Dr. Kay Brandner
Dr. Kay Brandner
Department of Applied Physics
Aalto University School of Science
P. O. Box 15100
FI-00076, Espoo Finland
Sep 2016 - present: Research Post as Postdoctoral Researcher by the Academy of Finland
Aug 2015 - present: Postdoc with Prof. Christian Flindt, Aalto University, Finland
2011-2015: PhD in Theoretical Physics with Prof. Udo Seifert, University of Stuttgart, Germany Thesis
2006-2011: Diploma in Physics, University of Stuttgart, Germany
Thermodynamics of quantum devices. Originally developed as a phenomenological theory of work and heat, classical thermodynamics provides a powerful tool to describe macroscopic machines such as Otto engines or household refrigerators. Nowadays, experimental progress makes it possible to miniaturize such devices down to the scale of nanometers. Modeling the operation cycles of these systems requires a new approach that takes into account the effect of thermal and quantum fluctuations on quantities such as applied work, exchanged charge or extracted heat.
Our general aim is to explore the fundamental principles that govern the dynamics of mesoscopic thermal machines operating far from equilibrium. We are thereby interested in both, the general theoretical framework and specific systems that can be realized experimentally through present-day quantum engineering. In particular, we investigate how inherently non-classical phenomena such as coherence affect typical performance figures of thermal devices like power, efficiency and precision. To this end, we combine a variety of methods from stochastic thermodynamics, the theory of open quantum systems and dynamical control theory.
 K. Brandner, T. Hanazato, K. Saito, Thermodynamic Bounds on Precision in Ballistic Multi-Terminal Transport, arXiv:1710.04928.
 K. Brandner, M. Bauer, U. Seifert, Universal Coherence-Induced Power Losses of Quantum Heat Engines in Linear Response, Phys. Rev. Lett. 119, 170602 (2017).
 K. Brandner, U. Seifert, Periodic thermodynamics of open quantum systems,
Phys. Rev. E 93, 062134 (2016).
Phase transitions and Lee-Yang zeros. Conventional phase transitions such as the condensation of a gas into a liquid are characterized by large fluctuations of thermodynamic observables and an anomalous behavior of the free energy. More than half a century ago, Lee and Yang realized that these exceptional phenomena can be understood from the complex values of an external control parameter, e.g. temperature, for which the partition function of a finite-sized system vanishes; in the thermodynamic limit, these Lee-Yang zeros approach the critical parameter value on the real axis. Over the last decades, this idea has been further developed into a powerful theoretical framework that covers, for example, non-equilibrium and dynamical phase transitions.
Here, we investigate the general laws that determine the trajectories of Lee-Yang zeros in the complex plane and their relation to physical quantities like the high-order cumulants of a stochastic process, which can be directly observed in an experiment. Applying tools from large-deviation theory, we could recently show that Lee-Yang zeros can, in principle, can be used to infer the behavior of a system in the thermodynamic limit from its fluctuations in the small-size regime. Further exploring the generality and consequences of this result, which suggests a quite remarkable duality between small and large systems, is a central topic in our current research.
 A. Deger, K. Brandner, C. Flindt, Lee-Yang zeros and large-deviation statistics of a molecular zipper,
 K. Brandner, V. F. Maisi, J. P. Pekola, J. P. Garrahan, C. Flindt, Experimental Determination of
Dynamical Lee-Yang Zeros, Phys. Rev. Lett 118, 180601 (2017).