# Theoretical Physics

Funded PhD position available for project on "Quantum computing with photonic networks" in close collaboration with the Oxford Quantum Technology Hub NQIT.

More information will be posted at a later date. If interested, please contact Almut Beige [A.Beige@leeds.ac.uk].

Absorption of light can cause large changes in molecular structure. This is important for biological function, for example in the first steps in vision. These processes happen on very fast time scales, and only recent experiments have been able to observe them in detail. Man made molecules can use similar principles to function as ultrafast photo switches.

This project aims at developing new theory to model motion in a molecule after light absorption. Detailed models of the interaction of an electronic state with a complex quantum mechanical environment will be made and their predictions will be compared with experiments. The project combines fundamental theory development in open quantum systems dynamics with practical applications.

In our group, we use models to understand how molecular systems use light to function. These models are compared with state of the art optical experiments, which allow us to probe the fundamental motions of electrons and nuclei that take place on femtosecond to picosecond time scales. Inspiration for our work comes from biological systems. Our work uses mathematics and computer programming. The projects are suitable for chemistry, physics and mathematics graduates. The first project is about photosynthesis. How is the energy that is collected by plants and bacteria from sunlight transported? It turns out that answering this question requires a detailed description of the pigment molecules that interact with the light, as well as of the protein and solvent environment. In this project, you will build a new model of the energy transport mechanism. The model will be based on quantum mechanics of an electronic system interacting with vibrations. A main goal of the project is to accurately determine the parameters that describe real systems, from either simulation or comparison to experiment.

The second project is about photo switching. Some of the fastest events in biology occur within the eye. As in photosynthesis, electrons are excited by light absorption, However, in the primary step in vision the nuclear motion induced by electronic excitation is very large. Cis-trans isomerization in the rhodopsin molecule completely changes the structure. The system clearly explores parts of the potential energy surface far away from equilibrium, such that a harmonic description is completely invalid. This is also the case in man-made photo switches. This is a challenging regime for models that treat both the electronic and the nuclear motion under the influence of the protein environment. This project aims at developing a new theory to describe quantum decoherence and friction outside the harmonic approximation.

References

[1] Dijkstra, A. G. and Tanimura. Y., New J. Phys. 14, 073027 (2012);

[2] Prokhorenko, V. I., Picchiotti, A., Pola, M., Dijkstra, A. G. and Miller, R. J. D., J. Phys. Chem. Lett. 7, 4445 (2016);

[3] Dijkstra, A. G., Wang, C., Cao, J. and Fleming, G. R., J. Phys. Chem. Lett. 6, 627 (2015).

Laser sideband cooling allows us to cool single particles to nanoKelvin temperatures. Its discovery opened the way for experiments, which test the foundations of quantum physics and have applications ranging from quantum metrology to quantum computing. Moreover, motivated by the imminent quantum technological applications of ultra-cold many-body quantum systems, quantum thermodynamics recently became a very active area of research. Nevertheless, applying laser cooling to large ensembles of particles is still not straightforward.

This project aims to enhance the cooling process of many particles through interactions. A quantum coherence with the ability to induce collective dynamics in an atomic gas has already been identified [1]. This coherence plays the same role as entropy in classical physics and would be strongly affected by interactions between particles. Linking thermodynamics and quantum physics and estimating efficiencies, we will establish more efficient approaches to cooling many particles than currently available.

[1] Cavity-mediated collective laser-cooling of an atomic gas inside an asymmetric trap, O. Kim, P. Deb and A. Beige, arXiv:1506.02910 (2016).

Disorder and interactions between particles are common features of most physical systems. What are the possible classes of behaviour that a closed quantum system can have in the presence of both disorder and interactions? Although easy to formulate, this fundamental question of quantum statistical physics remains open.

Most systems in nature reach thermal equilibrium during the course of their evolution because their microscopic dynamics is chaotic. However, this is not the only possibility. Recently, many-body localization [1] -- which arises due to quenched disorder and interactions – has come to attention as a generic mechanism that breaks ergodicity and prevents the system from thermalizing. This is in contrast to Anderson localization and integrable models, which also break ergodicity but in a non-generic way (i.e., they require either an absence of interactions or finely tuned coupling constants).

Many-body localized systems are promising for applications in quantum computing because they are able to “escape” thermalization even at “infinite” temperature. Because of this, their states can retain quantum coherence and are a promising venue to realize quantum order even at high temperature. In this sense, because they avoid dissipation, many-body localized systems can act as protected “quantum memories” for extended periods. Alternatively, many-body localization could boost the stability of topological phases of matter, which are usually fragile and realized only at low temperatures. The exotic non-local correlations in these phases, enhanced by many-body localization, would give another route to constructing protected quantum memory.

The implications of many-body localization are far-reaching: non-ergodic dynamics, the existence of novel phases and phase transitions that are forbidden by traditional statistical mechanics, possibility of designing robust quantum computing schemes, etc. Apart from many exciting theoretical prospects, recent experiments [2] have detected first evidence of many-body localization in optical lattices, while many other other experiments are currently under way.

**Project.** The goal of this project is to use quantum information techniques to study properties of many-body localized systems and, more generally, the dynamics of interacting disordered systems far from equilibrium. The project will focus on quantum entanglement in such systems and will include the development of numerical algorithms inspired by entanglement, such as tensor networks [3]. In addition to the theoretical understanding of many-body localized systems, we will identify routes by which complex entanglement structures and their dynamical evolution can be probed and possibly protected from decoherence in experiments. We will also investigate how intricate types of order, like topological order, can be made more robust due to many-body localization.

In the initial phase, we will set up a foundation for the project based on the theoretical progress on many-body localization since 2010. As the toy model, we will focus on the Heisenberg model of spins-1/2 in a random field. We will study the dynamics of entanglement in this model when the system is driven out of equilibrium. The characteristic logarithmic-in-time growth of entanglement will help us arrive at the effective theory of many-body localized phases which was established in 2013. This will allow us to understand some of the universal properties of localized phases that have been observed in experiment [2]. The second goal of the first phase of the project is a numerical implementation of a tensor-network algorithm describing a one-dimensional spin chain (e.g., the transverse-field Ising model). In doing so, we will learn how to study entanglement, dynamics and phase transitions using variational tensor network simulations.

**Note: the project is suitable for students with background (or strong interest) in the numerics (C++/Python...).**

[1] Rahul Nandkishore and David Huse, Many body localization and thermalization in quantum statistical mechanics, arXiv:1404.0686.

[2] Michael Schreiber et al., Observation of many-body localization of interacting fermions in a quasi-random optical lattice, Science 349, 842 (2015).

[3] Roman Orus, A Practical Introduction to Tensor Networks: Matrix Product States and Projected Entangled Pair States, Annals of Physics 349, 117-158 (2014).

The Casimir effect is an intriguing quantum effect which has attracted much attention since it was first discovered in 1948. It predicts an attractive force between plane mirrors, possibly caused by vacuum fluctuations. Initial experiments have already confirmed the existence of such a force but have also opened the door for a range of alternative explanations.

This project aims at improving our understanding of the Casimir effect as well as providing the theoretical background for further experimental tests. More concretely, we are interested in applying an alternative approach to electromagnetic field quantisation near semitransparent mirror surfaces [1,2] to optical cavities. This will allow us to derive the Casimir effect from the zero point energy of the electromagnetic field without relying on gauge-dependent models. It will also allow us to obtain general results, which take into account the properties of the cavity mirrors, such as their reflection rate and temperature.

[1] R. Bennett, T. M. Barlow, and A. Beige, Eur. J. Phys. 37, 014001 (2016).

[2] N. Furtak-Wells, L. A. Clark, R. Purdy and A. Beige, Photon emission from an atom in front of a semi-transparent mirror, to be submitted (2016).

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