Two physicists from the University of Stuttgart have proven that Carnot’s principle, a central law of thermodynamics, does not apply to objects of the size of atoms whose physical properties are linked (so-called correlated objects). This finding could, for example, advance the development of tiny, energy-efficient quantum motors. The scientific journal Science Advances published the derivation.( DOI: 10.1126/sciadv.adw8462 )
Internal combustion engines or steam turbines are heat engines: they convert thermal energy into mechanical motion - or in other words, heat into motion. In recent years, quantum mechanical experiments have made it possible to miniaturize heat engines down to the micro range. tiny motors, no bigger than a single atom, could become a reality in the future," says Eric Lutz from the Institute of Theoretical Physics I at the University of Stuttgart. "It is now also certain that these motors could achieve a higher maximum efficiency than larger heat engines." Professor Eric Lutz and Milton Aguilar, postdoctoral researcher at the Institute of Theoretical Physics I, have now shown why this is the case in a Science Advances paper. In a three-question interview, the two scientists get to the heart of their discovery.
What exactly did you find out?
The French physicist Sadi Carnot determined the maximum efficiency of heat engines almost exactly 200 years ago. Carnot’s principle, the second law of thermodynamics, was developed for large, macroscopic objects. It applies to steam turbines, for example. However, we have now been able to prove that Carnot’s principle must be extended for objects on the scale of atoms - for example, for strongly correlated molecular engines.
Why is that?
Carnot showed that the temperature difference has a decisive influence: The greater the difference between hot and cold, the higher the maximum possible efficiency of a heat engine. However, Carnot’s principle neglects the influence of so-called quantum correlations. These are special connections that arise between particles on a very small scale. We have derived generalizations of the laws of thermodynamics that fully take these correlations into account for the first time. We were able to show that thermal machines working at the atomic level can convert not only heat but also correlations into work. This means that more work can be produced - and the efficiency of a quantum engine can exceed the Carnot limit.
Our work deepens our knowledge of the world in atomic dimensions. The better we understand the laws of physics that apply in these dimensions, the sooner we can use them to develop technologies for tomorrow - such as tiny, highly efficient quantum motors that can perform work precisely at the nanoscale. Perhaps such motors will one day power medical nanobots or control machines that process materials at an atomic level? The potential is hugely diverse.
Video from the series "Physics that creates knowledge": Prof. Eric Lutz on quantum motors (2022)
