Absolute temperature, that is the fundamental temperature scale in thermodynamics, is usually bound to be positive. Under special con- ditions, however, negative temperatures - where high-energy states are more occupied than low-energy states - are also possible. In this talk, I will present a negative temperature state for motional degrees of freedom: By tailoring the Bose-Hubbard Hamiltonian we exper- imentally created an attractively interacting ensemble of ultracold bosons, which is stable against collapse for arbitrary atom numbers. In this negative temperature state, the quasi-momentum distribu- tion develops sharp peaks at the upper band edge, revealing thermal equilibrium and bosonic coherence over several lattice sites. Nega- tive temperatures imply negative pressures and open up new param- eter regimes for cold atoms, enabling fundamentally new many-body states and counterintuitive effects such as Carnot engines above unity efficiency. In addition, this system enabled us to study the dynam- ics of the phase transition from Mott insulator to superfluid and to experimentally investigate how fast phase coherence can spread.
Podden och tillhörande omslagsbild på den här sidan tillhör Michael Haack. Innehållet i podden är skapat av Michael Haack och inte av, eller tillsammans med, Poddtoppen.
