I always understood that the concept of absolute temperature stems from the third law of thermodynamics and that the lowest theoretical possible temperature is zero degrees Kelvin.
However, recently I came across an article published in Science (4th January 2013) titled "Negative Absolute Temperature for Motional Degrees of Freedom".
Given at the end of this question is the abstract of the said article.
This article was also covered on Nature.com in a somewhat more pop-science style, however I'm still unable to understand what the article is trying to cover.
What does negative absolute temperature mean in the given context?
Article's abstract:
Absolute temperature is usually bound to be positive. Under special conditions, however, negative temperatures—in which high-energy states are more occupied than low-energy states—are also possible. Such states have been demonstrated in localized systems with finite, discrete spectra. Here, we prepared a negative temperature state for motional degrees of freedom. By tailoring the Bose-Hubbard Hamiltonian, we created an attractively interacting ensemble of ultracold bosons at negative temperature that is stable against collapse for arbitrary atom numbers. The quasimomentum distribution develops sharp peaks at the upper band edge, revealing thermal equilibrium and bosonic coherence over several lattice sites. Negative temperatures imply negative pressures and open up new parameter regimes for cold atoms, enabling fundamentally new many-body states.
