A new theoretical approach helps connect quantum physics with classical thermodynamics by clarifying what counts as heat and useful work in tiny quantum machines.
When a machine is reduced to a single atom and particles of light, even basic concepts such as heat and useful work become difficult to define. This problem sits at the intersection of thermodynamics and quantum physics, and researchers at the University of Basel in Switzerland have developed a theoretical framework that brings the two descriptions into agreement.
Thermodynamics and quantum physics emerged to explain very different scales of nature. Thermodynamics took shape in the 19th century to describe machines such as steam engines, while quantum physics arose in the early 20th century to explain atoms and subatomic particles. Modern quantum technologies now bring those worlds together because tiny systems built from atoms and light particles (photons) can absorb, transform, and release energy, effectively operating as miniature quantum machines.
Physicists, therefore, need a description that works when the entire system is treated quantum mechanically and also in the semi-classical limit, where one part remains quantum while another can be described with classical physics. In Physical Review Letters, researchers in the University of Basel group led by Professor Patrick Potts present a theoretical approach designed to make those two regimes consistent.
A tiny quantum engine tests both theories
“Our calculations regard the concrete physical model of an atom that is placed in a cavity between two mirrors, where it can absorb and emit light particles,” says postdoc Marcelo Janovitch. A laser continuously adds photons to the cavity, while some light escapes through the partially reflecting mirrors. “This is a textbook example of a so-called driven-dissipative system that continuously receives energy and simultaneously loses it to the environment,” says the researcher. The setup provides a simple model for studying open quantum systems, with the atom acting much like a tiny heat engine or, in this case, a “light engine.”
Potts and his collaborators had previously shown that photons escaping the cavity should not automatically be treated as “waste heat” in a thermodynamic description. Some of the energy they carry can still perform useful work on another quantum system. The new research examines whether this distinction between heat and useful energy remains consistent as the model approaches the semi-classical limit.
Only one treatment survives the classical limit
In this limit, the atom still has discrete quantum energy levels, while the light is treated as a classical electromagnetic wave whose quantum effects can be neglected. “Treating the light classically makes it much easier to define which part of the energy can be used to perform work and which part is disordered heat,” says Janovitch. For the theory to remain consistent, however, this classical limit must follow naturally from the full quantum thermodynamic description.
Janovitch and his colleagues showed mathematically that their approach satisfies this requirement. When part of the emitted light is classified as work, the framework transitions consistently into the semi-classical limit. The conventional approach, which treats all energy escaping from the cavity as heat, does not. Their calculations also correctly predict how quantum effects reduce fluctuations in the emitted light.
These reduced fluctuations could be useful for quantum technologies because they suggest that heat, which normally creates disturbances in quantum systems, can instead serve as a resource. One possible application is producing specialized states of light for especially precise measurements in quantum metrology.
Reference: “Bridging Quantum and Semiclassical Thermodynamics in Cavity QED” by Marcelo Janovitch, Sander Stammbach, Matteo Brunelli and Patrick P. Potts, 11 August 2026, Physical Review Letters.
DOI: 10.1103/y6h7-sx93
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