Chilean physicists make important discoveries in quantum thermodynamics



Felipe Barra and Cristóbal Lledó, researchers from the Millennium Nucleus Physics of Active Matter of the University of Chile.

Quantum has become one of the areas of greatest interest for science and technology, due to the impact that promises to generate in the creation of supercomputers and machines. Unraveling the gaps and doubts that exist about the thermodynamics of these systems have become crucial to achieving these advances.
Felipe Barra, academic of the Department of Physics (FCFM) of University of Chile
and Cristóbal Lledó, student of the Science Master Program of the same academic unit, has just been published in Physical Review E magazine, with a study where they analyzed the evolution of a quantum system with the objective of characterizing their transformations and how the exchange of energy is divided between work and heat.
The result of his work allowed to describe previously unknown scenarios in the thermodynamics of these quantum systems, which does not only provide insights into the concepts and theories that are been discussed but also demonstrates that the range of applications could be greater than expected.

The findings
Felipe Barra, Ph.D. and associate researcher of the Physical Nucleus of Active Matter of the U. of Chile, explains that to study the thermodynamics of a quantum system they used a type of maps called thermal maps.
Thermal maps represent systems that reach thermodynamic equilibrium by themselves, which means, that is not necessarily an external agent that performs work on the system in order to achieve thermal equilibrium by exchanging heat with the environment.
The equilibrium state of these maps is represented by a quantity, called Gibbs state, which depends on the total energy of the system. This state has two properties that justify its appellation of equilibrium: its entropy (degree of molecular disorder of the universe) does not increase and the map does not change this amount.
What Chilean physicists found is that to have equilibrium it is not necessary that this amount depends on the total energy, as it was thought. Is enough with the fact that the other two properties fulfill for that to happen without being a state of Gibbs.
“What we find is that there are systems that have thermal equilibrium states only if these two properties are met, without being a state of Gibbs and that had not been seen before,” explains Barra.

It was not the only thing they found. The physicists of the Millennium Nucleus showed that, in the cases described above, the system will not reach thermal equilibrium by itself, as it was supposed to happen in thermal maps, but an external agent must work on the system for this to happen. “Like this is not usual, to convince ourselves, we not only focus on the average properties of thermodynamics, such as heat and work; but also, in fluctuating quantities, what is called esthotic thermodynamics”, says Barra.
A finding that extends the knowledge that we had about quantum thermodynamics.

Measure quantities
Finally, Barra and Lledó could also establish a correspondence between a map with equilibrium and a Lindblad equation, a formula well known in physics and used to calculate the state of a system that has a generalized detailed balance. “We realized that if we can obtain a Lindblad equation from a map with equilibrium, then it fulfills the generalized detailed balance property”, explains Barra.
This finding is not minor, because it allows knowing the thermodynamic quantities of the quantum systems in a simpler way.
In general, in order to evaluate these quantities it is necessary to have the information that is in both; inside the system and also, outside; in its environment, which in quantum can become a real headache.
The work of the Chilean physicists reveals that if a Lindblad equation has a generalized detailed balance it is not necessary to know the information of the environment to study the thermodynamic quantities of the quantum system. A finding that allows simplifying the study of these systems by eliminating variables that can complicate their study in the laboratory.
“With these findings what we do is help to understand and expand the theory. We explore contexts in which our scientific colleagues mistakenly think that certain things are going to happen because they have preconceived ideas. But we showed that in those scenarios the behavior is different and with it, we showed that actually, there is a much wider range of applications for the theory”, says Felipe Barra.

Stochastic thermodynamics of quantum maps with and without equilibrium.
Felipe Barra* and Cristóbal Lledó.
Más detalles: DOI: 10.1103/PhysRevE.00.00210024