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arXiv:1607.04650 · Physical Review A

Enhanced association and dissociation of heteronuclear Feshbach molecules in a microgravity environment

José Paulo D'Incao, Markus Krutzik, Ethan Elliott, Jason Williams

Published Jan 3, 2017• 17 citations
arXivHTMLPDF

Abstract

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

DOI
10.1103/physreva.95.012701

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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 41K\ensuremath-87Rb^{41}\mathrm{K}\text{\ensuremath{-}}^{87}\mathrm{Rb}41K\ensuremath-87Rb that allow optimal association or dissociation efficiency with minimal heating and loss to guide upcoming experiments with ultracold atomic gases in space.
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 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.
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.
We present a detailed analysis of the production efficiency of weakly bound heteronuclear 40K87Rb^{40}\mathrm{K}^{87}\mathrm{Rb}40K87Rb Feshbach molecules using radio-frequency association in a harmonic trap. The efficiency was measured in a wide range of temperatures, binding energies, and radio frequencies. A comprehensive analytical model is presented, explaining the observed asymmetric spectra and achieving good quantitative agreement with the measured production rates. This model provides a deep understanding of the molecule association process and paves the way for future experiments which rely on Feshbach molecules, e.g., for the production of deeply bound molecules.
This thesis has investigated the dynamics of ultracold atomic collisions in the presence of both static magnetic and oscillating radio-frequency (rf) fields. The bound state structure and scattering length of 39K+133Cs was examined in the presence of only a static magnetic field, where it was found that no Feshbach resonance of suitable width for magnetoassociation existed at a magnetic field where caesium can be cooled to degeneracy. We then showed that zero-energy Feshbach resonances may be engineered using an rf field in places where they did not previously exist. An rf field with frequency 79.9 MHz was chosen to induce a resonance near 21 G. The widths of such rf-induced Feshbach resonances increase quadratically with rf field strength. The resonances presented are lossless with circularly polarized rf, and the molecules created are long-lived even with plane-polarized RF. Collisional losses in rf-dressed magnetic traps were also investigated. An rf-induced loss mechanism that does not exist in the absence of rf radiation was identified. This mechanism is not suppressed by a centrifugal barrier in the outgoing channel, and can be much faster than spin relaxation, which is centrifugally suppressed. We explore the dependence of the rf-induced loss rate on singlet and triplet scattering lengths, hyperfine splittings and the strength of the rf field. The results were interpreted in terms of an adiabatic model of the collision dynamics, and calculate the corresponding nonadiabatic couplings. The loss rate can vary by 10 orders of magnitude as a function of singlet and triplet scattering lengths. 87Rb is a special case, where several factors combine to reduce rf-induced losses; as a result, they are slow compared to spin-relaxation losses. For most other alkali-metal pairs, rf-induced losses are expected to be much faster and may dominate. For heteronuclear mixtures an rf-modified spin-exchange mechanism was identified that results in loss rates orders of magnitude greater than the rf-induced loss rates in homonuclear cases. Fast loss is expected in mixtures where the Lande g-factors differ.
We report on the creation and characterization of heteronuclear 40K87Rb^{40}\text{K}^{87}\text{R}\text{b}40K87Rb Feshbach molecules in an optical dipole trap. Starting from an ultracold gas mixture of 40K^{40}\text{K}40K and 87Rb^{87}\text{R}\text{b}87Rb 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.