PHYSICS

Quantum demon

A Brazilian group can control the heat generated by atomic nuclei

Imagem: Fundamentals of Cybernetics | AY Lerner | Springer A 1975 paper by Russian mathematician Alexander Lerner includes a depiction of Maxwell’s demon inside a container of gas molecules to be selectedImagem: Fundamentals of Cybernetics | AY Lerner | Springer

The random generation of heat in the microscopic world is one of the main obstacles to advances in nanotechnology. As nanodevices become smaller and more complex, made of parts the size of molecules or even atoms, there is a greater chance that they will generate dangerous quantum fluctuations while operating. These fluctuations are abrupt, unpredictable energy variations governed by the probabilistic laws of quantum mechanics, and can damage nanomechanisms. A group of Brazilian physicists led by Roberto Serra, a professor at the Federal University of the ABC (UFABC), presented a technique able to attenuate the production of these heat fluctuations at the subatomic level in early December 2016 in Physical Review Letters.

The microscopic fluctuations in energy and heat would affect the nanomachines in a way similar to how uncontrolled overheating can damage a conventional macroscopic motor, like a car engine. During the Industrial Revolution in the 19th century, one of the motivations behind the development of classical thermodynamics—the area of physics that establishes how energy in the form of heat is converted into mechanical energy and vice-versa—was to understand the operation of pressure valves and refrigerators, devices that made the operation of steam and internal combustion engines safer and more efficient. Anticipating advances in nanotechnology, Serra and his colleagues are part of a community of physicists who have been developing a more general and detailed theory of thermodynamics, called non-equilibrium quantum thermodynamics, that can ensure the efficient operation of molecular- and atomic-scale devices in which quantum effects occur.

In order to develop a new control technique, Serra and his colleagues were inspired by “Maxwell’s demon,” a fantastic being invented by Scottish physicist and mathematician James Clerk Maxwell (1831-1879). Maxwell was one of the first to understand that the temperature of a given volume of gas depends on the average speed at which the molecules in it move. The faster the molecules, the hotter the gas. In an 1867 letter to his colleague Peter Tait, Maxwell posited a microscopic, intelligent being, able to measure and record the velocities of all of the gas molecules. In the mental experiment, the being controlled a valve separating two identical containers, both containing a gas at the same temperature. Opening and closing the valve quickly, the creature separated the molecules of gas, leaving the slower-than-average ones—which were thus colder—in one container and allowing the faster ones into the other container.

Imagem: Darling & Hulburt, American Journal of Physics, 23-7, 1955 In an article published in 1955 in the American Journal of Physics, the imaginary figure controls the flow of gas molecules from outside the containersImagem: Darling & Hulburt, American Journal of Physics, 23-7, 1955

Thermodynamic sin
In a Nature article in 1874, Irish physicist William Thomson, better known as Lord Kelvin, called the intelligent being a demon, to emphasize that the creature, by heating the gas in one container while cooling the gas in the other, committed the sin of violating the second law of thermodynamics.

This law states that an isolated body, or a set of isolated bodies, has a property called entropy, which always tends to increase over time. For a set of particles, entropy corresponds to the possible number of arrangements of all of them, in a given situation.

When ordering the gas molecules by their speed, the demon is supposedly reducing the number of possible arrangements of the system, thus reducing its entropy. After Thomson’s article, physicists still wondered: is the demon a mere fantasy, or does the mental experiment indicate a failure to understand the laws of thermodynamics?

More than a century later, in 1982, American physicist Charles Bennett, then a researcher at IBM, realized that in order to work in practice, Maxwell’s demon would need to record information on the speed of the gas molecules on a physical substrate, such as the memory bits of a computer. However, writing and erasing data in memory cannot be done without generating heat, something discovered years earlier by Rolf Landauer, another IBM researcher. And generating heat always increases entropy.

When assessing the increase and decrease of heat in the two stages of this process, one notes that the second law of thermodynamics is never violated: the demon can reduce the entropy inside the gas containers by selecting molecules, but the heat generated to record the speeds of the molecules in memory increases entropy outside the containers much more. The calculations show that Maxwell’s supernatural creature obeys all physics laws and that its function could be performed, in practice, by an automatic mechanism, controlled by computer memory.

Since then, researchers have created increasingly smaller mechanisms in the laboratory similar to that imagined by Maxwell. The current project being carried out by Serra and his colleagues is the first to design a completely quantum Maxwell’s demon. In a laboratory in the Brazilian Center for Physics Research (CBPF) in Rio de Janeiro, the researchers shot a pulse of electromagnetic waves at a solution of chloroform molecules—each one consisting of a carbon atom, a hydrogen atom, and three chlorine atoms (CHCl3). The pulse was adjusted to cause quantum fluctuations in the energy of the nuclei of the carbon atoms of the molecules. At the same time, physicists shot additional electromagnetic waves at the molecules with the goal of adjusting the interaction between the carbon and hydrogen nuclei of each molecule.

The researchers managed to use the hydrogen nucleus as a Maxwell’s demon, storing information about the state of the carbon nucleus. Depending on the energy level of the carbon nucleus, the hydrogen nucleus activated and restricted the energy fluctuations of its neighbor. The actions of the hydrogen nuclei caused the energy fluctuations of the carbon nuclei to occur in a way that produced the least possible entropy. “We designed this process based on a mathematical equation that we derived, relating information, entropy and energy,” says Serra. “The equation is quite general and can be applied to any quantum system, such as electrons and photons, not just to atomic nuclei.”

“It is an exciting study,” comments Vlatko Vedral, a physicist at Oxford University, England, who participated in an experiment carried out in 2016 in which laser beans were used to produce a Maxwell’s demon. “They tested a formula that describes the production of entropy in quantum systems under generic conditions. It is still unclear why the entropy of the Universe must always increase, and this approach could help explain the origins of the second law of thermodynamics.”

Project
National Institute of Quantum Information Science and Technology (nº 2008/57856-6); Grant Mechanism Thematic Project; Principal Investigator Amir Caldeira (Unicamp); Investment R$1,977,654.30 (for the entire project).

Scientific article
CAMATI, P. A. et al. Experimental rectification of entropy production by Maxwell’s demon in a quantum system. Physical Review Letters. V. 117. p. 240502. December 5, 2016.