Coherent photon coincidence spectroscopy of single quantum systems

Non-equilibrium photon correlations of coherently excited single quantum systems can reveal their internal quantum dynamics and provide spectroscopic access. Here we propose and discuss the fundamentals of a coherent photon coincidence spectroscopy based on the application of laser pulses with variable delay and the detection of an time-averaged two-photon coincidence rate. For demonstration, two simple but important cases, i.e., an exciton - biexciton in a quantum dot and two coupled quantum emitters, are investigated based on quantum dynamics simulations demonstrating that this nonlinear spectroscopy reveals information specific to the particular single quantum system

Improving Line Search Methods for Large Scale Neural Network Training

Kenneweg P, Kenneweg T, Hammer B. Improving Line Search Methods for Large Scale Neural Network Training. In: 2024 International Conference on Artificial Intelligence, Computer, Data Sciences and Applications (ACDSA). IEEE; 2024: 1-6

Faster Convergence for Transformer Fine-tuning with Line Search Methods

Kenneweg P, Galli L, Kenneweg T, Hammer B. Faster Convergence for Transformer Fine-tuning with Line Search Methods. In: 2023 International Joint Conference on Neural Networks (IJCNN). IEEE; 2023: 1-8.Recent works have shown that line search methods greatly increase performance of traditional stochastic gradient descent methods on a variety of datasets and architectures [1], [2]. In this work we succeed in extending line search methods to the novel and highly popular Transformer architecture and dataset domains in natural language processing. More specifically, we combine the Armijo line search with the Adam optimizer and extend it by subdividing the networks architecture into sensible units and perform the line search separately on these local units. Our optimization method outperforms the traditional Adam optimizer and achieves significant performance improvements for small data sets or small training budgets, while performing equal or better for other tested cases. Our work is publicly available as a python package, which provides a hyperparameter-free pytorch optimizer that is compatible with arbitrary network architectures

QDT - A Matlab toolbox for the simulation of coupled quantum systems and coherent multidimensional spectroscopy

Kenneweg T, Mueller S, Brixner T, Pfeiffer W. QDT - A Matlab toolbox for the simulation of coupled quantum systems and coherent multidimensional spectroscopy. Computer Physics Communications . 2024;296: 109031.We present QDT ("quantum dynamics toolbox"), an open-source Matlab software package that enables users to simulate coupled quantum systems in the subsystem energy eigenbasis using modular functions. QDT requires no user knowledge of operator matrix assembly and automatically performs all necessary operator constructions and Hilbert space expansions. Density matrix propagation is performed by numerically solving the Liouville-von-Neumann equation. In order to simulate dissipation and decoherence effects, the Lindblad formalism is implemented. Furthermore, QDT supplies practical analysis and plotting functions, such as visualization of density matrix and expectation value dynamics, that facilitate the evaluation of simulation results. QDT further provides a module for the simulation of coherent multidimensional spectroscopy

Fluorescence-Detected Two-Quantum and One-Quantum-Two-Quantum 2D Electronic Spectroscopy

We demonstrate two-quantum (2Q) coherent two-dimensional (2D)electronic spectroscopy using a shot-to-shot-modulated pulse shaper and fluorescence detection. Broadband collinear excitation is realized with the supercontinuum output of an argon-filled hollow-core fiber, enabling us to excite multiple transitions simultaneously in the visible range. The 2Q contribution is extracted via a three-pulse sequence with 16-fold phase cycling and simulated employing cresyl violet as a model system. Furthermore, we report the first experimental realization of one-quantum−two-quantum (1Q-2Q) 2D spectroscopy, offering less congested spectra as compared with the 2Q implementation. We avoid scattering artifacts and nonresonant solvent contributions by using fluorescence as the observable. This allows us to extract quantitative information about doubly excited states that agree with literature expectations. The high sensitivity and background-free nature of fluorescence detection allow for a general applicability of this method to many other systems

AMiRo-OS

Schöpping T, Korthals T, Herbrechtsmeier S, et al. AMiRo-OS. Bielefeld University; 2016.[AMiRo-OS](https://opensource.cit-ec.de/projects/amiro-os) is the operating system for the base version of the Autonomous Mini Robot (AMiRo). It utilizes [ChibiOS](http://chibios.org) (a real-time operating system for embedded devices developed by Giovanni di Sirio) as system kernel and extends it with platform specific functionalities. It also comprises a bootloader and flashing toolchain, based on [OpenBLT](http://feaser.com/en/openblt.php)

Observation of optical coherence in a disordered metal-molecule interface by coherent optical two-dimensional photoelectron spectroscopy

Aeschlimann M, Brixner T, Cinchetti M, et al. Observation of optical coherence in a disordered metal-molecule interface by coherent optical two-dimensional photoelectron spectroscopy. Physical Review B. 2022;105(20): 205415.Coherent multidimensional optical spectroscopy methods overcome some of the limitations found in their one-dimensional counterparts and allow us, for example, to resolve overlapped spectral features, to separate homogeneous and inhomogeneous broadening, and to track the energy transfer kinetics and coherent dynamics in complex quantum systems. In their most common configurations, signal detection is achieved by optical means, making these techniques widespread in studies of bulk systems but less common in the surface and interface sciences. In this paper, we demonstrate an inherently surface-sensitive two-dimensional coherent spectroscopy scheme, based on photoelectron detection, by studying the interface formed between tris(8-hydroxyquinolinato)aluminium (Alq(3)) and a ferromagnetic Co surface. Despite the inhomogeneous linewidth broadening (approximate to 800 meV) in the ensemble of disordered molecules, we resolve two narrow resonances (approximate to 10 meV linewidth) with an energy spacing of approximate to 80 meV. By combining experimental data and simulations, using the Lindblad master equation, we identify these resonances as lowest unoccupied molecular orbital (LUMO) to LUMO + 1 transitions in chemically decoupled second-layer Alq(3) molecules and deduce related optical coherence lifetimes of at least 120 and 240 fs. These observations establish that Alq(3) molecules in a disordered adsorbate layer exhibit well-defined and rather homogeneous internal electronic transitions, although the absolute energetic positions of the involved states with respect to the substrate reference are significantly affected by the disorder. The results indicate that inhomogeneous line broadening in a disordered Alq(3) layer and pure dephasing have only a minor impact on optical transitions in individual molecules. This opens interesting opportunities for coherent control schemes and emphasizes the importance of optical coherences for all electron dynamics, even in disordered hybrid metal-molecule interfaces