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Experiment reverses the direction of the heat flow

Publicado em 28 junho 2019

Heat flows from warm to cold objects. When a warm and a cold body are in thermal contact, they exchange heat energy until they reach a thermal equilibrium, whereby the warm body cools and the cold body warms up. This is a natural phenomenon that we experience all the time.

This is explained by the second law of thermodynamics, which states that the total entropy of an isolated system always tends to rise over time until it reaches a maximum. Entropy is a quantitative measure of the disorder in a system. Isolated systems evolve spontaneously towards more and more disordered states and lack of differentiation.

An experiment conducted by researchers from the Brazilian Center for Research in Physics (CBPF) and the Federal University of ABC (UFABC), as well as employees from other institutions in Brazil and elsewhere, has shown that quantum correlations influence the way entropy is distributed between 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 required by a household refrigerator. An article describing the experiment with theoretical considerations has just been published inNature communication.

The lead 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 through the Brazilian National Institute of Science and Technology in Quantum Information. FAPESP has also awarded two research grants to the project to another co-author, Gabriel Teixeira Landi, a professor at the Physics Institute of the University of São Paulo (IF-USP).

"Correlations can be seen as information shared by different systems. In the macroscopic world described by classical physics, the addition of energy from outside can reverse the heat flow in a system so that it flows from cold to hot. In an ordinary fridge for example, "Serra said.

"It is possible to say that in our nanoscopic experiment the quantum correlations had an analogous effect on that of added energy." The flow direction was reversed without violating the second law of thermodynamics, on the contrary, if we take into account elements of information theory when describing the transfer of heat, we find a generalized form of the second law and we 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 role in the experiment. We used radio frequency pulses to place the spin of each core at a different temperature, one cooler, another. were small, on the order of tens of trillionths of 1 Kelvin, but we now have techniques that allow us to manipulate and measure quantum systems with extreme precision. In this case, we measured the radio frequency fluctuations that the atomic nuclei, "said Serra .

The researchers explored two situations: in one the hydrogen and carbon nuclei did not start the process correlated, and in the other they were initially quantum correlated.

"In the first case, with the cores not correlated, we saw heat flowing in the usual direction, from warm to cold, until both cores were at the same temperature. In the second, with the cores that were initially correlated, we observed the heat flowing in the opposite direction, from cold to warm, the effect lasted for several thousandths of a second, until the original correlation was used up, "Serra explained.

The most striking aspect of this result is that it suggests a process of quantum cooling in which the addition of external energy (such as done in refrigerators and air conditioners to cool a specific environment) can be replaced by correlations, ie an exchange of information between objects.

Maxwell's demon

The idea that information can be used to reverse the direction of heat flow – in other words, to bring about a local decrease in entropy – originated in classical physics in the mid-nineteenth century, long before the information theory was invented.

It was a thought experiment proposed in 1867 by James Clerk Maxwell (1831-1879), who created the famous classical electromagnetism equations, among other things. In this thought experiment, which aroused a heated controversy at that time, the great Scottish physicist said that if there was a being able to know the speed of every molecule of a gas and manipulate all molecules on a microscopic scale, this being could separate them into two receivers, place faster than average molecules in one to create a hot compartment and slower than average molecules in the other to create a cool compartment. In this way, a gas that was initially in thermal equilibrium due to a mixture of faster and slower molecules would evolve into a differentiated state with less entropy.

With the thought experiment, Maxwell wanted to prove that the second law of thermodynamics was only statistical.

"The creature he proposed, capable of intervening in the material world on a molecular or atomic scale, became known as" Maxwell # 39; s demon. "It was a fiction invented by Maxwell to present his point of view. actually able to work on an atomic or even smaller scale, so that the usual expectations are adjusted, "Serra said.

The experiment conducted by Serra and staff and described in the just published article is a demonstration of this. Of course, it did not reflect Maxwell's thought experiment, but it produced an analogous result.

"When we talk about information, we don't mean something elusive. Information requires a physical substrate, a memory. If you want to erase 1 bit of memory from a flash drive, you must use 10,000 times a minimum amount of energy consisting of the Boltzmann constant times the absolute temperature. This minimum of energy required to erase information is known as the Landauer principle. This explains why erasing information generates heat. Notebook batteries consume more heat than anything else, # 39; Serra.

What the researchers saw was that the information present in quantum correlations can be used to perform work, in this case the transfer of heat from a colder to a hotter object, without using external energy.

"We can quantify the correlation of two systems by means of bits. Connections between quantum mechanics and information theory create what is known as quantum information science. From a practical point of view, the effect we have studied can be used one day to become part of a processor from the quantum computer, "Serra said.


About São Paulo Research Foundation (FAPESP)

The São Paulo Research Foundation (FAPESP) is a public institution with the mission to support scientific research in all areas of knowledge by awarding scholarships, scholarships and scholarships to researchers affiliated with higher education and research institutions in the state of São Paulo, Brazil. FAPESP is aware that the best research can only be done through international collaboration with the best researchers. That is why it has entered into partnerships with funding agencies, higher education, private companies and research organizations in other countries known for the quality of their research and encourages scientists who are funded by grants to further develop their international cooperation. You can find more information about FAPESP at http: // /NL and visit FAPESP news agency at http: // /NL to keep abreast of the latest scientific breakthroughs that FAPESP makes possible thanks to the many programs, prizes and research centers. You can also subscribe to the FAPESP news agency at http: // /subscribe.

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