Scientists discover how a one-atom quantum engine can turn ‘waste’ light into useful energy

A machine does not have to be large to behave like an engine. It can be built around a single atom, fed with light and still raise one of the oldest questions in physics: how much of the energy entering the system can actually be turned into useful work?

Researchers at the University of Basel in Switzerland are working on a new way to answer that question for quantum systems, where the familiar rules of thermodynamics meet the very different behaviour of atoms and photons.

The problem becomes particularly interesting when light escapes from a quantum device. Traditionally, energy leaving an open system can be treated as heat lost to the environment. But the Basel researchers argue that this picture can be too simple. Some of the outgoing light may retain enough structure to be used elsewhere rather than being written off as waste.


The work is part of a broader effort in quantum thermodynamics to understand energy, heat and work in microscopic machines. The University of Basel's Quantum Thermodynamics Group describes the field as an extension of conventional thermodynamics into a regime where quantum effects and fluctuations become important.

A single atom can behave like an engine

The system considered by the researchers is remarkably small.

At its centre is an atom positioned inside a cavity bounded by mirrors. A laser supplies energy to the cavity in the form of light. Photons interact with the atom, while some of the light eventually leaves through the partially reflecting mirrors.

That constant flow of energy in and out gives the setup the characteristics of a driven, open system.

In ordinary terms, it resembles a tiny engine that is continuously powered and continuously loses energy. But because the components are quantum mechanical, physicists cannot simply apply the same assumptions used to describe a conventional steam engine or other macroscopic machine.

The University of Basel has been developing theoretical tools for precisely this problem. Its research programme focuses on quantum heat engines, open quantum systems and the role of fluctuations in nanoscale devices.

The surprising problem with calling light ‘waste’

The most important question is what happens to the photons that leave the cavity.

It might seem obvious to classify all escaping energy as heat. After all, the light has left the machine and is no longer available to drive the process inside it.

But quantum mechanics makes the situation less straightforward.

If the emitted light retains usable properties, some of its energy could potentially be transferred to another quantum device. In that case, treating every escaping photon as waste heat would overlook part of the energy that can still perform useful work.

A related University of Basel research effort describes this distinction in terms of output light retaining a coherent component capable of doing work, rather than treating the entire output as dissipation.

That idea changes the way scientists can think about efficiency in microscopic machines.

Why the classical world still matters

Quantum physics describes the atom and the individual photons in the most complete version of the model. But researchers also need their equations to behave sensibly when quantum effects become negligible.

This is where the semi-classical limit comes in.

In that description, the atom continues to follow quantum rules, including its discrete energy levels, while the electromagnetic field can be treated more like a classical wave.

That transition is important because classical physics should emerge naturally from quantum mechanics when the conditions allow it. A theory that works only in the fully quantum regime would not provide a complete picture.

The Basel approach is designed to make that transition without creating a contradiction over what should be considered heat and what should count as work.

A different way to look at energy loss

The distinction may sound theoretical, but it changes a fundamental part of the energy accounting.

Imagine the cavity sends out two kinds of energy. One portion behaves like disordered heat and cannot readily be recovered as useful work. Another portion remains organised enough to interact productively with another quantum system.

If both are simply labelled "heat," the second portion effectively disappears from the useful-energy calculation.

The researchers' framework instead keeps track of the useful component of the emitted light.

This is particularly relevant for quantum technologies because many proposed devices operate in exactly this kind of open environment, where energy is constantly being supplied, transferred and lost.

The University of Basel research group has previously investigated related questions involving quantum heat engines, including how quantum fluctuations affect heat and work.

Quantum fluctuations could become an advantage

The research also points to a less intuitive possibility: fluctuations do not necessarily have to be the enemy of a quantum machine.

At the microscopic level, quantum fluctuations are unavoidable. They can introduce noise and make delicate states more difficult to control. But the researchers found that quantum effects can also alter the fluctuations in emitted light in ways that may be useful.

In particular, reduced fluctuations could help generate specially prepared states of light.

That matters for quantum metrology, where scientists use quantum systems to make extremely precise measurements. The ability to control noise in light could eventually help improve measurements that depend on detecting very small changes.

Rather than trying to eliminate every fluctuation, future quantum devices may therefore be designed to exploit certain fluctuations or energy distributions.

From steam engines to quantum batteries

The comparison with traditional engines is more than a convenient analogy.

Thermodynamics became one of the foundations of modern engineering because it provided a way to understand how machines convert energy. Today, researchers are asking whether those same principles can be reformulated for machines made from only a handful of quantum components.

The potential applications extend to technologies such as quantum batteries, sensors and other nanoscale devices.

The University of Basel has highlighted scenarios in which light emitted by a quantum system could perform useful work, including transferring energy to another quantum system. Its research programme specifically connects these questions to emerging quantum and nanotechnologies.

Why this could matter for future quantum devices

Quantum technology is often described in terms of computing power, secure communication or ultra-sensitive measurements. Behind all of those applications is a more basic problem: energy still has to be supplied, moved around and eventually released.

Understanding that energy flow becomes increasingly important as devices shrink.

A thermodynamic framework that can describe both quantum behaviour and its classical limit could give researchers a more consistent way to analyse those systems. It may also prevent useful energy from being automatically counted as loss simply because it leaves the original device.

The work does not mean scientists have built a practical one-atom engine that can power everyday technology. Its significance is theoretical: it provides a way of thinking about heat and work when the boundaries between a machine, its environment and its energy output become blurred.

And that may be exactly where future quantum machines operate — in a world where a beam of light leaving a tiny cavity is not necessarily the end of the energy story, but potentially the beginning of useful work somewhere else.