Optimizing Association of Heteronuclear Feshbach Molecules in Microgravity with a Magnetic Field Quench

Kirk Waiblinger

Published Jan 1, 2019• 0 citations

Abstract

This work presents a computational comparison of two magnetic field manipulation schemes for creating heteronuclear 87Rb41K Feshbach molecules. We focus on analysis of the parameters relevant to the NASA Cold Atom Laboratory experiments performing atom interferometry, which will require maximizing production of molecules at low densities, and extremely low temperatures (<1 nK). It is found that a scheme involving a magnetic field quench potentially offers substantial benefits over a direct linear magnetic field sweep across a Feshbach resonance.

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OpenAlex

Abstract

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^{87}

Rb and

^{41}

K) gas in a parameter regime enabled by a microgravity environment. This includes subnanokelvin temperatures and dual-species gases at densities as low as 10

^8

/cm

^3

. Our studies indicate that, in such a regime, traditional magneto-association schemes are inefficient due to the weak coupling between atomic and molecular states at low-densities, thus requiring extremely long magnetic field sweeps. To address this issue we propose a modified scheme where atoms are quenched to unitarity before proceeding with magneto-association. This substantially improves molecular formation, allowing for up to

80\%

efficiency, and within time-scales much shorter than those associated to atomic and molecular losses. We show that this scheme also applies at higher densities, therefore proving to be of interest to ground-based experiments.

(^{87}\mathrm{Rb} \mathrm{and} ^{41}\mathrm{K})

gas in a parameter regime enabled by a microgravity environment. It includes subnanokelvin temperatures and dual-species gases at densities as low as

{10}^{8}/{\mathrm{cm}}^{3}

. Our studies indicate that, in such a regime, traditional magneto-association schemes are inefficient due to the weak coupling between atomic and molecular states at low densities, thus requiring extremely long magnetic field sweeps. To address this issue we propose a modified scheme in which atoms are quenched to unitarity before proceeding with magneto-association. This modified scheme substantially improves molecular formation, allowing for up to

80%

efficiency within timescales much shorter than those associated with atomic and molecular losses. We show that this scheme also applies at higher densities, therefore proving to be of interest in ground-based experiments as well.

{10}^{8}/{\mathrm{cm}}^{3}

. We show that under such conditions, thermal and loss effects can be greatly suppressed, resulting in a high efficiency of both association and dissociation of Feshbach molecules with a mean size exceeding

{10}^{4}{a}_{0}

and allowing for the coherence in atom-molecule transitions to be clearly observable. Our theoretical model for heteronuclear mixtures includes thermal, loss, and density effects in a simple and conceptually clear manner. We derive the temperature, density, and scattering length regimes of $^{41}\mathrm{K}\text{\ensuremath{-}}^{87}\mathrm{Rb}$ that allow optimal association or dissociation efficiency with minimal heating and loss to guide upcoming experiments with ultracold atomic gases in space.

We report on the creation and characterization of heteronuclear

^{40}\text{K}^{87}\text{R}\text{b}

Feshbach molecules in an optical dipole trap. Starting from an ultracold gas mixture of

^{40}\text{K}

and

^{87}\text{R}\text{b}

atoms, we create as many as 25 000 molecules at 300 nK by rf association. Optimizing the association process, we achieve a conversion efficiency of 25%. We measure the temperature dependence of the rf association process and find good agreement with a phenomenological model that has previously been applied to Feshbach molecule creation by slow magnetic-field sweeps. We also present a measurement of the binding energy of the heteronuclear molecules in the vicinity of the Feshbach resonance and provide evidence for Feshbach molecules as deeply bound as 26 MHz.

We report on the observation of ultracold heteronuclear Feshbach molecules. Starting with a

^{87}\mathrm{Rb}

Bose-Einstein condensate and a cold atomic gas of

^{85}\mathrm{Rb}

, we utilize previously unobserved interspecies Feshbach resonances to create up to 25 000 molecules. Even though the

^{85}\mathrm{Rb}

gas is nondegenerate, we observe a large molecular conversion efficiency due to the presence of a quantum degenerate

^{87}\mathrm{Rb}

gas; this represents a key feature of our system. We compare the molecule creation at two different Feshbach resonances with different magnetic-field widths. The two Feshbach resonances are located at $265.44\ifmmode\pm\else\textpm\fi{}0.15\text{ }\mathrm{G}$ and $372.4\ifmmode\pm\else\textpm\fi{}1.3\text{ }\text{ }\mathrm{G}$. We also directly measure the small binding energy of the molecules through resonant magnetic-field association.