In a machine made entirely of atoms and photons, what qualifies as heat, and what can be considered useful work? As quantum technology advances, these questions are bringing together two major areas of physics that were originally developed for very different purposes. Researchers at the University of Basel in Switzerland have developed a theoretical framework that describes thermodynamics and quantum physics consistently within the same system.
Thermodynamics emerged in the 19th century to explain how large-scale machines, including steam engines, transfer and convert energy. Quantum physics, developed in the early 20th century, focuses on atoms, photons, and other subatomic particles. Today, these fields increasingly overlap. Tiny devices made from atoms and light particles can absorb, transform, and release energy, functioning as microscopic quantum machines.
The central challenge is developing a description that remains valid when the entire system is treated quantum mechanically and when it approaches the semiclassical limit. In this limit, some components are described using quantum physics while others can be treated using classical physics. Researchers in Professor Patrick Potts’ group at the University of Basel have proposed an approach that achieves this consistency, as detailed in a paper published in Physical Review Letters.
A small quantum light engine
“Our calculations consider a concrete physical model of an atom placed in a cavity between two mirrors, where it can absorb and emit photons,” says postdoctoral researcher Marcelo Janovic. In this setup, a laser continuously supplies photons to the cavity, while some of the light escapes through a partially reflective mirror.
“This is a textbook example of a so-called driven-dissipative system that continuously receives energy while simultaneously losing energy to its environment,” the researchers explain. The model allows physicists to investigate fundamental questions about open quantum systems. In this case, the atoms function like miniature heat engines, or more precisely, quantum light engines.
Potts and his collaborators previously demonstrated that photons escaping from the cavity should not automatically be classified as “waste heat” in thermodynamic models. Some of the energy carried by the emitted light can still be used to perform useful work in another quantum system.
In their latest study, the researchers examined how the distinction between heat and useful energy changes as the system approaches the semiclassical limit.
Separating useful energy from heat
In the semiclassical limit, the atoms inside the cavity remain quantum systems with discrete energy levels. However, light is described as a classical electromagnetic wave, meaning that its quantum effects can be neglected.
“When light is treated classically, it becomes much easier to identify which part of the energy can perform work and which part should be considered heat,” says Janovic. For the theory to be consistent, this classical behavior must emerge naturally from the complete quantum thermodynamic description.
Janovic and his colleagues showed mathematically that their approach achieves this result. Once some of the emitted light is identified as useful work, the theory transitions smoothly to the semiclassical limit.
Traditional approaches can produce different results. If all the energy leaving the cavity is classified as heat, the theory does not make the same consistent transition between quantum thermodynamics and classical physics.
Quantum fluctuations become a resource
The researchers also found that their calculations accurately describe how quantum effects can reduce fluctuations in the emitted photons.
Controlling these fluctuations could be valuable for emerging quantum technologies. Heat is often associated with disturbances that make quantum systems difficult to control. However, under the right conditions, thermal effects can also become a useful resource.
For example, controlled quantum fluctuations could help produce specialized light states for highly precise measurements in quantum metrology. The findings demonstrate how a clearer understanding of the boundary between heat and useful work could help scientists recover and harness energy that might otherwise be lost.
Source: www.sciencedaily.com