Negative Absolute Temperature for Motional Degrees of Freedom

<jats:title>Negative Is Hotter</jats:title> <jats:p> 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 above and below. <jats:bold> Braun <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="52" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1227831">52</jats:related-article> ; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6115" page="42" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1232558">Carr</jats:related-article> </jats:bold> ) achieved negative temperature for a system in which its spectrum was only bounded on one side. Starting with a gas of <jats:sup>39</jats:sup> K bosonic atoms with repulsive interactions in a dipole trap and an optical lattice, a final state with negative temperature was reached where the atoms attract each other. </jats:p>