Making a molecular gas in the quantum regime

Yu Chen, Hui Zhai, Zhenhua Yu

We consider the superradiant phase transition of a two-component Fermi gas in a cavity across a Feshbach resonance. It is known that quantum statistics plays a crucial role for the superradiant phase transition in atomic gases; in contrast to bosons, in a Fermi gas this transition exhibits strong density dependence. We show that across a Feshbach resonance, while the two-component Fermi gas passes through the BEC-BCS crossover, the superradiant phase transition undergoes a corresponding crossover from a fermionic behavior on the weakly interacting BCS side, to a bosonic behavior on the molecular Bose-Einstein condensate (BEC) side. This intricate statistics crossover makes the superradiance maximally enhanced either in the unitary regime for low densities, in the BCS regime for moderate densities close to Fermi surface nesting, or in the BEC regime for high densities.

J. P. Casquilho, P. I. C. Teixeira

Luc Thévenaz A, Sanghoon Chin, Isabelle Dicaire +3 more

The absorption of light by a gas molecule has been measured comparatively using light propagating in normal conditions and in a slow light regime. The experiment is designed to make the 2 measurements possible without modifying the interaction conditions, so that the sole effect of slow light is unambiguously observed. A 26% group velocity reduction induced by stimulated Brillouin scattering in a gas-filled microstructured fiber caused no observable change in the measured absorption, so that it is proved that material slow light does not enhance Beer-Lambert absorption and has a null impact on gas sensing or spectroscopic applications.

Jelena Stajic

Cold Molecules A dilute atomic gas cooled down to very cold temperatures can enter the so-called quantum degenerate regime, where quantum properties of the gas come to the fore. This regime has been achieved for both bosonic and fermionic atoms, but molecules, with their many internal states, present a special challenge. De Marco et al. cooled a bulk gas of fermionic potassium-rubidium molecules to quantum degeneracy (see the Perspective by Zelevinsky). The authors first cooled atomic potassium and rubidium gases separately, then bound them together into potassium-rubidium molecules, and finally brought the molecules down to their ground state. The density profile of the molecular gas revealed the system's quantum nature, which in turn kept the gas stable by suppressing chemical reactions. Science , this issue p. [853][1]; see also p. [820][2] [1]: /lookup/doi/10.1126/science.aau7230 [2]: /lookup/doi/10.1126/science.aav9149

Marco Merkli