Quantum Systems out of Equilibrium
Thermodynamics and Collective Phenomena
TEAM
Kay Brandner
Principal Investigator
Post-doctoral Research Fellow
​​Paul Nemec
PhD Student
TOPICS
Overview. Our subjects of interest are (mostly) quantum systems such as cold atoms, superconducting circuits or mesoscopic conductors. We use theoretical models to explore the non-equilibrium dynamics of such systems from a thermodynamic perspective with special emphasis on the role of collective phenomena that emerge through interactions between many degrees of freedom.
Quantum Thermal Devices. Classical thermodynamics was developed as a phenomenological theory to describe the equilibrium states of matter and transitions between them on the basis of a few fundamental laws. This theory makes it possible to characterize and optimize the operation cycles of macroscopic machines that use or generate heat fluxes such as heat engines and refrigerators. For applications in quantum technologies, such thermal machines must be realized on mesoscopic and atomistic length and energy scales. The modelling of these devices requires entirely new theoretical methods that are applicable far from equilibrium and account for strong thermal and quantum fluctuations. This framework is provided by stochastic and quantum thermodynamics.
The performance of quantum thermal devices is determined by a few figures of merit like efficiency, power and precision. A key aim of our work is to develop a comprehensive picture of the relationships between these figures and universal bounds that constrain them.
​
To explore the practical performance limits of quantum thermal devices, we use general models and methods from optimal control theory to find driving protocols that maximize their figures of merit. At the same time, we search for new mechanisms of energy conversion that exploit collective effects in many-body systems to enhance the output of thermal machines and investigate possible ways to connect them with other quantum technologies.
Open System Dynamics. Essentially any physical system is open in that its accessible degrees of freedom interact with inaccessible ones, which cannot be directly observed or controlled. In thermodynamics, the accessible degrees of freedom typically belong to working systems performing tasks like power generation or cooling, while the inaccessible ones form thermal reservoirs acting, for example, as heat sources. In information processing, the accessible degrees of freedom are usually quantum bits, while inaccessible degrees of freedom in their physical environment, which may belong, for example, to solid-state substrates, cause noise and decoherence, two of the biggest obstacles for the development of large-scale quantum technologies.
The theory of open (quantum) systems seeks to describe the time evolution of the observable degrees of freedom as accurately and with as little information about the inaccessible ones as possible. To this end, a large variety of methods has been developed over several decades, including a diverse range of quantum master equations, which, due to their relative simplicity, are currently among the most frequently applied tools in this area.
​
A central aim of our research is to develop rigorous and universally applicable techniques that make it possible to trace effective descriptions of open systems, like quantum master equations, back to physically transparent and mathematically well-defined conditions. In particular, we seek to provide systematic schemes to construct the dynamics of open systems from the underlying micro-dynamics of accessible and inaccessible degrees of freedom, and to quantify the error of necessary approximations involved in this process.
