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Atomes froids de chrome piegés magnétiquement et optiquement@ : premières étapes vers la condensation

Radu Chicireanu

Published Jan 1, 2007• 0 citations
OpenAlex

Abstract

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.

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Cooling and trapping Chromium Atoms

A. Pouderous, O. Gorceix

[Résumé en français]. Ce travail s inscrit dans la perspective d'obtenir des gaz quantiques d'atomes dipolaires : condensat de Bose-Einstein du 52Cr et mer de Fermi dégénérée du 53Cr. Le dispositif expérimental est décrit. Les solutions adoptées pour réaliser un four à haute température et un faisceau laser à 425,5 nm non modulé sont détaillées. Les pièges magnéto-optiques des deux isotopes sont caractérisés par un fort taux de collisions à deux corps et un taux de pertes à un corps dominé par une désexcitation vers des états métastables. Cette propriété permet d'accumuler en continu de l'ordre de dix millions d'atomes dans un piège magnétique. Un piège magnéto-optique du mélange 52Cr-53Cr est aussi étudié. La réalisation du premier piège optique d'atomes dans des états métastables est montrée pour le 52Cr. Ce résultat est une étape vers l'obtention d'un piège optique croisé où les atomes seront polarisés puis refroidis sous le seuil de condensation.

•Jan 1, 2007•0 citations

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

Quentin Beaufils, Radu Chicireanu, T. Zanon-Willette +5 more

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)

•Jul 1, 2008•0 citations

Scattering properties of ultra-cold chromium atoms

Piet O. Schmidt

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.

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Radu Chicireanu, A. Pouderous, R. Barbé +5 more

We report on magneto-optical trapping of fermionic $^{53}\mathrm{Cr}$ atoms. A Zeeman-slowed atomic beam provides loading rates up to $3\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{0.3em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. We present systematic characterization of the magneto-optical trap (MOT). We obtain up to $5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms in the steady-state MOT. The atoms radiatively decay from the excited $P$ state into metastable $D$ states, and, due to the large dipolar magnetic moment of chromium atoms in these states, they can remain magnetically trapped in the quadrupole field gradient of the MOT. We study the accumulation of metastable $^{53}\mathrm{Cr}$ atoms in this magnetic trap. We also report on the simultaneous magneto-optical trapping of bosonic $^{52}\mathrm{Cr}$ and fermionic $^{53}\mathrm{Cr}$ atoms. Finally, we characterize the light-assisted collision losses in this Bose-Fermi cold mixture.

arXiv:quant-ph/0603028•Physical Review A•May 24, 2006•53 citations

Studies of cold chromium atoms in magnetic and optical traps: steps towards Bose-Einstein condensation

Radu Chicireanu, O. Gorceix

[Résumé français] Le travail de cette thèse s inscrit dans la perspective d étudier les interactions dipo-laires dans des gaz de bosons et de fermions quantiquement dégénérés. Dans ce manuscrit on présente la réalisation d une experience pour refroidir et piéger des atomes de Cr, ayant un fort moment magnétique de 6 B.Nos experiences ont permis d obtenir et d étudier des pièges agneto-optiques pour les deux isotopes majoritaires du Cr. Des experiences de chargement en continu des atomes métastables de Cr, dans des pièges agnétiques de profondeur finie (tronquespar des champs RF) ont permis d obtenir des densités dans l espace des phases de 6 . 10 , dans des temps inférieurs à la seconde. Les propriétés collisionnelles des atomes métastables de Cr ont été étudiés en detail. Finalement, une nouvelle méthode de chargement d un piège optique avec plus d un 52 million d atomes méta stables de Cr, à100 K, été mis en evidence. La polarization de ces atomes dans l état minimal en énergie ouvre des perspectives pour commencer le 52 oidissement évaporatif, afin d atteindre la condensation de Bose-Einstein du Cr.

•Jan 1, 2007•1 citation

Ultracold chromium atoms: from Feshbach resonances to a dipolar Bose–Einstein condensate

J. Stühler, Axel Griesmaier, Jörg Werner +3 more

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.

Journal of Modern Optics•Aug 23, 2006•10 citations
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.
[Résumé en français]. Ce travail s inscrit dans la perspective d'obtenir des gaz quantiques d'atomes dipolaires : condensat de Bose-Einstein du 52Cr et mer de Fermi dégénérée du 53Cr. Le dispositif expérimental est décrit. Les solutions adoptées pour réaliser un four à haute température et un faisceau laser à 425,5 nm non modulé sont détaillées. Les pièges magnéto-optiques des deux isotopes sont caractérisés par un fort taux de collisions à deux corps et un taux de pertes à un corps dominé par une désexcitation vers des états métastables. Cette propriété permet d'accumuler en continu de l'ordre de dix millions d'atomes dans un piège magnétique. Un piège magnéto-optique du mélange 52Cr-53Cr est aussi étudié. La réalisation du premier piège optique d'atomes dans des états métastables est montrée pour le 52Cr. Ce résultat est une étape vers l'obtention d'un piège optique croisé où les atomes seront polarisés puis refroidis sous le seuil de condensation.
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 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 magneto-optical trapping of fermionic 53Cr^{53}\mathrm{Cr}53Cr atoms. A Zeeman-slowed atomic beam provides loading rates up to $3\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{0.3em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. We present systematic characterization of the magneto-optical trap (MOT). We obtain up to $5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms in the steady-state MOT. The atoms radiatively decay from the excited PPP state into metastable DDD states, and, due to the large dipolar magnetic moment of chromium atoms in these states, they can remain magnetically trapped in the quadrupole field gradient of the MOT. We study the accumulation of metastable 53Cr^{53}\mathrm{Cr}53Cr atoms in this magnetic trap. We also report on the simultaneous magneto-optical trapping of bosonic 52Cr^{52}\mathrm{Cr}52Cr and fermionic 53Cr^{53}\mathrm{Cr}53Cr atoms. Finally, we characterize the light-assisted collision losses in this Bose-Fermi cold mixture.
[Résumé français] Le travail de cette thèse s inscrit dans la perspective d étudier les interactions dipo-laires dans des gaz de bosons et de fermions quantiquement dégénérés. Dans ce manuscrit on présente la réalisation d une experience pour refroidir et piéger des atomes de Cr, ayant un fort moment magnétique de 6 B.Nos experiences ont permis d obtenir et d étudier des pièges agneto-optiques pour les deux isotopes majoritaires du Cr. Des experiences de chargement en continu des atomes métastables de Cr, dans des pièges agnétiques de profondeur finie (tronquespar des champs RF) ont permis d obtenir des densités dans l espace des phases de 6 . 10 , dans des temps inférieurs à la seconde. Les propriétés collisionnelles des atomes métastables de Cr ont été étudiés en detail. Finalement, une nouvelle méthode de chargement d un piège optique avec plus d un 52 million d atomes méta stables de Cr, à100 K, été mis en evidence. La polarization de ces atomes dans l état minimal en énergie ouvre des perspectives pour commencer le 52 oidissement évaporatif, afin d atteindre la condensation de Bose-Einstein du Cr.
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.