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Quantum physics: Hot and cold at the same time

Date:
April 9, 2015
Source:
Vienna University of Technology
Summary:
Temperature is a statistical concept. Very small systems, consisting of a small number of particles, are not usually described statistically. Scientists have now measured how quantum systems reach a state with well defined statistical properties -- and surprisingly, they found out that quantum systems can have several temperatures at once. The connection between small quantum systems and large systems obeying the laws of classical physics is one of the big open questions in physics.
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This is the atom chip for capturing and cooling clouds of atoms.
Credit: TU Wien

Temperature is a very useful physical quantity. It allows us to make a simple statistical statement about the energy of particles swirling around on complicated paths without having to know the specific details of the system. Scientists from the Vienna University of Technology together with colleagues from Heidelberg University have now investigated, how quantum particles reach such a state where statistical statements are possible. The result is surprising: a cloud of atoms can have several temperatures at once. This is an important step towards a deeper understanding of large quantum systems and their exotic properties.

Statistics Helps where Things get Complicated

The air around us consists of countless molecules, moving around randomly. It would be utterly impossible to track them all and to describe all their trajectories. But for many purposes, this is not necessary. Properties of the gas can be found which describe the collective behaviour of all the molecules, such as the air pressure or the temperature, which results from the particles' energy. On a hot summer's day, the molecules move at about 430 meters per second, in winter, it is a bit less.

This statistical view (which was developed by the Viennese physicist Ludwig Boltzmann) has proved to be extremely successful and describes many different physical systems, from pots of boiling water to phase transitions in liquid crystals in LCD-displays. However, in spite of huge efforts, open questions have remained, especially with regard to quantum systems. How the well-known laws of statistical physics emerge from many small quantum parts of a system remains one of the big open questions in physics.

Hot and Cold at the Same Time

Scientists at the Vienna University of Technology have now succeeded in studying the behaviour of a quantum physical multi-particle system in order to understand the emergence of statistical properties. The team of Professor Jörg Schmiedmayer used a special kind of microchip to catch a cloud of several thousand atoms and cool them close to absolute zero at -273°C, where their quantum properties become visible.

The experiment showed remarkable results: When the external conditions on the chip were changed abruptly, the quantum gas could take on different temperatures at once. It can be hot and cold at the same time. The number of temperatures depends on how exactly the scientists manipulate the gas. "With our microchip we can control the complex quantum systems very well and measure their behaviour," says Tim Langen, leading author of the paper published in "Science." There had already been theoretical calculations predicting this effect, but it has never been possible to observe it and to produce it in a controlled environment.

The experiment helps scientists to understand the fundamental laws of quantum physics and their relationship with the statistical laws of thermodynamics. This is relevant for many different quantum systems, maybe even for technological applications. Finally, the results shed some light on the way our classical macroscopic world emerges from the strange world of tiny quantum objects.


Story Source:

Materials provided by Vienna University of Technology. Note: Content may be edited for style and length.


Journal Reference:

  1. T. Langen, S. Erne, R. Geiger, B. Rauer, T. Schweigler, M. Kuhnert, W. Rohringer, I. E. Mazets, T. Gasenzer, J. Schmiedmayer. Experimental observation of a generalized Gibbs ensemble. Science, 2015; 348 (6231): 207 DOI: 10.1126/science.1257026

Cite This Page:

Vienna University of Technology. "Quantum physics: Hot and cold at the same time." ScienceDaily. ScienceDaily, 9 April 2015. <www.sciencedaily.com/releases/2015/04/150409143037.htm>.
Vienna University of Technology. (2015, April 9). Quantum physics: Hot and cold at the same time. ScienceDaily. Retrieved May 26, 2017 from www.sciencedaily.com/releases/2015/04/150409143037.htm
Vienna University of Technology. "Quantum physics: Hot and cold at the same time." ScienceDaily. www.sciencedaily.com/releases/2015/04/150409143037.htm (accessed May 26, 2017).

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