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arXiv:0808.4077 · Physical Review A

Association of ultracold double-species bosonic molecules

Claudia Weber, Giovanni Barontini, Jacopo Catani, Gregor Thalhammer, M. Inguscio, F. Minardi

Published Dec 3, 2008• 85 citations
arXivHTMLPDF

Abstract

We report on the creation of heterospecies bosonic molecules, associated from an ultracold Bose-Bose mixture of $^{41}\mathrm{K}$ and $^{87}\mathrm{Rb}$, by using a resonantly modulated magnetic field close to two Feshbach resonances. We measure the binding energy of the weakly bound molecular states versus the Feshbach field and compare our results to theoretical predictions. We observe the broadening and asymmetry of the association spectrum due to the thermal distribution of the atoms and a frequency shift occurring when the binding energy depends nonlinearly on the Feshbach field. A simple model is developed to quantitatively describe the association process. This work represents a required step toward Bose-Einstein condensates of dipolar molecules.

DOI
10.1103/physreva.78.061601

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We report on the creation of heterospecies bosonic molecules, associated from an ultracold Bose-Bose mixture of 41K^{41}\mathrm{K}41K and 87Rb^{87}\mathrm{Rb}87Rb, by using a resonantly modulated magnetic field close to two Feshbach resonances. We measure the binding energy of the weakly bound molecular states versus the Feshbach field and compare our results to theoretical predictions. We observe the broadening and asymmetry of the association spectrum due to the thermal distribution of the atoms and a frequency shift occurring when the binding energy depends nonlinearly on the Feshbach field. A simple model is developed to quantitatively describe the association process. This work represents a required step toward Bose-Einstein condensates of dipolar molecules.
We report on the observation of ultracold heteronuclear Feshbach molecules. Starting with a 87Rb^{87}\mathrm{Rb}87Rb Bose-Einstein condensate and a cold atomic gas of 85Rb^{85}\mathrm{Rb}85Rb, we utilize previously unobserved interspecies Feshbach resonances to create up to 25 000 molecules. Even though the 85Rb^{85}\mathrm{Rb}85Rb gas is nondegenerate, we observe a large molecular conversion efficiency due to the presence of a quantum degenerate 87Rb^{87}\mathrm{Rb}87Rb 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.
Ultracold Feshbach molecules are a crucial intermediate step for the creation of quantum degenerate gases of strongly dipolar molecules. After coherent transfer to the rovibrational ground state, these dimers can realize stable dipolar gases with strong, tunable long-range interactions. Here, we report the creation of bosonic Na 23 K 41 Feshbach molecules by radio-frequency (RF) association. An RF pulse applied on the molecular side of an interspecies Feshbach resonance at 73.6(1) G associates up to 1.1 ( 1 ) × 10 4 molecules from a thermal mixture of Na 23 and K 41 atoms. Measurements of the binding energy reveal a broad resonance width of 5.1(2) G, facilitating robust control over interspecies interactions. The molecule lifetime in the presence of background atoms exceeds 2 ms and increases to approximately 7 ms after removal of Na 23 . These results constitute a key step toward the production of ultracold Na 23 K 41 ground-state molecules and for pursuing shielded evaporative cooling in the dipolar regime.
The route towards a Bose–Einstein condensate (BEC) of dipolar molecules requires the ability to efficiently associate dimers of different chemical species and transfer them to the stable rovibrational ground state. Here, we report on recent spectroscopic measurements of two weakly bound molecular levels and newly observed narrow d-wave Feshbach resonances. The data are used to improve the collisional model for the Bose–Bose mixture 41 K 87 Rb, one of the most promising candidates to create a molecular dipolar BEC.
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.
Ultracold bialkali polar molecules play a leading part at the frontline of quantum physics. They recently attract a lot of attention in the field of ultracold quantum chemistry, quantum many-body physics and quantum simulations. The key for their success is the rich internal level structure with rotational and vibrational degrees of freedom and their large electric dipole moments. Still, only a handful of molecular species are available at ultracold temperatures until now, although it is highly desirable to produce new molecular species to further expand the range of applications. Besides direct laser cooling methods for molecules, the assembly of heteronuclear ground-state molecules from ultracold atomic mixtures is the most promising approach for the creation of polar molecules. It includes the formation of weakly bound Feshbach molecules from the diatomic mixture and the subsequent two-photon stimulated Raman adiabatic passage (STIRAP) transfer to the rovibrational ground state. This creation strategy has been successfully demonstrated for the first time in the pioneering experiments at JILA with ultracold 40K87Rb molecules. Since then, only a few more molecular species from different alkali atoms have been created, namely 6Li23Na, 23Na40K, 23Na87Rb and 87Rb133Cs. In this thesis, I report the successful creation of a new species of ultracold polar ground-state molecules: 23Na39K. Starting from an ultracold mixture of bosonic 23Na and 39K atoms, weakly bound molecules are created. For this purpose, a Feshbach resonance in a high angular momentum scattering channel is chosen, experimentally identified and characterized. Close to this resonance the weakly bound Feshbach molecules are formed using resonant radio frequency radiation. For the two-photon ground-state transfer, a unique, highly specialized two-color laser system is designed and realized. It is used for one- and two-photon spectroscopy to identify the relevant transitions for the ground-state transfer. Based on the obtained data, a local model of the singlet-triplet mixed excited state manifolds is developed, with which the hyperfine structure and the magnetic field dependence is predicted with high accuracy. According to these findings, a suitable pathway to a single hyperfine ground state is chosen considering selection rules and experimental conditions such as laser polarization and beam alignment. To precisely determine the two-photon resonance condition for STIRAP, electromagnetically induced transparency measurements are performed. The ground-state transfer is then performed using STIRAP. The experimental findings regarding the STIRAP are successfully supported theoretically by a model based on a five-level master equation. The pure molecular gas shows evidence for two-body dominated loss mechanisms, such as sticky four-body collisions. The molecule-atom mixture of 23Na39K+39K reveals an unexpectedly low loss rate coefficient although sticky three-body collisions are assumed to occur. This behavior demands further investigations regarding different hyperfine collisional chan-nels and magnetic field regions.
We create atom-molecule superpositions in a Bose-Fermi mixture of 87Rb^{87}\text{R}\text{b}87Rb and 40K^{40}\text{K}40K atoms. The superpositions are generated by ramping an applied magnetic field near an interspecies Fano-Feshbach resonance to coherently couple atom and molecule states. Rabi- and Ramsey-type experiments show oscillations in the molecule population that persist as long as $150\text{ }\ensuremath{\mu}\text{s}$ and have up to 50% contrast. The frequencies of these oscillations are magnetic-field dependent and consistent with the predicted molecule binding energy. This quantum superposition involves a molecule and a pair of free particles with different statistics (i.e., bosons and fermions) and furthers exploration of atom-molecule coherence in systems without a Bose-Einstein condensate.