Christoph Kreisbeck



Dr. Christoph Kreisbeck

Postdoc
Department of Chemistry and Chemical Biology
Harvard University

Office: Cv-B21

E-mail: christophkreisbeck[at]fas.harvard.edu
E-mail: christophkreisbeck[at]gmail.com
Address: 12 Oxford Street, Mailbox 302, Cambridge, MA 02138

News
    our GPU-HEOM tool to simulate spectra of light-harvesting complexes in now available at nanohub.org. Feel free to build, test and simulate your own exciton system …
Education
Research Interests
    LightHarvesting

    Exciton dynamics in light-harvesting complexes (LHCs)

    Understanding natural antenna systems

    Recent advances in two-dimensional (2d) electronic spectroscopy (ES) reveal remarkable energy transport properties in light-harvesting complexes (LHCs). In particular, oscillatory components in 2d echo-spectra of the Fenna-Matthews-Olson (FMO) complex are considered as evidence for the relevance of quantum wave like motion in natural photosynthesis.
    The how and why robust electronic coherence emerges under the noisy conditions in biological systems is not yet completely understood. The picture gets more complicated because LHCs exhibt several strongly coupled underdamped vibrational modes. The entangled motion of the exciton system and the surrounding protein mask affects the outcome of the experimental signal in a similar way as electronic coherence. Drawing a final conclusion on how to interpret the wiggles in the 2d echo-spectra requires a realistic simulation of the energy transfer dynamics. My research focuses on separating mechanisms that shield electronic coherence from the fluctuations induced by the environment.

    LightHarvesting

    Performing accurate simulations for the parameter regime of LHCs means to tackle several problems, we need to treat the coupling to the environment in a non-perturbative way, we need to incorporate the finite time-scale of the protein motion and most importantly we need to be able to do the simulations within reasonable computation times for large systems at physiological temperatures. Tobias Kramer and I developed an efficient implementation of the hierarchically coupled equation of motion approach (HEOM) where we use the immense computation power of graphics processing units (GPUs). GPU-HEOM is on-line available at nanohub.org. Feel free to build, test and simulate your own exciton system …

    Towards the design of artificial devices

    protein structure

    In order to guide experimentalists in the fabrication process it is of importance to unambiguously characterize and probe the intended transport properties of the designed functional unit. There are many degrees of freedom, such as the exciton energies, dipole orientations, etc., to manipulate and optimize excitonic systems. One corner stone towards the design of new excitonic structures is given by our GPU-HEOM implementation that provides a powerful tool to screen a wide range of parameters and simulate the corresponding dynamics. The ability to accurately simulate the 2d echo-spectra allows us to extract in what ways specific functional features manifest in the experimental signal. The latter is indispensable for the actual testing of the functional units of artificial devices.
    The missing gap that needs to be closed is to extract the system parameter from the protein structure. We need to understand how to manipulate the microscopic structures to gain a certain advantage for the exciton dynamics. Theoretically such a picture can be provided with the help of quantum mechanics (QM)/ molecular mechanics (MM) methods. The difficulty of QM/MM is not only in the numerical complexity but we need also to decide what parts of the protein at hand are influenced by the laws of quantum mechanics and what parts can be described within molecular mechanics. Our idea is to combine GPU-HEOM with QM/MM and to follow the complete path from the protein structure to the 2d-echo signal.

    Ballistic transport in semiconductor heterostructures

    In nano-electronics experiments are typically performed in dilution refrigerators at very low temperatures down to few milli-Kelvin. One focus of current research lies on establishing conceptually new devices based on ballistic electron transport in high mobility AlGaAs/AlGa samples. In the ballistic regime, the transport characteristics are determined by coherent interference effects. Ballistic transport offers the prospect of developing fast and low power consuming devices. Possible applications range from switching devices, magneto-ballistic rectifiers to quantum interference transistors.

    AB-Ring

    Since the intrinsic device properties are sensitive to changes in the geometry, a careful modeling of the potential landscape within the two dimensional electron gas (2DEG) of the AlGaAs/AlGa heterostructure becomes necessary. We develop a time-dependent approach, based on wave-packet propagation that allows us to simulate complex-shaped device geometries with an extension of up to a few micrometers. As an application we discuss electron transfer through a four-terminal Aharonov-Bohm interferometer. The corresponding experimental work is done by Prof. Dr. Saskia Fischer, Prof. Dr. Ulrich Kunze and co-workers. The idea is to use such interferometers as measurement devices to gain information about the transmission properties of an embedded quantum systems, such as a quantum dot.

    High-performance graphics processing unit (GPU) computing

    Over the last decades the computational power of a single central processing unit (CPU) grew exponentially. This is the best situation for programmers. There is nothing to worry about, just wait for the next generation of computers and your programs run faster automatically. However, this situation has changed in recent years. The performance of a single CPU saturated and the paradigm changed from single-core to multi-core chips. Now, algorithms need to be divided into smaller units that are executed at the different cores at the same time. Today’s programmer need to think parallel.
    Modern graphics processing units (GPUs) are optimized to stream heavy calculations over hundreds or thousand of cores. This brings great opportunities. You can carry around your own super computer basically in your Laptop. The challenge is how to extract the computational power provided by the GPUs. For programmers this means to ensure an efficient work distribution, since we need to keep our little processors busy. Splitting the code into small tasks is only half of the story, the other is memory bandwidth. For many applications memory bandwidth is the factor that limits the performance of our algorithm. This calls for a clever memory management where we need to take advantage of different hierarchies of memory access. Don’t be disappointed if things don’t run faster immediately. The performance will grow rapidly once you put your intelligence and creativity into your code.
    Right now the hardware layouts change and so does the way of how to make the most of it. This is why the field of high-performance computing is so exciting!!! The answer of intel to the success of GPU computing has come promptly. I look forward to getting my first hands on the intel Xeon Phi …

Published Software

Exciton Dynamics Lab for Light-Harvesting Complexes (GPU-HEOM)
C. Kreisbeck and T. Kramer
http://nanohub.org/resources/gpuheompop (2013)

Publications
  1. Disentangling electronic and vibronic coherences in two-dimensional echo spectra
    C. Kreisbeck , T. Kramer and A. Aspuru-Guzik
    J. Phys. Chem. B (2013)
    article arxiv NASA
  2. Long-lived electronic coherence in dissipative exciton-dynamics of light-harvesting complexes
    C. Kreisbeck and T. Kramer
    J. Phys. Chem. Lett., 3, 2828 (2012)
    article arxiv NASA
  3. Modelling of oscillations in two-dimensional echo-spectra of the Fenna-Matthews-Olson complex
    B. Hein, C. Kreisbeck, T. Kramer and M. Rodriguez
    New J. Phys., 14, 023018 (2012)
    article arxiv NASA
  4. High-performance solution of hierarchical equations of motion for studying energy transfer in light-harvesting complexes
    C. Kreisbeck, T. Kramer, M. Rodriguez and B. Hein
    J. Chem. Theory Comput. 7, 2166 (2011)
    article arxiv NASA
  5. Phase shifts and phase pi jumps in four-terminal waveguide Aharonov-Bohm interferometers
    C. Kreisbeck, T. Kramer, S. S. Buchholz, S. F. Fischer, U. Kunze, D. Reuter and A. D. Wieck
    Phys. Rev. B, 82, 165329 (2010)
    article arxiv NASA
  6. Wave packet approach to transport in mesoscopic systems
    T. Kramer, C. Kreisbeck and V. Krueckl
    Phys. Scr., 82, 038101 (2010)
    article arxiv NASA
  7. Phase readout of a charge qubit capacitively coupled to an open double quantum dot
    C. Kreisbeck, F. J. Kaiser and S. Kohler
    Phys. Rev. B, 81, 125404 (2010)
    article arxiv NASA
  8. Theory of the quantum Hall effect in finite graphene devices
    T. Kramer, C. Kreisbeck, V. Krueckl, E. J. Heller, R. E. Parrott and C.-T. Liang
    Phys. Rev. B, 81, 081410(R) (2010)
    article arxiv NASA