Breaking Absolute Zero

Figure from Braun, S. et al. Science 339, 52–55 (2013).

Since Absolute Zero Kelvin is defined by the average energy of the observed particles. Typically more particles are in the lower energy states, and less in a higher energy states. If these states can be evened out than "Absolute Zero" is reached, if we can put more particles in the higher energy state than the lower energy state we reach negative Kelvin. Scientists at Ludwig-Maximilians-Universität München and the Max-Planck-Institut für Quantenoptik have successfully observed such a state in a atomic Potassium-39 gas. You can check out this link "Ultra-cold atoms give way to negative Kelvin" at nature.com to read more and if you have access you can read their paper in the journal Science here.