Negative Absolute Temperature for Motional Degrees of Freedom

TL;DR

This paper demonstrates the creation of a stable negative temperature state for motional degrees of freedom in ultracold bosons, revealing new thermodynamic regimes and many-body phenomena.

Contribution

It introduces a method to prepare and stabilize negative temperature states for motional degrees of freedom in ultracold atoms, expanding the scope of negative temperature physics.

Findings

Sharp peaks at the upper band edge in momentum distribution

Thermal equilibrium and bosonic coherence observed

Negative pressures and new many-body states enabled

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.