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arXiv:0908.1794 · Physical Review A

Improving the efficiency of ultracold dipolar molecule formation by first loading onto an optical lattice

James K. Freericks, Maciej M. Maśka, Anzi Hu, Thomas M. Hanna, Carl J. Williams, Paul S. Julienne, R. Lemański

Published Jan 28, 2010• 25 citations
arXivHTMLPDF

Abstract

Ultracold ground-state dipolar $^{40}\mathrm{K}$$^{87}\mathrm{Rb}$ molecules recently have been produced in a loose harmonic trap by employing a magnetic field sweep across a Feshbach resonance followed by stimulated Raman adiabatic passage [K.-K. Ni et al., Science 322, 231 (2008)]. The overall experimental efficiency for molecule formation was around $20%$. We show that the efficiency can be increased to nearly $100%$ if one first loads the atomic gases into an optical lattice of the appropriate depth and tunes the interspecies attraction to have exactly one atom of each species at an occupied lattice site. Our proposed scheme provides a large enhancement to the dipolar molecule density, even at relatively high temperatures, and avoids three-body recombination loss by preventing lattice sites from being triply occupied.

DOI
10.1103/physreva.81.011605

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Ultracold ground-state dipolar ^{40}\mathrm{K}$$^{87}\mathrm{Rb} molecules recently have been produced in a loose harmonic trap by employing a magnetic field sweep across a Feshbach resonance followed by stimulated Raman adiabatic passage [K.-K. Ni et al., Science 322, 231 (2008)]. The overall experimental efficiency for molecule formation was around 2020%20. We show that the efficiency can be increased to nearly 100100%100 if one first loads the atomic gases into an optical lattice of the appropriate depth and tunes the interspecies attraction to have exactly one atom of each species at an occupied lattice site. Our proposed scheme provides a large enhancement to the dipolar molecule density, even at relatively high temperatures, and avoids three-body recombination loss by preventing lattice sites from being triply occupied.
Ultracold paramagnetic and polar diatomic molecules are among the promising systems for quantum simulation of lattice-spin models. Unfortunately, their experimental observation is still challenging. Based on our recent ab initio calculations, we analyze the feasibility of all-optical schemes for the formation of ultracold 87Rb84Sr^{87}\mathrm{Rb}^{84}\mathrm{Sr}87Rb84Sr bosonic molecules. A first possibility is photoassociation followed by spontaneous emission. The photoassociation rate coefficients toward electronic states converging to the 87Rb(5s2S1/2)+84Sr(5s5p3P0,1,2^{87}\mathrm{Rb}(5s\phantom{\rule{0.16em}{0ex}}^{2}S_{1/2})+^{84}\mathrm{Sr}(5s5p\phantom{\rule{0.16em}{0ex}}^{3}P_{0,1,2}87Rb(5s2S1/2​)+84Sr(5s5p3P0,1,2​) asymptotes are particularly small for vibrational levels close to the asymptote. The creation of molecules would be more interesting by using deeply bound levels which preferentially relax to the ${v}^{\ensuremath{'}\ensuremath{'}}=0$ level of the ground state. On the other hand, the photoassociation rate coefficients toward electronic states correlated to the Rb(5p2P1/2,3/2)+Sr(5s21S0\mathrm{Rb}(5p\phantom{\rule{0.16em}{0ex}}^{2}P_{1/2,3/2})+\mathrm{Sr}(5{s}^{2}\phantom{\rule{0.16em}{0ex}}^{1}S_{0}Rb(5p2P1/2,3/2​)+Sr(5s21S0​) are significant for levels close to the asymptote. The spontaneous emission thus creates weakly bound molecules in a single vibrational level. A second option relies on stimulated Raman adiabatic passage implemented in a tight optical trap. It efficiently creates weakly bound ground-state molecules in a well-defined level, thus providing a promising alternative to magnetic Feshbach resonances for further population transfer toward the absolute ground state of the RbSr molecule.
We report on the creation of ultracold (84)Sr(2) molecules in the electronic ground state. The molecules are formed from atom pairs on sites of an optical lattice using stimulated Raman adiabatic passage (STIRAP). We achieve a transfer efficiency of 30% and obtain 4×10(4) molecules with full control over the external and internal quantum state. STIRAP is performed near the narrow (1)S(0)-(3)P(1) intercombination transition, using a vibrational level of the 1(0(u)(+)) potential as an intermediate state. In preparation of our molecule association scheme, we have determined the binding energies of the last vibrational levels of the 1(0(u)(+)), 1(1(u)) excited-state and the X (1)Σ(g)(+) ground-state potentials. Our work overcomes the previous limitation of STIRAP schemes to systems with magnetic Feshbach resonances, thereby establishing a route that is applicable to many systems beyond alkali-metal dimers.
We report efficient all-optical creation of an ultracold gas of alkaline-earth-metal dimers, 88 Sr 2 , in their absolute ground state. Starting with weakly bound singlet molecules formed by narrow-line photoassociation in an optical lattice, followed by stimulated Raman adiabatic passage (STIRAP) via a singlet-dominant channel in the excited potential, we prepare pure samples of more than 5500 molecules in . We observe two-body collisional loss rates close to the universal limit for both the least bound and most bound vibrational states in . We demonstrate the enhancement of STIRAP efficiency in a magic-wavelength optical lattice where thermal decoherence is eliminated. Our results pave the way for the use of alkaline-earth-metal dimers for high-precision spectroscopy, and indicate favorable prospects for robust quantum state preparation of ultracold molecules involving closed-shell atoms, as well as molecule assembly in deep optical traps tuned to a magic wavelength.
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 produce an ultracold and dense sample of rovibronic ground state Cs 2 molecules close to the regime of quantum degeneracy, in a single hyperfine level, in the presence of an optical lattice. The molecules are individually trapped, in the motional ground state of an optical lattice well, with a lifetime of 8 s. For preparation, we start with a zero-temperature atomic Mott-insulator state with optimized double-site occupancy and efficiently associate weakly-bound dimer molecules on a Feshbach resonance. Despite extremely weak Franck-Condon wavefunction overlap, the molecules are subsequently transferred with >50% efficiency to the rovibronic ground state by a stimulated four-photon process. Our results present a crucial step towards the generation of Bose-Einstein condensates of ground-state molecules and, when suitably generalized to polar heteronuclear molecules such as RbCs, the realization of dipolar many-body quantum-gas phases in periodic potentials.
There is great interest in the use of ultracold polar molecules confined in op- tical lattices for quantum simulation of complex many-body problems. The objective of our work is to realise a system of ultracold fermionic 40K133Cs molecules in a 1D optical lattice consisting of an array of 2D pancake traps. Such a system will enable a wide range of future investigations, including the study of novel regimes of interlayer superfluidity [1]. To create polar molecules we will employ magnetoassociation on an interspecies Feshbach resonance followed by transfer to the rovibrational ground state using STIm- ulated Raman Adiabatic Passage (STIRAP). This thesis reports the progress towards this ambitious goal focusing on the initial laser cooling stage. We present a simple single cell vacuum system that can be used to locate the interspecies Feshbach resonances essential for magnetoassociation. We give a description of the laser setups for both species, including details of the laser spectroscopy used for frequency stabilisation. We then present results on the optimisation of the magneto optical trap (MOT) for each species (focusing on the most abundant 39K isotope). The effects of the intensities and detun- ings of both the cooling and repump light and the magnetic field gradient are investigated in order to maximise the number of trapped atoms. Finally, we present preliminary observations of simultaneously loading overlapped MOTs for both species.