Creation of ultracold molecules from a Fermi gas of atoms

TL;DR

This paper reports the creation and detailed characterization of ultracold fermionic molecules from a quantum degenerate Fermi gas, demonstrating control over molecular binding energies and reversible atom-molecule conversion.

Contribution

It presents the first quantitative creation and characterization of ultracold $^{40}$K$_2$ molecules from a Fermi gas using Feshbach resonance techniques.

Findings

Over a quarter million molecules created from a Fermi gas

Molecular binding energy controlled by Feshbach detuning

Reversible conversion between atoms and molecules demonstrated

Abstract

Since the realization of Bose-Einstein condensates (BEC) in atomic gases an experimental challenge has been the production of molecular gases in the quantum regime. A promising approach is to create the molecular gas directly from an ultracold atomic gas; for example, atoms in a BEC have been coupled to electronic ground-state molecules through photoassociation as well as through a magnetic-field Feshbach resonance. The availability of atomic Fermi gases provides the exciting prospect of coupling fermionic atoms to bosonic molecules, and thus altering the quantum statistics of the system. This Fermi-Bose coupling is closely related to the pairing mechanism for a novel fermionic superfluid proposed to occur near a Feshbach resonance. Here we report the creation and quantitative characterization of exotic, ultracold $^{40}$K$_2$ molecules. Starting with a quantum degenerate Fermi gas of atoms at T < 150 nanoKelvin we scan over a Feshbach resonance to adiabatically create over a quarter million trapped molecules, which we can convert back to atoms by reversing the scan. The small binding energy of the molecules is controlled by detuning from the Feshbach resonance and can be varied over a wide range. We directly detect these weakly bound molecules through rf photodissociation spectra that probe the molecular wavefunction and yield binding energies that are consistent with theory.