Talks

Activity, phase separation and nuclear architecture

I will describe recent work in which we use computational descriptions of large-scale nuclear architecture to model the biophysics of chromatin organization and nucleolus assembly in eukaryotic cells. These models combine relatively new concepts in the description of cell-scale biological structuring - activity and phase separation - illustrating how the overlap of physics, biology and computation can provide quantitative approaches to old problems.

Quantum Dynamics Seminar: On the Role of Nuclear-Spin Statistics in Quantum Molecular Dynamics

Identical particles’ permutation symmetry is a central pillar of quantum mechanics, and one of the most fundamental laws of nature. The corresponding quantum symmetrization postulate stipulates that the total wavefunction of any quantum system, be it an atom, a molecule, or even a solid, must be antisymmetric under permutation of two identical fermions, and symmetric under exchange of two indistinguishable bosons. In the former case, the identical particles have half-integral spin and obey Fermi-Dirac statistics, whereas in the latter, they have integral spin and obey Bose-Einstein statistics. For molecules, the principle of the indistinguishability of identical particles should be applied both to electrons and identical nuclei. In the case of electrons, the corresponding Pauli exclusion principle is a prerequisite for any electronic structure theory. Using Slater determinants as basis functions, this precondition is naturally implemented in all computer programs of quantum chemistry. The same cannot be said about the identical nuclei of molecular systems. Indeed, while parity and nuclear permutation symmetry are the two main building blocks of theoretical high-resolution molecular spectroscopy, the latter symmetry is often overlooked in molecular dynamics calculations, whether their focus is on electronic or even nuclear dynamics. Due to the so-called curse of dimensionality, most of these computations rely on reduced descriptions of quantum dynamics, which include only some (supposedly) relevant degrees of freedom, and neglect all the other molecular modes that are considered to be spectators to the dynamical process under study. These reduced-dimensionality models do not generally obey nuclear-spin statistics. This usually leads to incomplete and sometimes even incorrect pictures of iconic molecular processes. In this talk, I will give some concrete examples of reduced descriptions that do not comply with the indistinguishability of identical nuclei. I will then bring to the fore the powerful machinery of complete-nuclear-permutation-inversion (CNPI) groups and their molecular symmetry (MS) groups, which facilitate the implementation of nuclear-spin statistics. The effect of the latter on quantum molecular dynamics will be exemplified by electronic and/or nuclear motion in highly symmetric molecules. The main object of the dispute between these reduced and fully-symmetrized descriptions is whether nuclear permutation symmetry allows (dynamical) localization of the electronic, vibrational, or even rotational density on a specific molecular substructure (or configuration) rather than on another one that is identical (indistinguishable). This will be illustrated by electron dynamics in photo- dissociation of the dihydrogen molecular cation, electronic Kekulé dynamics in benzene, umbrella inversion of ammonia, and atom-diatom inelastic and reactive collisions.

Quantum Dynamics Seminar: On the Role of Nuclear-Spin Statistics in Quantum Molecular Dynamics

Identical particles’ permutation symmetry is a central pillar of quantum mechanics, and one of the most fundamental laws of nature. The corresponding quantum symmetrization postulate stipulates that the total wavefunction of any quantum system, be it an atom, a molecule, or even a solid, must be antisymmetric under permutation of two identical fermions, and symmetric under exchange of two indistinguishable bosons. In the former case, the identical particles have half-integral spin and obey Fermi-Dirac statistics, whereas in the latter, they have integral spin and obey Bose-Einstein statistics. For molecules, the principle of the indistinguishability of identical particles should be applied both to electrons and identical nuclei. In the case of electrons, the corresponding Pauli exclusion principle is a prerequisite for any electronic structure theory. Using Slater determinants as basis functions, this precondition is naturally implemented in all computer programs of quantum chemistry. The same cannot be said about the identical nuclei of molecular systems. Indeed, while parity and nuclear permutation symmetry are the two main building blocks of theoretical high-resolution molecular spectroscopy, the latter symmetry is often overlooked in molecular dynamics calculations, whether their focus is on electronic or even nuclear dynamics. Due to the so-called curse of dimensionality, most of these computations rely on reduced descriptions of quantum dynamics, which include only some (supposedly) relevant degrees of freedom, and neglect all the other molecular modes that are considered to be spectators to the dynamical process under study. These reduced-dimensionality models do not generally obey nuclear-spin statistics. This usually leads to incomplete and sometimes even incorrect pictures of iconic molecular processes. In this talk, I will give some concrete examples of reduced descriptions that do not comply with the indistinguishability of identical nuclei. I will then bring to the fore the powerful machinery of complete-nuclear-permutation-inversion (CNPI) groups and their molecular symmetry (MS) groups, which facilitate the implementation of nuclear-spin statistics. The effect of the latter on quantum molecular dynamics will be exemplified by electronic and/or nuclear motion in highly symmetric molecules. The main object of the dispute between these reduced and fully-symmetrized descriptions is whether nuclear permutation symmetry allows (dynamical) localization of the electronic, vibrational, or even rotational density on a specific molecular substructure (or configuration) rather than on another one that is identical (indistinguishable). This will be illustrated by electron dynamics in photo- dissociation of the dihydrogen molecular cation, electronic Kekulé dynamics in benzene, umbrella inversion of ammonia, and atom-diatom inelastic and reactive collisions.

A Variational Ansatz for the Ground State of the Quantum Sherrington-Kirkpatrick Model

We present an ansatz for the ground states of the Quantum Sherrington-Kirkpatrick model, a paradigmatic model for quantum spin glasses. Our ansatz, based on the concept of generalized coherent states, very well captures the fundamental aspects of the model, including the ground state energy and the position of the spin glass phase transition. It further enables us to study some previously unexplored features, such as the non-vanishing longitudinal field regime and the entanglement structure of the ground states. We find that the ground state entanglement can be captured by a simple ensemble of weighted graph states with normally distributed phase gates, leading to a volume law entanglement, contrasting with predictions based on entanglement monogamy.

Activity suppressed phase separation and active interface dynamics

The phase diagram of equilibrium systems can change radically by introducing activity. Examples of this include motility induced phase separation, flocking induced by self propulsion and alignment interactions, and turbulent flows produced by extensile active liquid crystals, to name a few. I will describe a different process, complementary to MIPS, by which activity strongly suppresses phase separation in a binary mixture of active and passive particles. I will motivate the study experimentally with very recent experiments on mixtures of microtubules with DNA condensates, and introduce a continuum theory in which active turbulent flows produced by active liquid crystals provide a stirring force capable of mixing an immiscible mixture, shifting the critical point to a lower temperature. I will further explain how the phase separated regime, which is a highly dynamic form of micro phase separation, can be understood by looking at the linear dynamics of the interfaces between the active and passive species.

IMPRS seminar: An Overview of Trajectory guided Coherent State Basis sets Methods of Quantum Dynamics with Examples of Applications in Photochemistry and in Physics

An overview of existing methods, which use trajectory guided grids of Coherent States, will be given and their technical details will be discussed. In chemistry the trajectory guided random grids of Gaussian Coherent States are routinely used to simulate quantum dynamics in ultrafast photochemistry in a manner similar to classical molecular dynamics, but with the difference that an ensemble of trajectories is used instead of a single trajectory1. With the right sampling techniques they can yield well converged results for molecules with tens of vibrations, treating all nuclear degrees of freedom on a fully quantum level2. Similar methods based on Gaussian Coherent States can be used in physics to simulate the dynamics of ensembles of Bose particles described by second quantisation Hamiltonians3 or to dynamics of electrons in strong field4, 5. Other types of Coherent States, such as Coherent States of two level systems can be used to describe fermions6 and to obtain Born-Oppenheimer electronic energies and coupled qubits. 1. Makhov, D. V.; Symonds, C.; Fernandez-Alberti, S.; Shalashilin, D. V., Ab initio quantum direct dynamics simulations of ultrafast photochemistry with Multiconfigurational Ehrenfest approach. Chemical Physics 2017, 493, 200-218. 2. Symonds, C.; Kattirtzi, J. A.; Shalashilin, D. V., The effect of sampling techniques used in the multiconfigurational Ehrenfest method. 2018, 148 (18), 184113. 3. Green, J. A.; Shalashilin, D. V., Simulation of the quantum dynamics of indistinguishable bosons with the method of coupled coherent states. Phys. Rev. A 2019, 100 (1), 013607. 4. Kirrander, A.; Shalashilin, D. V., Quantum dynamics with fermion coupled coherent states: Theory and application to electron dynamics in laser fields. Phys. Rev. A 2011, 84 (3), 13. 5. Symonds, C.; Wu, J.; Ronto, M.; Zagoya, C.; Figueira de Morisson Faria, C.; Shalashilin, D. V., Coupled-coherent-states approach for high-order harmonic generation. Phys. Rev. A 2015, 91 (2), 023427. 6. Shalashilin, D. V., Zombie states for description of structure and dynamics of multi-electron systems. Journal of Chemical Physics 2018, 148 (19), 194109.

IMPRS seminar: Basis sets for light-matter interaction: from static coherent states to moving Gaussians

We develop a computationally efficient way of employing Gaussian wave packets to study laser-induced electron dynamics in atomic and molecular systems by directly solving the time-dependent Schr\"odinger equation (TDSE). First, we investigate charge migration (treating the nuclei classically), high-order harmonic generation (HHG), and single-isolated attosecond pulse generation in the Hydrogen molecular ion subjected to intense laser fields in a different range of frequencies with a basis of static coherent states (SCS). Then, seeking a smarter way of constructing and guiding a minimal set of time-dependent basis functions, we introduce a fast and accurate approach for optimizing s-type Gaussian type orbitals (GTOs) and apply it to calculate electronic states of different 1D and 3D time-independent systems. Finally, we apply our optimization approach to time-dependent problems. With our approach, we obtain excellent agreement with the exact results for HHG spectra of the 1D Hydrogen atom and molecular ion exposed to intense laser fields, which is not possible even with a much larger basis of static s-type GTOs.

IMPRS Career talk: From A PhD to a Team Master at TraceTronic GmbH Dresden

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Condmat for dummies: Introduction to quantum spin ice

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Nonequilibrium transport phenomena in biochemical systems

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Condmat for dummies: BCS v. BDG: How to get the ground state from excitations

Quantum simulation with ultracold atoms – from Hubbard models to gauge theories

Well-controlled synthetic quantum systems, such as ultracold atoms in optical lattices, offer intriguing possibilities to study complex many-body problems in regimes that are beyond reach using state-of-the-art classical computations. The basic idea is to construct and use a well-controlled quantum many-body system in order to study its in- and out-of-equilibrium properties and potentially use it to develop more efficient tailored numerical methods that can then be applied to other systems that are not directly accessible with the simulator. An important future quest concerns the development of novel experimental techniques that allow us to expand the range of models that can be accessed. I will demonstrate this using the example of topological lattice models, which in general do not naturally appear in cold-atom experiments. I will show how the technique of periodic driving, also known as Floquet engineering, facilitates their realization and show how charge-neutral atoms in lattices can mimic the behavior of charged particles in the presence of an external magnetic field. A key ingredient for quantum simulation is the degree of control one has over the individual particles and the microscopic parameters of the model. We have recently succeeded to not only use the technique of periodic driving to emulate physical systems that we know exist in nature, but to take this idea one step further and realize completely new topological regimes that do not have any static analog. Moreover, we are currently developing a novel hybrid optical lattice platform, where tightly focused optical tweezers are used to locally control the motion of the atoms in the lattice, paving the way towards quantum simulation of simplified lattice gauge theories, which play a fundamental role in a variety of research areas including high-energy physics and topological quantum computation.

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