaction of light and electrons with atoms and molecules is of critical importance, for example, in the modelling of large-scale plasma dynamics. These plasma dynamics can range from small-scale industrial plasmas in plasma lasers used for EUV lithography, through the modelling of plasmas throughout an entire fusion reactor (from the hot core to the divertor region to the reactor walls), including the presence of contaminants due to wall degradation, up to supernova modelling in astrophysics.

Atomic and molecular physics plays a key role in the development and advancement of new technology through the provision of diagnostics. This is of particular relevance for large-scale infrastructure projects, such as FELs. In the initial application of new technology, simple systems need to be investigated so that the performance of the facility can be established. Computational methods developed by CCPQ have been proved to provide the accuracy needed to enable facility researchers to assess performance.

In many-body physics it is the primary aim of the CCPQ's TNT library to become the backbone of the UK's computational methods for the study of strongly-correlated systems for the next 15 years and more. In addition we expect that as the TNT approach grows and crosses new scientific frontiers, it will significantly push what is achievable with the ever-advancing capabilities of HPC and will aid the strategic driver of widely spreading HPC skills. Beyond these direct and immediate impacts for research based on well-established TNT algorithms, we further anticipate that the low level tensor software routines will have far-reaching benefits. Specifically, because of its generality it can serve as the foundation for the development of novel tensor network algorithms that are now widely believed by computer scientists to provide the much sought after extension of standard linear algebra algorithms to multidimensional data analysis. Given the huge importance of complex classical systems in natural sciences, operations research and industry as yet unforeseen applications of TNT could emerge in the near future.