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arXiv:2105.03763 · Physical Review A

Quenched magneto-association of ultracold Feshbach molecules

Kirk Waiblinger, Jason Williams, José Paulo D'Incao

Published Sep 8, 2021• 3 citations
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Abstract

We study enhanced magneto-association of atoms into weakly bound molecules near a Feshbach resonance using a quench preparatory stage. In anticipation of experiments with NASA's Cold Atom Laboratory aboard the International Space Station, we assume as a baseline a dual-species $(^{87}\mathrm{Rb} \mathrm{and} ^{41}\mathrm{K})$ 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.

DOI
10.1103/physreva.104.033310

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We study enhanced magneto-association of atoms into weakly bound molecules near a Feshbach resonance using a quench preparatory stage. In anticipation of experiments with NASA's Cold Atom Laboratory aboard the International Space Station, we assume as a baseline a dual-species (87Rband41K)(^{87}\mathrm{Rb} \mathrm{and} ^{41}\mathrm{K})(87Rband41K) gas in a parameter regime enabled by a microgravity environment. It includes subnanokelvin temperatures and dual-species gases at densities as low as 108/cm3{10}^{8}/{\mathrm{cm}}^{3}108/cm3. 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 8080%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.
We study enhanced magneto-association of atoms into weakly-bound molecules near a Feshbach resonance using a quench preparatory stage. In anticipation of experiments with NASA's Cold Atom Laboratory aboard the International Space Station, we assume as a baseline a dual-species (87^{87}87Rb and 41^{41}41K) gas in a parameter regime enabled by a microgravity environment. This includes subnanokelvin temperatures and dual-species gases at densities as low as 108^88/cm3^33. 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%80\%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.
We study the association and dissociation dynamics of weakly bound heteronuclear Feshbach molecules using transverse radio-frequency fields for expected parameters accessible through the microgravity environment of NASA's Cold Atom Laboratory (CAL) aboard the International Space Station, including subnanokelvin temperatures and atomic densities as low as 108/cm3{10}^{8}/{\mathrm{cm}}^{3}108/cm3. 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 104a0{10}^{4}{a}_{0}104a0​ 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.
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.
Enhanced magnetoassociation of 6^66Li in the quantum degenerate regime
We study magnetic Feshbach resonance of ultracold 6^66Li atoms in a dipole trap close to quantum degeneracy. The experiment is carried out by linearly ramping down the magnetic field from the BCS to the BEC side around the broad resonance at Br=834.1B_r=834.1Br​=834.1G. The Feshbach molecule formation efficiency depends strongly on the temperature of the atomic gas and the rate at which the magnetic field is ramped across the Feshbach resonance. The molecular association process is well described by the Landau-Zener transition while above the Fermi temperature, such that two-body physics dominates the dynamics. However, we observe an enhancement of the atom-molecule coupling as the Fermionic atoms reach degeneracy, demonstrating the importance of many-body coherence not captured by the conventional Landau-Zener model. We develop a theoretical model that explains the temperature dependence of the atom-molecule coupling. Furthermore, we characterize this dependence experimentally and extract the atom-molecule coupling coefficient as a function of temperature, finding qualitative agreement between our model and experimental results. Accurate measurement of this coupling coefficient is important for both theoretical and experimental studies of atom-molecule association dynamics.
We investigate magnetoassociation of ultracold fermionic Feshbach molecules in a mixture of 40{}^{40}40K and 87{}^{87}87Rb atoms, where we can create 40{}^{40}40K87{}^{87}87Rb molecules with a conversion efficiency as high as 45%. In the limit of fast magnetic sweeps and small molecule fraction, we find that the time scale of molecule production is accurately predicted by a parameter-free model that uses only the atomic densities, masses, and the known Feshbach resonance parameters. We find that molecule production efficiency saturates for slower magnetic sweeps, at which point the maximum number of Feshbach molecules depends primarily on the atoms' phase-space density. At higher temperatures, our measurements agree with a phenomenological model that successfully describes the formation of bosonic molecules from either Bose or Fermi gases. However, for quantum degenerate atom gas mixtures, we measure significantly fewer molecules than this model predicts.
We report the creation of ultracold bosonic 23 Na 87 Rb Feshbach molecules via magneto-association. By ramping the magnetic field across an interspecies Feshbach resonance (FR), at least 4000 molecules can be produced out of the near degenerate ultracold mixture. Fast loss due to inelastic atom–molecule collisions is observed, which limits the pure molecule number, after residual atoms removal, to 1700. The pure molecule sample can live for 21.8(8) ms in the optical trap, long enough for future molecular spectroscopy studies toward coherently transferring to the singlet ro-vibrational ground state, where these molecules are stable against chemical reaction and have a permanent electric dipole moment of 3.3 Debye. We have also measured the Feshbach molecule's binding energy near the FR by the oscillating magnetic field method and found these molecules have a large closed-channel fraction.