Thermodynamics is a phenomenological theory that allows the investigation of equilibrium properties of macroscopic objects. Based on fundamental measurable quantities – heat and work – it provides a universal framework to study the conversion of different forms of energy. Introduced 150 years ago, at the beginning of the industrial revolution, to analyze and improve the performance of the newly invented steam engine, it has been successfully applied since then to design a great variety of useful devices, from car engines and refrigerators to power plants and solar cells. In a famous quote, Einstein once hailed it as “the only physical theory of universal content concerning which I am convinced that, within the framework of the applicability of its basic concepts, it will never be overthrown”.
Now, with technological progress reaching into the nano and quantum regime, and in view of the fundamentally different rules of quantum mechanics, there is an imminent need to understand thermodynamics at the microscopic scale where thermal fluctuations compete with quantum fluctuations. Our research revolves around three main topics that are organized in working groups (WGs). They range from fundamental questions on the nature of equilibration (WG1), to theory that aims to provide practical methods of characterising thermodynamics and non-equilibrium processes in the quantum regime (WG2) to experimental techniques of controlling, manipulating and exploring quantum systems in and out-of equilibrium and test predictions of the theory (WG3).
How do complex quantum systems come to equilibrium, once pushed out of it? On what timescale can one expect this to happen? And how do properties such as thermal states dynamically emerge from a microscopic quantum description?
While statistical physics has recipes for treating quantum systems in equilibrium, and how thermal states can be grasped as maximum entropy states given suitable constraints, a long-standing open question is to resolve the tension between a microscopic description and the one offered by equilibrium statistical mechanics. Some mechanisms, that are relevant for processes of equilibration and thermalisation, have been identified early on, yet key questions still remain wide open.
To be applicable for nano-scale systems thermodynamics must be revised to account for, in addition to thermal fluctuations, quantum fluctuations. These effects are becoming measurable as rapid experimental progress in controlling and manipulating the dynamics of small systems is being made. Extremely small at the macroscale, and therefore neglected in the usual formulation of thermodynamics, they dominate at the micro and nanoscale.
A multitude of questions open up, and these will need to be addressed in order to make optimal use of the new technologies on the horizon: What are the correct/most useful definitions of work and heat for closed and open quantum processes? How does quantum measurement affect them? Can quantum engines be build that explore quantum fluctuations to provide technological advantages over classical ones? If entanglement allows the extraction of work could one use the abundance of entanglement in the world for that purpose? Finally, WG 2 will aim to find a general description of non-equilibrium quantum processes, for example by generalising the extremely successful classical theory of fluctuation theorems and combining it with insights from the study of quantum channels and information theoretic relations.
Given the generality of classical thermodynamic principles and their importance for engineering in the macroscopic world, it is of utmost importance to address the corresponding phenomena in various quantum realizations of modern experimental systems.
It is now that advances in producing and controlling a range of different man-made quantum systems is starting to provide ideal experimental techniques to test and explore thermodynamics and energy statistics in the quantum regime. Experimenters in mesoscopic electron systems, cold atoms, trapped ions and quantum optics, all share the common interest of understanding relaxation, thermalisation, non-equilibrium and general thermodynamic properties of their systems. WG3 will aim to reconcile quantum information and thermodynamic techniques to test and explore thermodynamic and non-equilibrium relations at the classical-quantum boundary and into the quantum regime.