Negative Absolute Temperature for Motional Degrees of Freedom
Abstract
Absolute temperature is usually bound to be positive. Under special conditions, however, negative temperatures—in which highenergy states are more occupied than lowenergy 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 BoseHubbard 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 manybody states.
 Publication:

Science
 Pub Date:
 January 2013
 DOI:
 10.1126/science.1227831
 arXiv:
 arXiv:1211.0545
 Bibcode:
 2013Sci...339...52B
 Keywords:

 PHYSICS;
 Condensed Matter  Quantum Gases;
 Condensed Matter  Statistical Mechanics
 EPrint:
 5 pages, 4 figures + supplementary material