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All-optical Bose-Einstein condensation of chromium atoms and rf spectroscopy of cold Cr{sub 2} molecules

Quentin Beaufils, Radu Chicireanu, T. Zanon-Willette, B. Laburthe-Tolra, É. Maréchal, L. Vernac, J. Keller, O. Gorceix

Published Jul 1, 2008• 0 citations
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Abstract

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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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)•Jun 1, 2009•0 citations
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)
In the perspective of studying dipole-dipole interactions in quantum degenerate Bose and Fermi gases, we have built an experimental setup for cooling and trapping Chromium (Cr) atoms. In this respect, one needs to use appropriate technological solution, such as a high-temperature oven (∼ 1500◦C) and a laser system providing high laser power at 425nm. Our experiments allow us to obtain magneto-optical traps for the two main isotopes of Cr (i.e. the bosonic 52Cr and the fermionic 53Cr). These traps are characterized by large light-assisted collisional loss rates, which have been studied both experimentally and theoretically. The existence of radiative decay channels towards two long-lived metastable states allows the magnetic trapping and accumulation of fairly large atom numbers of cold Cr atoms. We have studied the possibility of modifying the shape of these traps with the use of RF magnetic fields, which allowed the study of collisional properties of Cr atoms in the metastable states. Finally, we have demonstrated a new continuous loading technique of an optical dipole trap with more than one million metastable 52Cr atoms, at 100μK. The transfer into the ground state and the polarization in the lowest-energy Zeeman sublevel opens the perspective for reaching the Bose-Einstein condensation of Cr through evaporative cooling.
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.
We create Bose-Einstein Condensates of52Cr atoms in an optical trap. Intense radio-frequency fields are used to induce degeneracy between the Zeeman sub-states of the ground state atoms in the BEC despite an ambient static field of 100 mG. We demonstrate this effect by analyzing atom trajectories under the influence of dressed magnetic potentials. We investigate the problem of adiabaticity in the dressing process. In a further set of experiments, we transfer the atoms from the absolute ground state into the excited Zeeman sub-state and study the dipolar relaxation in ultra-cold gases of chromium atoms above and below the condensation threshold. We observe a minimum in the dipolar relaxation rate for a finite magnetic field value which enables a new determination of the scattering length.
In this work a gas of ultra-cold chromium atoms in a magnetic trap has been prepared and its elastic and inelastic scattering properties have been investigated with regard to Bose-Einstein condensation. Bose-Einstein condensation of dilute atomic gases is achieved using different cooling and trapping techniques. Deviating from the standard way we were able to devise a continuous loading mechanism for a magnetic trap as a result of the spectroscopic properties and the large magnetic dipole moment of chromium. It not only allows us to trap more atoms but also facilitates the subsequent preparation steps. After loading, the magnetic trap is compressed to increase the atomic density. In doing so we also increases the temperature of the atomic cloud. Doppler cooling of the atoms in the compressed trap reduces the temperature and increases the density. Subsequent evaporative cooling further reduces the temperature. The efficiency of the cooling mechanism is determined by the elastic and inelastic scattering properties of atomic species. Ultra-cold collisions between ground state atoms are dominated by s-wave collisions. They are characterized by a single parameter, the scattering length a. We were able to determine the temperature dependence of the elastic collision rate for the two bosonic isotopes 52Cr and 50Cr in a relaxation experiment. Comparing our results with the effective range theory allowed us not only to extract the magnitude of the scattering length, but also its sign. The sign of the scattering length is important since only condensates with a positive scattering length are stable. Our efforts to achieve Bose-Einstein condensation in chromium by evaporative cooling in a magnetic trap resulted in a maximum phase space density of 0.04. Further cooling reduced the phase space density due to increased loss of atoms from dipolar relaxation. We were able to obtain preliminary results on the magnetic field dependence of dipolar relaxation. Our experimental data is in excellent agreement with theory. These experiments confirm the theoretical prediction that the dipolar relaxation rate is independent from the details of the interaction potential but rather scales with the magnetic offset field and the magnetic dipole moment of the atoms. The findings of this work mark an important step towards the realization of a Bose-Einstein condensate with chromium atoms. Especially the knowledge of the elastic and inelastic scattering properties allow now to devise a successful strategy.
We report on experiments exploring the physics of dipolar quantum gases using a 52 Cr Bose-Einstein condensate (BEC). By means of a Feshbach resonance, it is possible to reduce the effects of short range interactions and reach a regime where the physics is governed by the long-range, anisotropic dipole-dipole interaction between the large (6 µB) magnetic moments of Chromium atoms. Several dramatic effects of the dipolar interaction are observed: the usual inversion of ellipticity of the condensate during time-of flight is inhibited, the stability of the dipolar gas depends strongly on the trap geometry, and the explosion following the collapse of an unstable dipolar condensate displays d-wave like features.