Two physicists at the University of Stuttgart have shown that Carnot’s principle, a fundamental rule of thermodynamics, does not fully apply at the atomic scale when particles are physically connected (so-called correlated objects). Their findings suggest that this long-term efficiency limit breaks down for small systems driven by quantum effects. The work could help accelerate progress toward extremely small and energy-efficient quantum engines. The team published their mathematical proof in a journal Scientific advances.
Traditional heat engines, such as internal combustion engines and steam turbines, work by converting heat energy into mechanical motion, or simply by converting heat into motion. Over the past few years, advances in quantum mechanics have allowed researchers to scale heat engines down to microscopic dimensions.
“Small motors, no larger than one atom, could become a reality in the future,” says Professor Eric Lutz of the Institute for Theoretical Physics I at the University of Stuttgart. “It is also now clear that these engines can achieve higher peak efficiencies than larger heat engines.”
Professor Lutz and Dr. Milton Aguilar, a postdoctoral researcher at the same institute, describes the physics behind this surprising result in their Scientific advances paper. In a three-question interview, they outline what they discovered and why it matters.
Rethinking the 200-year-old efficiency limit
Almost two centuries ago, the French physicist Sadi Carnot established the theoretical maximum efficiency that any heat engine could achieve. Carnot’s principle, which later became part of the second law of thermodynamics, was formulated for large-scale systems such as steam turbines.
The Stuttgart researchers have now demonstrated that this principle must be extended when applied to atomic-scale systems. This is especially true for strongly correlated molecular engines, where particles are tightly coupled in ways not accounted for in classical thermodynamics.
The hidden role of quantum correlations
Carnot’s original work showed that efficiency depends on temperature differences, with larger gaps between hot and cold resulting in higher potential efficiency. What the classical formulation does not include is the effect of quantum correlations. These are subtle connections that occur between particles when systems become extremely small.
For the first time, scientists have derived generalized laws of thermodynamics that fully encompass these correlations. Their results show that atomic-scale heat machines can convert not only heat into work, but also quantum correlations themselves. With this added contribution, such machines can generate more work than classical theory allows, meaning that the efficiency of a quantum engine can exceed the traditional Carnot limit.
What this means for the technology of the future
In addition to improving basic physics, the research opens up new possibilities for future applications. A deeper understanding of how the laws of physics work at the atomic level could accelerate the development of next-generation technologies, including ultra-small and highly efficient quantum engines capable of performing precise tasks at the nanoscale.
Such motors could one day power medical nanobots or guide machines that manipulate materials atom by atom. The range of potential uses is huge, showing how rethinking basic scientific principles can lead to entirely new technological horizons.
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