I was under the impression that temperature was a measurement of the movement of particles and, as such, a temperature below absolute zero was impossible by definition. The article didn't really clarify what a temperature below absolute zero entailed (or, in fact, how it was measured). Can anyone shed some light on either of these for me?
Temperature has a lot of different definitions. It can be the average kinetic energy of the particles of a system, or the change in heat with respect to the change in entropy (This can be derived through both thermodynamic and statistical analysis). In the paper, they arrive at their conclusion of "negative temperature" by manipulating the distribution of energy, such that the particles are more likely to occur at higher energies. Normally, this distribution would only be transient, and the crystal lattice of particles they had set up would collapse, eventually giving the distribution on the left side of that image. However, through some strange quantum magic that I do not understand, they managed to keep that energy distribution stable. In the last paragraph of the publication, they give a short comment that the relevance of these temperature definitions is still being investigated: If you want to read the actual publication and need past the paywall, PM me.In ultracold quantum gases, the basic concepts of thermodynamics, positive or negative temperature, or whether a temperature concept is even relevant, are under intense and profound exploration.
A temperature below absolute zero results in entropy being reduced with the addition of energy to the system. I only understand the basics when it comes to "negative temperatures", unfortunately. It was most likely measured using a mathematical definition for temperature, such as T= (1/T) = dS/dE. Where dS is the rate of change of entropy and dE is the rate of change of energy.
There's a really great documentary on absolute zero here. It's from 2009 so it doesn't include any of this "negative absolute temperature" stuff, but it's still relevant.
Whenever I read about these kinds of discoveries I wonder about the circumstances under which the experiment was dreamed up. "Hey, I had this idea about temperatures... What if we got a shitton of lasers and magnetic fields and used them to constrain the movement of already extremely cold atoms while simultaneously increasing the amount of energy they have. That'd... that'd technically work, right?" "Dude, I don't know. Have another hit."
They mention in the paper that similar experiments have been done before, albeit without the temperature trick to keep the lattice from imploding:Then they tune the interactions to be attractive and at the same time turn their trap upside down to be an anti-trap. Finally, they melt the Mott insulator to obtain an attractive superfluid. These anti-traps have been used before, to create a self-propagating pulse of atoms that does not disperse (a bright solution) from attractive gases in one dimension (7, 8)