An atom in a cavity between two mirrors (left) acts as a heat engine in a driven-dissipative quantum system in which energy is continuously added and lost to the environment. The fluctuations in the escaping light (center) are reduced (right) if only the atom, and not the light, is described quantum mechanically.
Illustration Credit: Enrique Sahagún, Scixel, University of Basel
Scientific Frontline: Extended "At a Glance" Summary: Quantum Thermodynamics and Light Engines
The Core Concept: Researchers have experimentally realized a miniature "engine" that uses light as the working medium instead of a conventional fluid, allowing the study of thermodynamics in the quantum realm.
Key Distinction/Mechanism: Unlike macroscopic engines that use moving parts (like pistons) to compress and expand a gas, this quantum engine uses photons trapped within a laser-created optical resonator. A single atom acts as the "piston," interacting with the photons. Work is extracted by changing the properties of the trapped light (e.g., its energy or phase).
Origin/History: This research is an ongoing theoretical and experimental pursuit bridging the gap between classical thermodynamics (established in the 19th century) and quantum mechanics (developed in the 20th century). The specific study referenced involves the University of Basel and the University of Stuttgart.
Major Frameworks/Components:
- Quantum Resonator: An optical cavity that confines light (photons) to a small space.
- Single-Atom "Piston": A single atom interacts with the light field, allowing for the exchange of energy (work and heat).
- Quantum Work and Heat: The precise definitions and measurements of these classical concepts when applied to a system of only a few photons and a single atom.
- Thermodynamic Cycles: The implementation of analogous cycles (like the Carnot or Otto cycle) in a quantum system to analyze efficiency and power output.
Branch of Science: Quantum Physics, Thermodynamics, Optics.
Future Application: Understanding how energy is converted at the quantum level is crucial for the development of highly efficient, microscopic devices, including quantum computers, sensors, and nanomachines. This could lead to more energy-efficient information processing.
Why It Matters: This research addresses a fundamental question in physics: how do the rules of macroscopic thermodynamics emerge from the microscopic laws of quantum mechanics? It demonstrates that thermodynamic principles can be applied and tested in highly controlled quantum systems, paving the way for advanced quantum technologies.
What are heat and useful work when a machine consists only of an atom and light particles? In modern quantum technologies, this kind of question connects thermodynamics with quantum physics. Researchers at the University of Basel have developed a theoretical approach that can reconcile both theories.
The physical theories of thermodynamics and quantum physics could not be more different. While thermodynamics was developed in the nineteenth century to explain the working principles of large steam engines, quantum physics emerged at the beginning of the twentieth century to describe the properties of atoms and subatomic particles. Nevertheless, in modern quantum technologies, the two theories meet again. In fact, tiny systems made of atoms and light particles (photons) can also absorb energy, convert it, and release it, thus acting as tiny quantum machines.
The challenge facing physicists involves finding a treatment for such systems that works for a completely quantum-mechanical system as well as in the semiclassical limit. The latter is the limiting case in which one part of the system is treated quantum mechanically, while classical physics is sufficient for the other part. In the scientific journal Physical Review Letters, researchers at the University of Basel, led by Professor Patrick Potts, have now presented a theoretical approach that addresses precisely this challenge.
Miniature Heat Engines in a Cavity
"Our calculations focus on the concrete physical model of an atom placed in a cavity between two mirrors, where it can absorb and emit light particles," says postdoctoral researcher Marcelo Janovitch. A laser continuously pumps additional photons into the cavity, while light can escape through the partially reflecting mirrors. "This is a textbook example of a driven-dissipative system, which continuously receives energy and simultaneously loses it to the environment," the researcher says. Such a model can be used to study fundamental questions about open quantum systems. In this context, the atom acts similarly to a tiny heat engine—or, in this case, a "light engine."
Recently, Potts and his collaborators demonstrated that the light particles escaping from the cavity should not generally be regarded as "waste heat" in thermodynamic treatments. Rather, a portion of their energy can still be used to perform useful work on another quantum system. In their new paper, the researchers investigated how this distinction between heat and useful energy affects the semiclassical limit.
In the semiclassical limit, the atom in the cavity is still viewed as a quantum system with discrete energy levels, while the light is modeled as a classical electromagnetic wave, such that quantum effects can be neglected. "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," says Janovitch. An important point is that this limiting case should be consistently derivable from the quantum-thermodynamic treatment.
Reduced Fluctuations as a Resource
This is precisely what Janovitch and his colleagues have now mathematically demonstrated. Their approach, in which part of the emitted light is counted as work, transitions to the semiclassical limit without any problems. By contrast, the conventional method, which regards all the energy escaping from the cavity as heat, fails in this limit. Moreover, the researchers’ calculations correctly predict how quantum effects lead to a reduction in the fluctuations of the light particles.
These reduced fluctuations are particularly interesting for applications in quantum technologies. They make it possible to use heat—which normally leads to disturbances in quantum systems—as a resource for specific purposes. For instance, researchers can create particular states of light for highly precise measurements in quantum metrology.
Published in journal: Physical Review Letters
Title: Bridging Quantum and Semiclassical Thermodynamics in Cavity QED
Authors: Marcelo Janovitch, Sander Stammbach, Matteo Brunelli, and Patrick P. Potts
Source/Credit: University of Basel | Oliver Morsch
Edited by: Scientific Frontline
Reference Number: qs081826_01
