AIBRIEF
A University of Basel paper in Physical Review Letters shows that, for one atom trapped between two mirrors by laser light, the definition of 'heat' determines which theoretical limits on how efficiently a quantum engine can turn heat into work hold.
When the engine is a single atom between two mirrors, deciding what counts as "heat" decides which physics holds. A new paper from the University of Basel, published in Physical Review Letters on 11 August 2026 by the group of Patrick Potts, makes that choice explicit.
One atom sits inside an optical cavity, a small gap between two partially reflecting mirrors, and is continuously driven by a laser. Photons bounce back and forth; some escape through the mirrors. The setup is "driven-dissipative": the laser feeds the system, the escaping photons drain it.
The paper shows that in the semi-classical limit, where quantum mechanics slides toward classical physics, two ways of dividing the outgoing photon flux into "heat loss" and "useful work" give different answers. Only the bookkeeping that treats part of the escaping light as a power source, not pure loss, lets the standard thermodynamic uncertainty relations hold. The authors illustrate the point with a three-level version of the atom.
"This result shows how subtle the definition of heat can be in driven-dissipative systems," Marcelo Janovitch, a postdoc in the Potts group, said in the University of Basel release.
The result is theoretical, and the authors prove it in the semi-classical limit, not the fully quantum case. A ScienceDaily re-report on 19 August framed the work as "useful energy hiding in waste heat." The paper itself does not claim any measured efficiency gain, and no device has been built.