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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Ultracold collisions in chromium: D-wave Feshbach resonance and rf-assisted molecule association

Quentin Beaufils, T. Zanon-Willette, A. Crubellier, B. Laburthe-Tolra, É. Maréchal, L. Vernac, O. Gorceix

Published Jun 1, 2009• 0 citations
Publisher

Abstract

A detailed investigation of an unusual Feshbach resonance in a magneto-optical trap involving an incoming d-wave collision channel is reported. Bose-Einstein condensates of chromium atoms using an all-optical procedure is created. The set-up induces a reduction of inelastic losses in collisions between atoms and can cool and trap fermionic atoms.

DOI
10.1109/cleoe-eqec.2009.5192360

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Full text: The study of quantum gases made of chromium atoms is compelling for several reasons. Being accessible to laser manipulation, chromium has a most abundant bosonic isotope {sup 52}Cr and a 9-percent abundant fermionic isotope {sup 53}Cr. Most importantly, Cr atoms carry an exceptionally large magnetic moment of 6 {mu}{sub B}. Consequently, Cr provides a valuable tool to study the physics of dipolar quantum gases as demonstrated elsewhere. We present our recent achievement of a chromium Bose-Einstein Condensation (Cr-BEC) using an all-optical procedure along with two innovative techniques: continuous accumulation of metastable {sup 52}Cr atoms in a mixed optical and magnetic trap; fast and intense rf sweeps to average to zero the magnetic potential and optimize the transfer efficiency from the Cr-MOT to the optical trap. We also report on the rf spectroscopy and association of weakly bound Cr{sub 2} molecules in the decatriplet {sup 13}{sigma}{sub g}{sup +} state. These latter experiments are performed in the vicinity of a d-wave Feshbach resonance at low magnetic field. Though the association rate is at present fairly low, we can study the spectroscopic properties of these cold trapped high-spin chromium molecules. (author)

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arXiv:0710.2479•Physical Review Letters•Apr 8, 2008•131 citations
A detailed investigation of an unusual Feshbach resonance in a magneto-optical trap involving an incoming d-wave collision channel is reported. Bose-Einstein condensates of chromium atoms using an all-optical procedure is created. The set-up induces a reduction of inelastic losses in collisions between atoms and can cool and trap fermionic atoms.
We report on experiments with ultracold chromium atoms. Preparing a cloud of 52Cr atoms in a crossed optical dipole trap (CODT) and applying magnetic fields between 4 and 600 G, we observe 14 Feshbach resonances by the occurrence of increased atom loss at distinct magnetic field values. A comparison with theory taking only dipole–dipole coupling into account shows very good agreement between experimental and theoretical resonance positions and allows us to extract the s-wave scattering lengths a 6=112(14) a 0, a 4= 58(6) a 0, a 2=−7(20) a 0 of the involved molecular potentials as well as the dispersion coefficients C 6= 733(70) au and au. The strongest resonance at 589 G has a calculated width of 1.7 G and reveals a three-body loss coefficient below L 3, max∼3×10−36 m6 s−1. Further evaporative cooling within the CODT leads to the formation of a Bose–Einstein condensate (BEC) with up to 100 000 condensed atoms. The magnetic dipole–dipole interaction between the Cr atoms is so strong that, as a first mechanical manifestation of dipole–dipole interaction, we observe a modification of the condensate expansion which depends on the alignment of the atomic magnetic dipoles with respect to the axis of the CODT. This magnetostrictive effect is in very good agreement with the theory of dipolar quantum gases and shows that a Cr-BEC is an excellent model system to study dipole–dipole interactions in degenerate quantum gases.
Full text: The study of quantum gases made of chromium atoms is compelling for several reasons. Being accessible to laser manipulation, chromium has a most abundant bosonic isotope {sup 52}Cr and a 9-percent abundant fermionic isotope {sup 53}Cr. Most importantly, Cr atoms carry an exceptionally large magnetic moment of 6 {mu}{sub B}. Consequently, Cr provides a valuable tool to study the physics of dipolar quantum gases as demonstrated elsewhere. We present our recent achievement of a chromium Bose-Einstein Condensation (Cr-BEC) using an all-optical procedure along with two innovative techniques: continuous accumulation of metastable {sup 52}Cr atoms in a mixed optical and magnetic trap; fast and intense rf sweeps to average to zero the magnetic potential and optimize the transfer efficiency from the Cr-MOT to the optical trap. We also report on the rf spectroscopy and association of weakly bound Cr{sub 2} molecules in the decatriplet {sup 13}{sigma}{sub g}{sup +} state. These latter experiments are performed in the vicinity of a d-wave Feshbach resonance at low magnetic field. Though the association rate is at present fairly low, we can study the spectroscopic properties of these cold trapped high-spin chromium molecules. (author)
We analyze a narrow Feshbach resonance with ultracold chromium atoms colliding in ddd wave. The resonance is made possible by dipole-dipole interactions, which couple an incoming l=2l=2l=2 collision channel with a bound molecular state with l=0l=0l=0. We find that three-body losses associated with this resonance increase with temperature, and that the loss feature width as a function of magnetic field also increases linearly with temperature. The analysis of our experimental data shows that the Feshbach coupling is small compared both to the temperature and to the density limited lifetime of the resonant bound molecular state. One consequence is that the three-body loss rate is proportional to the square of the number of atoms, and that we can directly relate the amplitude of the losses to the Feshbach coupling parameter. We compare our measurement to a calculation of the coupling between the collisional channel and the molecular bound state by dipole-dipole interactions, and find a good agreement, with no adjustable parameter. An analysis of the loss line shape is also performed, which enables to precisely measure the position of the resonance.
This talk reviews some of the basic concepts in cold collisions, and gives examples of calculations directed towards understanding the following areas: (1) collisions of Na atoms trapped in Zeeman components of the F=1 hyperfine sublevel, and their control by magnetically-induced Feshbach resonances; (2) elastic and inelastic collisions of Cs atoms in the double spin-polarized F=4, M=4 level; and (3) stimulated Raman photoassociation of two colliding atoms in a Bose-Einstein condensate (BEC) to make translationally cold molecules in a selected target level.
We report the first creation of a bulk sample of ultracold heteronuclear p-wave Feshbach molecules in an optically trapped Bose-Bose mixture of 23Na and 87Rb atoms. Using loss spectroscopy and binding energy measurements, we systematically characterize the interspecies p-wave Feshbach resonances near 284 G. Leveraging this understanding, we use magneto-association to form p-wave NaRb Feshbach molecules, producing both pure samples and mixtures of molecules in different p-wave orbitals. We further measure the molecular lifetime and identify atom-molecule and molecule-molecule collisions as the dominant loss mechanisms. This work establishes a previously unavailable ultracold molecule platform that combines orbital anisotropy with heteronuclear constituents, represents a significant step toward realizing tunable p-wave interactions in Bose-Bose mixtures, and provides a foundation for exploring non-zero angular momentum molecules.
Using a Feshbach resonance, we create ultracold fermionic molecules starting from a Bose-Fermi atom gas mixture. The resulting mixture of atoms and weakly bound molecules provides a rich system for studying few-body collisions because of the variety of atomic collision partners for molecules; either bosonic, fermionic, or distinguishable atoms. Inelastic loss of the molecules near the Feshbach resonance is dramatically affected by the quantum statistics of the colliding particles and the scattering length. In particular, we observe a molecule lifetime as long as 100 ms near the Feshbach resonance.