Negative Absolute Temperature for Motional Degrees of Freedom

Abstract

Absolute temperature, the fundamental temperature scale in thermodynamics, is usually bound to be positive. Under special conditions, however, negative temperatures - where high-energy states are more occupied than low-energy states - are also possible. So far, such states have been demonstrated in localized systems with finite, discrete spectra. Here, we were able to prepare 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 quasi-momentum 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 and counterintuitive effects such as Carnot engines above unity efficiency.

5 pages, 4 figures + supplementary material

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• Topology of a dissipative spin: dynamical Chern number, bath induced non-adiabaticity and a quantum dynamo effect
• Comment on "Consistent thermostatistics forbids negative absolute temperatures"
• Mean-field dynamics to negative absolute temperatures in the Bose-Hubbard model
• Critique of the Gibbs volume entropy and its implication
• Reply to Schneider et al. [arXiv:1407.4127v1]
• Gravity as a thermodynamic phenomenon
• Thermodynamic laws in isolated systems
• On the definition of the measurement unit for extreme quantity values: some considerations on the case of temperature and the kelvin scale
• Study of Bose-Einstein condensation using generalized canonical partition function