Negative Absolute Temperature for Motional Degrees of Freedom

@article{Braun2013NegativeAT,
  title={Negative Absolute Temperature for Motional Degrees of Freedom},
  author={Simon Braun and Jens Philipp Ronzheimer and Michael Schreiber and Sean S. Hodgman and Tim Rom and Immanuel Bloch and Ulrich Schneider},
  journal={Science},
  year={2013},
  volume={339},
  pages={52 - 55}
}
Negative Is Hotter A common-sense perception of temperature tells us that the lower the temperature, the colder it is. However, below absolute zero, there is a netherworld of negative temperatures, which are, counterintuitively, hotter than positive temperatures. Usually, such states are achieved in the laboratory and are characterized by a higher occupation of high-energy versus low-energy states. This is most easily done for systems that have a finite spectrum of energy states, bounded from… 
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