Universität Stuttgart

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Subject: search.filters.subject.530Institute: search.filters.UBSInstitut.Institut für Theoretische Physik IInstitute: search.filters.UBSInstitut.Fakultätsübergreifend / Sonstige EinrichtungStart date: 2021End date: 2025

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Now showing 1 - 9 of 9
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    Werner Eissner (1930-2022) : a pioneer in computational atomic physics
    (2023) Bhatia, Anand K.; Lynas-Gray, Anthony E.; Mendoza, Claudio; Nahar, Sultana; Nussbaumer, Harry; Pradhan, Anil K.; Seaton, Anthony M.; Wunner, Günter; Zeippen, Claude J.
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    Analysis of the fine structure of the D‐exciton shell in cuprous oxide
    (2021) Heckötter, Julian; Rommel, Patric; Main, Jörg; Aßmann, Marc; Bayer, Manfred
    The exciton states in cuprous oxide show a pronounced fine structure splitting associated with the crystal environment and the resulting electronic band structure. High‐resolution spectroscopy reveals an especially pronounced splitting of the yellow D excitons with one state pushed above any other state with the same principal quantum number. This large splitting offset is related to a strong mixing of these D states with the 1S exciton of the green series, as suggested by previously published calculations. Here, a detailed comparison of this theory with experimental data is given, which leads to a complete reassignment of the experimentally observed D exciton lines. The origin of different amounts of green admixture to D‐envelope states is deduced by analyzing the different terms of the Hamiltonian. The yellow-green mixing leads to level repulsion and induces an exchange interaction splitting to D‐envelope states, from which one of them becomes the highest state within each multiplet. Furthermore, the assignment of D exciton states according to their total angular momentum F is given and corrects an earlier description given in a former study.
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    A quantum heat engine driven by atomic collisions
    (2021) Bouton, Quentin; Nettersheim, Jens; Burgardt, Sabrina; Adam, Daniel; Lutz, Eric; Widera, Artur
    Quantum heat engines are subjected to quantum fluctuations related to their discrete energy spectra. Such fluctuations question the reliable operation of thermal machines in the quantum regime. Here, we realize an endoreversible quantum Otto cycle in the large quasi-spin states of Cesium impurities immersed in an ultracold Rubidium bath. Endoreversible machines are internally reversible and irreversible losses only occur via thermal contact. We employ quantum control to regulate the direction of heat transfer that occurs via inelastic spin-exchange collisions. We further use full-counting statistics of individual atoms to monitor quantized heat exchange between engine and bath at the level of single quanta, and additionally evaluate average and variance of the power output. We optimize the performance as well as the stability of the quantum heat engine, achieving high efficiency, large power output and small power output fluctuations.
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    Quantum cooling : thermodynamics and information
    (2023) Soldati, Rodolfo R.; Lutz, Eric (Prof. Dr.)
    The theory of cooling is an important corner of thermodynamics, underlying many modern technological applications. As the field of quantum thermodynamics advances, refrigeration techniques must keep pace to fuel the innovations of quantum technologies. We study quantum cooling from its foundations to laboratory implementations within the specific paradigm of heat bath algorithmic cooling. Our study includes a detail modeling of experimental imperfections and establishes the fundamental cooling limits of the model, consolidating the algorithm as a viable quantum refrigeration method. Next, by developing the notion of virtual qubits, we demonstrate a cooling-boost protocol fueled by quantum coherences which is robust to experimental implementations. Aiming at aiding in the progress of refrigeration technologies, we conclude by studying the zero temperature equilibrium properties of a many-body system that can accommodate an autonomous quantum absorption refrigerator, and calculate its entanglement and critical properties, two features that, like quantum coherences, promise to improve the performance of quantum coolers.
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    Generalized Clausius inequalities in a nonequilibrium cold-atom system
    (2023) Mayer, Daniel; Lutz, Eric; Widera, Artur
    Thermodynamic inequalities, such as the Clausius inequality, characterize the direction of nonequilibrium processes. However, the latter result presupposes a system coupled to a heat bath that drives it to a thermal state. Far from equilibrium, the Clausius inequality can be generalized using information-theoretic quantities. For initially isolated systems that are moved from an equilibrium state by a dissipative heat exchange, the generalized Clausius inequality is predicted to be reversed. We here experimentally investigate the nonequilibrium thermodynamics of an initially isolated dilute gas of ultracold Cesium atoms that can be either thermalized or pushed out of equilibrium by means of laser cooling techniques. We determine in both cases the phase-space dynamics by tracing the evolution with position-resolved fluorescence imaging, from which we evaluate all relevant thermodynamic quantities. We confirm the validity of the generalized Clausius inequality for the first process and of the reversed generalized Clausius inequality for the second transformation.
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    Electrostatic all-passive force clamping of charged nanoparticles
    (2025) Tuna, Yazgan; Al-Hiyasat, Amer; Kashkanova, Anna D.; Dechant, Andreas; Lutz, Eric; Sandoghdar, Vahid
    In the past decades, many techniques have been explored for trapping microscopic and nanoscopic objects, but the investigation of nano-objects under arbitrary forces and conditions remains nontrivial. One fundamental case concerns the motion of a particle under a constant force, known as force clamping . Here, we employ metallic nanoribbons embedded in a glass substrate in a capacitor configuration to generate a constant electric field on a charged nanoparticle in a water-filled glass nanochannel. We estimate the force fields from Brownian trajectories over several micrometers and confirm the constant behavior of the forces both numerically and experimentally. Furthermore, we manipulate the diffusion and relaxation times of the nanoparticles by tuning the charge density on the electrode. Our highly compact and controllable setting allows for the trapping and force-clamping of charged nanoparticles in a solution, providing a platform for investigating nanoscopic diffusion phenomena.
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    Fluctuation theorems for non-Gaussian processes and disordered baths : quantum and classical
    (2025) Faria, Arthur M.; Lutz, Eric (Prof. Dr.)
    Non-Gaussian noise is omnipresent in systems where the central-limit theorem is inapplicable. We investigate the stochastic thermodynamics of small systems described by a general Kramers-Moyal equation with both Gaussian and non-Gaussian white noise, obtaining detailed and integral fluctuation relations for non-equilibrium entropy production in the small-noise regime. These results extend to phase space, where we examine whether the integral fluctuation theorem holds for Markovian non-Gaussian noises and characterize their non-equilibrium thermodynamics. These theoretical results manifest in inhomogeneous environments with finite-range system-bath coupling, where finite correlation lengths induce non-Gaussian Markovian noise via higher-order terms in the Kramers-Moyal equation; analytical solutions for the overdamped harmonic oscillator in this regime explicitly demonstrate non-Gaussian diffusion and non-trivial steady-state statistics. Building on these findings, we numerically study a driven system with localized bath interactions, including overdamped or underdamped particles in dragged harmonic potentials, where systematically reducing the interaction range leads to two main effects: dramatic enhancement of non-Gaussian signatures accompanied by strong suppression of mean entropy production. Finally, in the overdamped regime, we further discuss a generalized detailed-balance condition that defines a dynamic structure factor for the environment’s non-Gaussian density fluctuations, making it directly measurable in scattering experiments
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    Noise-induced quantum synchronization with entangled oscillations
    (2025) Tao, Ziyu; Schmolke, Finn; Hu, Chang-Kang; Huang, Wenhui; Zhou, Yuxuan; Zhang, Jiawei; Chu, Ji; Zhang, Libo; Sun, Xuandong; Guo, Zechen; Niu, Jingjing; Weng, Wenle; Liu, Song; Zhong, Youpeng; Tan, Dian; Yu, Dapeng; Lutz, Eric
    Random fluctuations can lead to cooperative effects in complex systems. We here report the observation of noise-induced quantum synchronization in a chain of superconducting transmon qubits with nearest-neighbor interactions. The application of Gaussian white noise to a single site leads to synchronous oscillations in the entire chain. We show that the two synchronized end qubits are entangled, with nonzero concurrence, and that they belong to a class of generalized Bell states known as maximally entangled mixed states, whose entanglement cannot be increased by any global unitary. We further demonstrate the stability against frequency detuning of both synchronization and entanglement by determining the corresponding generalized Arnold tongue diagrams. Our results highlight the constructive influence of noise in a quantum many-body system, and initiate the exploration of collective synchronization effects with stronger than classical correlations.
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    Fundamental limits on nonequilibrium sensing
    (2025) Dechant, Andreas; Lutz, Eric
    The performance of equilibrium sensors is restricted by the laws of equilibrium thermodynamics. Here, we investigate the physical limits on nonequilibrium sensing in bipartite systems with both reciprocal and nonreciprocal couplings. We show that one of the subsystems, acting as a Maxwell demon, can significantly suppress the fluctuations of the other subsystem relative to its response to an external perturbation. The importance of nonreciprocal interactions for such negative violations of the fluctuation-dissipation relation to occur is identified. We further demonstrate that these violations can considerably improve the signal-to-noise ratio above its corresponding equilibrium value, allowing the subsystem to operate as an enhanced sensor. In addition, we find that the nonequilibrium signal-to-noise ratio of linear systems may be arbitrarily large at low frequencies after proper parameter optimization, even at a fixed overall amount of dissipation. These results indicate that highly accurate nonreciprocal sensors can be designed at a finite energetic cost.