Production Efficiency of Ultracold Feshbach Molecules in Bosonic and Fermionic Systems

TL;DR

This study examines how efficiently ultracold molecules are produced from bosonic and fermionic atomic gases when sweeping across a Feshbach resonance, revealing phase space density as a key factor and introducing a new predictive model.

Contribution

The paper presents a novel, physically grounded model that accurately predicts molecule conversion efficiency in both bosonic and fermionic ultracold gases during adiabatic magnetic field sweeps.

Findings

Conversion efficiency depends on phase space density during adiabatic sweeps.

The new model accurately predicts efficiency for both bosonic and fermionic gases.

In non-adiabatic regimes, efficiency follows a Landau Zener model based on density and magnetic field sweep rate.

Abstract

We investigate the production efficiency of ultracold molecules in bosonic $^{85}$Rb and fermionic $^{40}$K when the magnetic field is swept across a Feshbach resonance. For adiabatic sweeps of the magnetic field, the conversion efficiency of each species is solely determined by the phase space density of the atomic cloud, in contrast to a number of theoretical predictions. Our novel model for the adiabatic pairing process, developed from general physical principles, accurately predicts the conversion efficiency for {\it both} ultracold gases of bosons and of fermions. In the non-adiabatic regime our measurements of the $^{85}$Rb molecule conversion efficiency follow a Landau Zener model, with a conversion efficiency that is characterized by the density divided by the time derivative of the magnetic field.