Physicists at the University of Basel have shown that a tiny engine built from a single atom and light can be bookkept in a way that matches classical thermodynamics, a step toward bridging two fields that have long described the world at very different scales. The work, published in Physical Review Letters and led by Patrick Potts, suggests that how researchers separate heat from work in quantum systems determines whether their results hold up.
The two fields have underpinned major technological eras. Thermodynamics describes how large collections of particles behave, a framework that powered the steam engine, while quantum mechanics explains the physics of atoms and subatomic particles, which underlies modern electronics. Building a coherent bridge between them remains an open problem at the frontier of physics, and the researchers chose a light engine as a test case.
The device consists of an atom placed inside a cavity formed by two mirrors. A laser shines on the atom, prompting it to emit photons that bounce between the mirrors. Because the mirrors are slightly leaky, photons eventually escape. The system therefore forms what physicists call a driven-dissipative system: like a conventional engine, it absorbs energy in the form of light and releases it to its surroundings.
Defining heat and work is central to thermodynamics. A spinning wind turbine can do useful work with its kinetic energy, while a box of hot gas holding the same energy delivers that energy mostly as heat that is harder to use. Light follows a similar logic: a one-watt laser beam can perform work, whereas a one-watt incandescent bulb produces mostly heat. In quantum systems, however, there is still no consensus on how to make that distinction.
« In this work, we take two existing treatments of light leaving the cavity and show that only one of them is consistent with the classical thermodynamic treatment of light », said Marcelo Janovitch, a postdoctoral researcher at the University of Basel. « Since there is no ambiguity in the classical thermodynamic description, the comparison helps us select the right quantum model. »
In the standard approach, all light leaving the cavity is counted as waste heat. The team tested this against a fully quantum model of the same engine, then increased the incoming laser power to reach a semi-classical limit, where the laser is treated classically and the atom remains quantum. In that limit, the results diverge from the standard thermodynamic model, which the researchers say invalidates that accounting.
An alternative description splits the leaking light into two parts: a coherent, laser-like mean and fluctuations around it. Treating the coherent part as work and the fluctuations as heat produces results that agree with thermodynamics in the same calculations. Janovitch said this matters because correct accounting underpins thermodynamic uncertainty relations, which are often used to characterise how quantum a system is. Incorrect bookkeeping could cause researchers to miss genuine quantum signatures.
The stakes are practical as well as theoretical. Atom-cavity systems are already used for precision measurement, where quantum effects provide the extra sensitivity. As the researchers frame it, an accounting error is not merely a bookkeeping problem; it could hide the very quantum effect an experiment was designed to exploit.
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