University of Basel Researchers Develop Framework Bridging Thermodynamics and Quantum Physics

Researchers at the University of Basel in Switzerland have introduced a theoretical framework that aligns thermodynamics and quantum physics, two branches of physics that were historically developed for distinct purposes. This innovative work aims to resolve key questions about energy transfer in microscopic systems made from atoms and light particles, known as photons.

Thermodynamics originated in the 19th century to explain energy conversion in large machines, such as steam engines, while quantum physics emerged in the early 20th century, focusing on the behavior of atoms and subatomic particles. With the rise of quantum technologies, the intersection of these fields is increasingly significant as tiny systems can now absorb, transform, and release energy, functioning as miniature quantum machines.

A critical challenge in this area is establishing a valid description that accommodates both quantum mechanical treatment of an entire system and the semi-classical limit, where part of the system is quantum and the other part is classical. The research group led by Professor Patrick Potts addressed this issue in a study published in Physical Review Letters.

A Tiny Quantum Light Engine

Postdoc Marcelo Janovitch explains that their calculations focus on an atom placed in a cavity between two mirrors, enabling it to absorb and emit light particles. In this configuration, a continuous laser supplies additional photons to the cavity, while some light escapes through 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 Janovitch. This model allows physicists to investigate fundamental questions surrounding open quantum systems, likening the atom’s behavior to a miniature heat engine, or “light engine.”

The team previously demonstrated that the photons escaping the cavity should not simply be classified as “waste heat” in thermodynamic terms, as some of this light energy can perform useful work on a quantum system.

Separating Useful Energy From Heat

In their latest study, the researchers examined how the distinction between heat and useful energy shifts as the system nears the semi-classical limit. In this case, while the atom continues to exhibit quantum properties with discrete energy levels, the light is treated as a classical electromagnetic wave, thus allowing for the neglect of its quantum effects. “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,” notes Janovitch.

The researchers mathematically validated that their approach enables a smooth transition into the semi-classical limit when part of the emitted light is classified as useful work. In contrast, the conventional method, which counts all energy leaving the cavity as heat, results in an inconsistent transition.

Quantum Fluctuations Become a Resource

Additionally, the researchers found that their calculations effectively describe how quantum effects can minimize fluctuations in the emitted light particles. These reduced fluctuations possess potential value for quantum technologies since heat often introduces disturbances that complicate the control of quantum systems. However, under specific conditions, these fluctuations could transform into a resource.

For instance, the researchers suggest that such effects could foster the generation of particular light states beneficial for precise measurements in quantum metrology. Their findings indicate that a refined understanding of the interplay between heat and useful work could enable researchers to harness energy that might otherwise be deemed lost.