Collective Quantum Thermodynamics: Quantum vs Classical

Thermal machines like car engines, airplane turbines and household refrigerators have long been essential to our modern society. By converting heat into mechanical work or vice versa, they set cars and airplanes in motion, drive the generators that deliver electricity to our computers and cool our food, living spaces and data centers. None of these modern applications would be possible without one fundamental theory that emerged 200 years ago and has since then enabled engineers to develop more and more advanced machines: thermodynamics. Equipped with a few elementary concepts and laws, this theory lays down the basic rules that govern the performance of James Watt's 18th century steam engine and today's car engines alike.

With the next technological revolution underway in the nano and quantum world, there is now an increasing need to develop a new generation of thermal machines that operate on extremely small length-scales to propel nano-robots or cool the building blocks of quantum computers that require ultra-low working temperatures. The last decade has seen a series of landmark experiments, in which ever smaller thermal machines were realized down to the level of single atoms. Such tiny objects are no longer bound by the rules of the classical world; they can occupy two places at the same time or influence each other at a distance without direct interaction. These phenomena are manifestations of the quantum laws of motion that govern the world at atomic scales. The discipline that aims to describe thermal machines operating in this world and seeks to harness their technological potential has been called quantum thermodynamics and forms my main area of research.

Technological applications of quantum thermal machines are still facing major conceptual and practical challenges. One of these challenges is their limited energy turnover, which is too small to match the needs of most currently envisaged applications by several orders of magnitude. The key idea underpinning my fellowship is to address this problem by harnessing the properties of collective states of matter, which emerge when large numbers of quantum objects begin to behave in a coordinated way, somewhat similar to a flock of birds. Laying the theoretical groundwork to realize new types of quantum thermal machines that exploit these phenomena to enhance their performance is the central aim of my research program.

Building on our results so far, my team, my partners in theory and experiment and I are working on three major topics, which are connected by the theme of seeking synergies between quantum and classical physics. First, to develop the methods required to describe quantum systems hosting collective effects, we investigate classical analogues of these systems, which can be efficiently simulated with classical computers; this idea is similar to using classical water waves as models for the wave character of quantum particles. Second, with the aim of integrating collective quantum thermal machines with classical consumers of their output, we investigate how thermodynamic quantities, like the work produced by a heat engine, can be transmitted from the quantum world into the classical one. Third, to find quantitative measures for the thermodynamic advantage generated by collective quantum effects, we explore how these phenomena make it possible to overcome general trade-off relations that constrain the power, efficiency and precision of classical small-scale thermal machines such as molecular motors.

Quantum technologies are widely expected to shape our century in a similar way as the industrial revolution changed 19th and 20th century. Collective quantum thermal machines, for the development of which we are helping to lay the conceptual foundations, have the potential of becoming the steam engines of this development. They will not move our future cars, but they might well help to run our quantum computers and encryption devices.