This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: What is heat, and what is useful work if 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, Switzerland, 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 19th century to explain the working principle of large steam engines, quantum physics dealt with the properties of atoms and subatomic particles at the beginning of the 20th century.
Nevertheless, the two theories meet again in modern quantum technologies: 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 is finding a treatment of 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 journal Physical Review Letters, researchers at the University of Basel in the group of professor Patrick Potts have presented a theoretical approach that addresses precisely this challenge. "Our calculations concern the specific physical model of an atom placed in a cavity between two mirrors, where it can absorb and emit light particles," says postdoc Marcelo Janovitch. A laser continuously pumps additional photons into the cavity, while light can escape outside through the 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 the researcher. 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 showed that the light particles escaping from the cavity must not generally be regarded as "waste heat" in the thermodynamic treatment.
Rather, part 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 now taken to be a classical electromagnetic wave, such that quantum effects can be neglected.
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