Negative Absolute Temperature for Motional Degrees of Freedom

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 above and below. Braun et al. (p. 52 ; see the Perspective by Carr ) achieved negative temperature for a system in which its spectrum was only bounded on one side. Starting with a gas of 39 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.