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Physicists observe entropy production in two intermediate – scale quantum systems, indicating that the systems have undergone an irreversible process.
M. Brunelli / University of Cambridge
In the continuum from quantum to classical, there is a paradox. Conventional machines such as heaters can not run in reverse – and act effectively as refrigerators – without additional energy. Such a scenario would violate the second law of thermodynamics, according to which entropy must always increase. In contrast, the equations of quantum mechanics imply that quantum processes can run in the opposite direction. At present, researchers do not know how to reconcile the two frameworks. To this end, Matteo Brunelli of the University of Cambridge in the UK and his colleagues experimentally studied two intermediate-scale quantum systems: a Bose-Einstein condensate composed of 100,000 rubidium atoms and an optomechanical cavity weighing less than one millionth of a gram. They put each system in contact with two heat tanks. In both cases, they found that entropy increased as energy went in and out of the system. The result indicates that intermediate-scale quantum systems behave irreversibly in the manner of conventional machines.
As entropy production can not be measured directly, the team first developed a theoretical framework for calculating entropy from energy. Then, they used this framework to infer the entropy output resulting from the energy exchange between the BEC and two heat reservoirs and between the cavity and two reservoirs. Measuring the energies of both systems, they found that entropy increased in both. The team studied these two systems because they both look mathematically at coupled quantum harmonic oscillators in contact with two thermal reservoirs. The team said that in the future, it planned to monitor the entropy of the two systems with a finer temporal resolution in order to observe the systems approaching the steady state.
This research is published in Letters of physical examination.
-Sophia Chen
Sophia Chen is an independent science writer based in Tucson, Arizona.
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