Heat flows from hot to cold objects. When a hot and a cold body are in thermal contact, they exchange heat energy until they reach thermal equilibrium, with the hot body cooling down and the cold body warming up. This is a natural phenomenon we experience all the time.
It is explained by the second law of thermodynamics, which states that the total entropy of an isolated system always tends to increase over time until it reaches a maximum. Entropy is a quantitative measure of the disorder in a system. Isolated systems evolve spontaneously toward increasingly disordered states and lack of differentiation.
An experiment conducted by researchers at the Brazilian Center for Research in Physics (CBPF) and the Federal University of the ABC (UFABC), as well as collaborators at other institutions in Brazil and elsewhere, has shown that quantum correlations affect the way entropy is distributed among parts in thermal contact, reversing the direction of the so-called “thermodynamic arrow of time.”
In other words, heat can flow spontaneously from a cold object to a hot object without the need to invest energy in the process, as is required by a domestic fridge. An article describing the experiment with theoretical considerations has just been published in Nature Communications.
The first author of the article, Kaonan Micadei, completed his PhD under the supervision of Professor Roberto Serra and is now doing postdoctoral research in Germany. Serra, also one of the authors of the article, was supported by São Paulo Research Foundation — FAPESP via Brazil’s National Institute of Science and Technology in Quantum Information. FAPESP also awarded two research grants linked to the project to another coauthor, Gabriel Teixeira Landi, a professor at the University of São Paulo’s Physics Institute (IF-USP).
“Correlations can be said to represent information shared among different systems. In the macroscopic world described by classical physics, the addition of energy from outside can reverse the flow of heat in a system so that it flows from cold to hot. This is what happens in an ordinary refrigerator, for example,” Serra told.
“It’s possible to say that in our nanoscopic experiment, the quantum correlations produced an analogous effect to that of added energy. The direction of flow was reversed without violating the second law of thermodynamics. On the contrary, if we take into account elements of information theory in describing the transfer of heat, we find a generalized form of the second law and demonstrate the role of quantum correlations in the process.”
The experiment was performed with a sample of chloroform molecules (a hydrogen atom, a carbon atom and three chlorine atoms) marked with a carbon-13 isotope. The sample was diluted in solution and studied using a nuclear magnetic resonance spectrometer, similar to the MRI scanners used in hospitals but with a much stronger magnetic field.
“We investigated temperature changes in the spins of the nuclei of the hydrogen and carbon atoms. The chlorine atoms had no material…