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Trapping and Cooling of Fermionic Alkali Atoms to Quantum Degeneracy

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Trapping and Cooling of Fermionic Alkali Atoms to Quantum Degeneracy Trapping and cooling of fermionic alkali atoms to quantum degeneracy. Sub-Doppler cooling of Potassium-40 and Lithium-6 in gray molasses Diogo Rio Fernandes To cite this version: Diogo Rio Fernandes. Trapping and cooling of fermionic alkali atoms to quantum degeneracy. Sub- Doppler cooling of Potassium-40 and Lithium-6 in gray molasses. Quantum Physics [quant-ph]. Uni- versité Pierre et Marie Curie, 2014. English. tel-01082122 HAL Id: tel-01082122 https://tel.archives-ouvertes.fr/tel-01082122 Submitted on 12 Nov 2014 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Département de physique Laboratoire Kastler Brossel École Normale Supérieure THÉSE de DOCTORAT de l’UNIVERSITÉ PARIS VI Spécialité : Physique Quantique présentée par Diogo RIO FERNANDES pour obtenir le grade de Docteur de l’Université Pierre et Marie Curie (Paris VI) Trapping and cooling of fermionic alkali atoms to quantum degeneracy. Sub-Doppler cooling of Potassium-40 and Lithium-6 in gray molasses. Soutenue le 29 septembre 2014 devant le jury composé de : Denis BOIRON . Rapporteur Philippe BOUYER . Rapporteur Frédéric CHEVY . Directeur de thèse Roland COMBESCOT . Examinateur Bruno LABURTHE-TOLRA . Examinateur Giacomo ROATI . Examinateur Christophe SALOMON . Membre invité 2 À minha avó Lilinha, aos meus pais e à minha irmã "[. ] Eles não sabem, nem sonham, que o sonho comanda a vida, que sempre que um homem sonha o mundo pula e avança como bola colorida entre as mãos de uma criança." António Gedeão 4 Contents 1 Introduction 15 1.1 Prospects for ultracold fermionic mixtures . 17 1.2 State of the art in Fermi-Fermi experiments . 18 1.3 Outline of this thesis ............................. 20 2 Experimental apparatus 23 2.1 Vacuum chamber ............................... 23 2.1.1 MOT chamber ............................. 25 2.1.2 Transport section ........................... 26 2.2 Laser systems ................................. 29 2.2.1 Atomic levels and cooling transitions . 30 2.2.2 Setup for cooling in the D2 and D1 transitions . 31 2.2.3 Optical setup around the science cell . 32 2.3 Absorption imaging ............................. 33 2.3.1 Evaluation of the imaging data ................... 34 2.3.2 Imaging optical setup ........................ 36 2.3.3 High magnetic field imaging .................... 38 2.4 Computer control system .......................... 40 Appendix 2.A Vacuum parts ........................... 43 Appendix 2.B 40K Laser table .......................... 45 Appendix 2.C 6Li Laser table ........................... 46 Appendix 2.D Imaging detunings for high-field imaging . 47 3 Dual-species MOT 49 3.1 Double MOT current status and improvements . 50 Appendix 3.A Publications ............................ 53 5 Contents 4 Gray molasses 81 4.1 Introduction .................................. 81 4.2 Principle .................................... 82 4.3 The case of 40K: semi-classical calculations . 86 4.4 Experimental setup .............................. 89 4.5 Results and discussion ............................ 90 4.5.1 Characterization of the 40K gray molasses . 91 4.5.2 Role of the repumper: study of the Raman detuning . 95 4.6 Gray molasses cooling of 6Li ......................... 98 4.6.1 Experimental setup - offset lock ................... 98 4.7 Conclusions .................................. 102 Appendix 4.A Publications ............................ 103 5 Magnetic trapping and magnetic transport 123 5.1 Principles of magnetic trapping of neutral atoms . 124 5.2 Optical pumping and transferring atoms into the magnetic quadrupole trap ....................................... 129 5.3 Magnetic transport .............................. 135 5.3.1 Working principle . 136 5.3.2 Optimization and performance . 139 5.4 Conclusions .................................. 148 Appendix 5.A Electromagnets . 151 5.A.1 Description of the coils . 151 5.A.2 Electronics ............................... 156 6 Evaporative cooling in a plugged magnetic quadrupole trap 159 6.1 Introduction .................................. 159 6.2 Cold elastic collisions ............................. 160 6.2.1 S-wave collisions . 162 6.2.2 P-wave collisions . 164 6.3 Loss mechanisms ............................... 166 6.3.1 Majorana losses ............................ 166 6.3.2 Spin-exchange collision rate . 167 6 Contents 6.4 Evaporation dynamics ............................ 168 6.4.1 Evaporation rate . 169 6.4.2 Evaporation dynamics . 171 6.5 Experimental techniques . 173 6.5.1 Radio-frequency and micro-wave sources . 174 6.5.2 Probing density and energy distributions in a magnetic trap . 176 6.5.3 Optical plug .............................. 178 6.5.4 Study of the plugged magnetic quadrupole trap . 180 6.6 Evaporation in the plugged trap . 181 6.7 Sympathetic cooling of 6Li . 184 6.7.1 Thermalization of a gas of 6Li with 40K . 184 6.7.2 Thermalization experiments in the MOT chamber . 185 6.7.3 Sympathetic cooling of 6Li . 186 6.7.4 Spin-exchange interspecies collision rate . 187 6.8 Summary .................................... 190 Appendix 6.A Corrective factor for evaporation rate . 191 Appendix 6.B Construction of the antennae . 193 6.B.1 Potassium-40 hyperfine frequency (MW) antenna . 194 6.B.2 Radio-frequency antenna . 195 6.B.3 Lithium-6 hyperfine frequency (RF) antenna . 198 6.B.4 Stub technique ............................ 198 7 Evaporative cooling in an optical dipole trap 201 7.1 Trapping in an ODT .............................. 201 7.2 Experimental setup .............................. 204 7.2.1 Laser setup .............................. 205 7.2.2 Laser power control with a logarithmic amplifier . 206 7.3 Loading of the ODT .............................. 207 7.3.1 Model ................................. 207 7.3.2 Experimental results . 210 7.4 Axial confinement ............................... 212 7.5 Evaporative cooling to quantum degeneracy . 215 7.5.1 Introduction .............................. 215 7 Contents 7.5.2 Experimental results . 216 7.5.3 Transfer to the lower Zeeman states . 218 7.5.4 Degenerate Fermi gas . 221 7.6 Cooling of the 6Li-40K mixture . 223 7.7 Conclusions .................................. 225 Appendix 7.A Gravitational sag . 227 Appendix 7.B Evaporation rate of a double atomic gas with interspecies collisions .................................... 227 8 Conclusions and outlook 229 8.1 Summary of the results ............................ 229 8.2 Perspectives .................................. 232 Appendix 8.A Optical lattice for 40K . 235 Index 237 Bibliography 239 8 Abstract This thesis describes the design, construction and characterization of an apparatus capable of trapping and cooling fermionic atoms of 6Li and 40K to ultracold tempera- tures. The study of mixtures of degenerate Fermi gases opens the door for the creation of new many-body quantum systems. We present a novel laser cooling technique able to simultaneously cool 6Li and 40K to the sub-Doppler regime based on the gray molasses scheme operating on the D1 atomic transition. This strategy enhances the phase space density of both atomic 4 6 species to 10− , the highest value reported in the literature for laser cooled Li and 40K. The optimization of a device able to transport a magnetically trapped atomic cloud from the MOT chamber to a science cell is described. In this cell evaporative cooling is performed first in a plugged magnetic quadrupole trap and then in an optical dipole trap. We report the production of a quantum degenerate Fermi gas of 1.5 105 atoms 40 × K in a crossed dipole trap with T/TF = 0.17, paving the way for the creation of strongly interacting superfluids of 40K. 9 10 Résumé Ce mémoire décrit la conception, la construction et la caractérisation d’un appareil capable de piéger et refroidir des atomes fermioniques de 6Li et 40K à des tempéra- tures ultrabasses. L’étude des mélanges des gazes de Fermi dégénérés ouvre la porte vers la création des nouveaux systèmes quantiques à N corps. Nous présentons une nouvelle technique de refroidissement laser capable de refroi- dir simultanément 6Li et 40K à des températures sub-Doppler basée sur un schéma de molasses grises fonctionnant sur la transition atomique D1. Cette stratégie améliore 4 la densité dans le espace des phases des deux espèces atomiques à 10− , la valeur la plus élevée rapportée dans la littérature pour le refroidissement laser du 6Li et du 40K. L’optimisation d’un dispositif capable de transporter un nuage atomique piégé ma- gnétiquement de l’enceinte MOT à une cellule de science est décrite. Dans cette cellule on effectue du refroidissement évaporatif d’abord dans un piège magnétique quadri- polaire dont le zéro du champ est interdit par un potentiel répulsif et après dans un piège optique dipolaire. Nous rapportons la production d’un gaz quantique de Fermi dégénéré de 1.5 105 atomes de 40K dans un piège dipolaire croisé avec T/T = 0.17, × F ouvrant la voie à la création des superfluides de 40K en interactions fortes. 11 12 Acknowledgements I start by thanking Denis Boiron, Philippe Bouyer, Roland Combescot, Bruno Laburthe-Torla and
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