Experiment reverses the direction of heat flow - and time

Schematic of the experimental setup. a Heat flows from the hot to the cold spin (at thermal contact) when both are initially uncorrelated. This corresponds to standard thermodynamic. For initially quantum-correlated spins, heat is spontaneously transferred from the cold to the hot spin. The direction of heat flow is here reversed. b View of the magnetometer used in our NMR experiment. A superconducting magnet, producing a high-intensity magnetic field (B0) in the longitudinal direction, is immersed in a thermally shielded vessel in liquid He, surrounded by liquid N in another vacuum separated chamber. The sample is placed at the center of the magnet within the radio-frequency coil of the probe head inside a 5-mm glass tube. c Experimental pulse sequence for the partial thermalization process. The blue (black) circle represents x (y) rotations by the indicated angle. The orange connections represents a free evolution under the scalar coupling, HHCJ=(πℏ/2)JσHzσCz, between the 1H and 13C nuclear spins during the time indicated above the symbol. We have performed 22 samplings of the interaction time τ in the interval 0 to 2.32 ms. Credit: Nature Communications, from: Reversing the direction of heat flow using quantum correlations

Inversion of heat and time

A crucial experiment that had a major impact on the scientific environment, published here  was published by a peer-reviewed scientific journal.

The delay in publication may be explained by the effects of the international team led by Brazilian physicists: they have reversed the sense of heat and in doing so have shown that the concept of the time arrow can be seen as a relative concept , not necessarily traveling inescapably from the past into the future.

Thermodynamic time arrow

Heat flows from hot objects to cold ones. When a hot object comes into thermal contact with a cold, both evolve into an equilibrium configuration. The hot cools and the cold gets hot. This is a phenomenon of nature as evidenced by daily experience and explained by the second law of thermodynamics.

According to this law, the entropy of any single system always tends to increase with time until it reaches a maximum value. Entropy is the greatness that describes the degree of undifferentiation of a system. Isolated systems evolve spontaneously into increasingly undifferentiated states.

The experiment showed that quantum correlations affect the way the entropy is distributed between the parts in thermal contact, changing the sense of the so-called "thermodynamic arrow of time". In other words, heat can flow spontaneously from the cold to the warm body without the need to invest energy in the process, as it does in an ordinary refrigerator.

"In the macroscopic world described by classical physics, the input of external energy can invert the direction of a system's heat flow, causing it to flow from the cold to the hot. which occurs in a common refrigerator, for example.

"It is possible to say that in our nanoscopic experiment, quantum correlations produced an effect analogous to that of energy." The direction of flow was inverted, without this being a violation of the second law of thermodynamics. information in the description of heat transport, we find a generalized form of the second law, unraveling the role of quantum correlations in the process, "explains Professor Roberto Serra of UFABC.

Reversing heat and time

The experiment was carried out with a sample of chloroform molecules (one hydrogen atom, one carbon and three chlorine atoms) labeled with the carbon isotope 13. This sample was diluted in solution and studied by means of a nuclear magnetic resonance device, similar to those used in hospitals for imaging tests, but with a much more intense magnetic field.

"We investigated changes in the temperature of the spins of the hydrogen and carbon nuclei." Chlorine atoms did not play a relevant role in the experiment. Using radiofrequency pulses, we put the spins of each of the hydrogen and carbon nuclei at different temperatures, one more The temperature differences were very small, in the order of tens of billionths of a kelvin, but modern techniques make it possible to manipulate and measure quantum systems with extreme accuracy, in which case the radiofrequency oscillations produced by atomic nuclei, "said Serra.

The researchers explored two situations: one in which the two nuclei (hydrogen and carbon) started the process uncorrelated and another in which both were correlated in a quantum form.

"In the first case, of the uncorrelated nuclei, we observe the heat flowing in the usual sense, from hot to cold, until the two nuclei are at the same temperature. In the second case, with the two nuclei initially correlated, we observe the heat flowing in the opposite direction , from the cold to the hot.The effect lasted a few thousandths of a second until the initial correlation was consumed, "he said.

The most interesting thing about this result is that it allows us to think of a quantum cooling process in which the external energy input (which is the resource used in refrigerators and air conditioners to cool a given environment) is replaced by correlations, by exchanging information between objects.

Maxwell's Demon

The idea that information could be used to reverse the sense of heat flow - that is, to promote the local decrease in entropy - arose in classical physics at the end of the 19th century, at a time when there was not even an information theory. 

This occurred in a mental experiment proposed by James Clerk Maxwell (1831-1879), author, among other things, of the famous equations of classical electromagnetism. In this mental experiment, Maxwell stated that if there was a being able to know the individual velocity of each molecule of a gas and act on it on a microscopic scale, it could separate those molecules into two containers. On the one hand, it would put the molecules faster, creating a warm compartment. On the other, it would put the molecules slower, creating a cold compartment. In this way, the gas, initially in thermal equilibrium due to the mixture of fast and slow molecules, would evolve to a differentiated state, therefore, of less entropy. 

Maxwell's idea with this mental experiment was to prove that the second law of thermodynamics had a purely statistical character. 

"Being proposed by him, capable of intervening in the material world on a molecular or atomic scale, became known as ' Maxwell's Devil .' He was a fictional figure Maxwell invented to present his point of view. to act on these scales and even on smaller scales, modifying the usual expectations, "said Professor Serra.

The experiment that motivated the now published article is proof of this. The study did not reproduce Maxwell's mental experiment, but produced an analogous result.

"When we talk about information, we are not referring to something imponderable. Information needs a physical substrate, a memory. Today, to erase a bit of memory from a pendrive it is necessary to spend 10 thousand times a minimum amount of energy constituted by This minimum of energy required to erase information is known as the Landauer Principle, and therefore erasing information generates heat. Heating is what consumes most the battery of notebooks, "said Serra.

What the researchers observed was that the information present in the quantum correlations can be used to produce a task that, in this case, was to transfer heat from a colder object to a warmer object without external energy consumption.

"We can quantify the correlation of two systems by means of bits." Connections between quantum mechanics and information theory are now creating what the scientific community has called quantum information science . to be employed to cool part of a quantum computer processor in the future. "


Reversing the thermodynamic arrow of time using quantum correlations
Kaonan Micadei, John PS Peterson, Alexandre M. Souza, Roberto S. Sarthour, Ivan S. Oliveira, Gabriel T. Landi, Tiago B. Batalhão, Roberto M. Serra, Eric Lutz
Nature Communications
Vol. 1711.03323
DOI: 10.1038 / s41467-019-10333-7


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