This thesis describes the advances made towards a dual-species magneto-optical trap (MOT) of Li and Rb for use in photoassociation spectroscopy, Feshbach resonance studies, and, as long-term aspirations, the formation of ultracold heteronuclear polar molecules. The initial discussion will focus on a brief theoretical overview of laser cooling and trapping and the production of ultracold molecules from a cold atom source. Subsequently, details of the experimental system, including those pertaining to the required laser light, the vacuum chamber, and the computer control system will be presented. Finally, preliminary optimization and characterization measurements showing the performance of a single species Li MOT are introduced. These measurements demonstrated the loading of over 8 x 107 Li atoms directly into a MOT without the need for a Zeeman slower.

This thesis describes the advances made towards a dual-species magneto-optical trap (MOT) of Li and Rb for use in photoassociation spectroscopy, Feshbach resonance studies, and, as long-term aspirations, the formation of ultracold heteronuclear polar molecules. The initial discussion will focus on a brief theoretical overview of laser cooling and trapping and the production of ultracold molecules from a cold atom source. Subsequently, details of the experimental system, including those pertaining to the required laser light, the vacuum chamber, and the computer control system will be presented. Finally, preliminary optimization and characterization measurements showing the performance of a single species Li MOT are introduced. These measurements demonstrated the loading of over 8 x 107 Li atoms directly into a MOT without the need for a Zeeman slower.

This thesis describes the advances made towards a dual-species magneto-optical trap (MOT) of Li and Rb for use in photoassociation spectroscopy, Feshbach resonance studies, and, as long-term aspirations, the formation of ultracold heteronuclear polar molecules. The initial discussion will focus on a brief theoretical overview of laser cooling and trapping and the production of ultracold molecules from a cold atom source. Subsequently, details of the experimental system, including those pertaining to the required laser light, the vacuum chamber, and the computer control system will be presented. Finally, preliminary optimization and characterization measurements showing the performance of a single species Li MOT are introduced. These measurements demonstrated the loading of over 8 x 107 Li atoms directly into a MOT without the need for a Zeeman slower. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Ultracold bialkali polar molecules present a wealth of opportunities in quantum science research and technology; including fields such as quantum simulation, quantum chemistry, quantum metrology, precision measurement and quantum computation. A great deal of interest lies in their rich internal rotational and vibrational state structure and their large electric dipole moment. However, the additional complexity also provides significant challenges. To date, only a limited number of molecular species are available at ultracold temperatures below 1 microkelvin. The assembly of heteronuclear ground state molecules from ultracold atoms has emerged as a promising approach for creating ultracold molecules. In this thesis, I will present the creation of the first ultracold gases of NaCs ground state molecules. First, we produce an ultracold mixture of Na and Cs. Second, we associate weakly bound molecular pairs from the Na-Cs mixture. Finally, we apply a two-photon stimulated Raman adiabatic passage (STIRAP) pulse to transfer the weakly bound NaCs molecules into the deeply bound rovibrational ground state. I report on the construction of a new apparatus that produces ultracold mixtures of Na and Cs. We use this apparatus to assemble weakly bound NaCs molecules and successfully transfer up to 20,000 ultracold dipolar NaCs molecules to their rovibrational ground state in each experimental run. On the way to these results, we demonstrated a pathway towards creating the first quantum degenerate mixtures of Na and Cs. We identified and characterized an interspecies Feshbach resonance at 864.12(5) G, adiabatically sweeping across it to form weakly bound NaCs Feshbach molecules. We characterized the Feshbach molecule formation in various parameter regimes. Next, we performed a study of accessible NaCs excited states and identified a pathway to the rovibrational ground state using one- and two-photon spectroscopy. Finally, we demonstrated STIRAP to the rovibrational ground state, and investigated basic properties of the ground state molecules.

The immaculate control of atoms and molecules with light is the defining trait of modern experiments in ultracold physics. The rich internal degrees of freedom afforded by molecules enrich the toolbox of precision spectroscopy for fundamental physics, and hold great promise for applications in quantum simulation and quantum information science. A vibrational molecular lattice clock with systematic fractional uncertainty at the 14th decimal place is demonstrated for the first time, matching the performance of the earliest optical atomic clocks. Van der Waals dimers of strontium are created at ultracold temperatures and levitated by an optical standing wave, whose wavelength is finely tuned to preserve the delicate molecular vibrational coherence. Guided by quantum chemistry theory refined by highly accurate frequency-comb-assisted laser spectroscopy, record-long Rabi oscillations were demonstrated between vibrational molecular states that span the entire depth of the ground molecular potential. Enabled by the narrow molecular clock linewidth, hertz-level frequency shifts were resolved, facilitating the first characterization of molecular hyperpolarizability in this context. In a parallel effort, deeply bound strontium dimers are coherently created using the technique of stimulated Raman adiabatic passage. Ultracold collisions of alkaline-earth metal molecules in the absolute ground state are studied for the first time, revealing inelastic losses at the universal rate. This thesis reports one of the most accurate measurement of a molecule's vibrational transition frequency to date, which may potentially serve as a secondary representation of the SI unit of time in the terahertz (THz) band where standards are scarce. The prototypical molecular clock lays the important groundwork for future explorations into THz metrology, quantum chemistry, and fundamental interactions at atomic length scales.

We study the nonlinear mean-field dynamics of diatomic molecule formation at coherent photo- and magneto-association of ultracold atoms focusing on the case when the system is initially in the all-atomic state. We show that in the limit of strongly nonlinear interaction between an ultra-cold atomic-molecular system and a quasi-resonant electromagnetic field, the molecule formation process, depending on the characteristics of the associating field, may evolve according two different scenarios, namely, weak- and strong-oscillatory regimes. In the first case the number of molecules increases without pronounced oscillations of atom-molecule populations, while in the second case high-amplitude Rabi-type oscillations arise. Assuming an arbitrary external field configuration, we construct analytical solutions to describe the system's temporal dynamics in the both interaction regimes. Further, we investigate the influence of inter-particle elastic scattering on the dynamics of coherent molecule formation subject to an external field configuration of the resonance-crossing Landau-Zener model. We derive an approximate solution which for the first time describes the whole temporal dynamics of the molecule formation in this general case.

La thèse se positionne dans le domaine des molécules ultra-froides, c’est-à-dire des molécules qui ont des vitesses correspondant à des températures de l’ordre du µK. L’obtention de gaz dilués moléculaires à ces températures peut ouvrir la porte à des applications importantes en simulation ou en informatique quantique. La thèse s’intéresse plus particulièrement à la formation de molécules dipolaires électriques et magnétiques. Celles-ci sont présagées pour être un système idéal dans l’optique d’un simulateur quantique du système réseau-spin, permettant de décrire le magnétisme dans les solides. Nous avons choisi l’exemple de la molécule RbSr qui fait l’objet actuellement d’une expérience à Amsterdam. Nous avons donc exploré plusieurs alternatives basées sur l’emploi de laser pour la formation de molécules RbSr ultra-froides Nous avons d’abord considéré la photoassociation dont le principe est de coupler l’état de collision initial avec un état rovibrationnel d’un état électronique excité. L’étape d’émission spontanée qui suit forme des molécules dans l’état électronique fondamental. Nous avons également considéré le problème des pertes supplémentaires d’atomes lorsque le laser de photoassociation est intense et focalisé, mises en évidence dans une expérience à Bangalore. Dans la suite de la thèse, nous avons exploré des méthodes cohérentes. Nous avons montré que des molécules faiblement liées de RbSr peuvent être formées à l’aide d’un STIRAP en partant de paires d’atomes isolées et confinées dans un isolant de Mott. Nous avons ensuite étudié leur stabilisation vers le niveau le plus profond de l’état fondamental de la molécule à l’aide d’un second STIRAP. Enfin, nous avons étudié des méthodes se déroulant uniquement dans l’état électronique fondamental. La formation est induite par l’utilisation d’une impulsion à dérive de fréquence induisant un passage adiabatique ou à l’aide d’une impulsion-pi. En plus, nous avons découvert que cette méthode formation peut être reliée à une résonance de Feshbach dans la représentation habillée par les photons, que nous avons appelée Résonance de Feshbach auto-induité assistée par Laser (LASIFR en anglais). Nous montrons qu’elles sont un outil prometteur et puissant pour le contrôle des propriétés de mélange de gaz d’atomes ultra-froids, comme par exemple la longueur de diffusion. / The thesis is positioned in the ultracold domain, i.e molecules which have velocities corresponding to microkelvin temperatures. The formation of molecular diluted gas at these temperatures is promising for important applications in quantum simulation, quantum information or in precision measurements.More particularly, the thesis is focused on the formation of molecules which are polar and paramagnetic. Some recent works are predicted that these molecules could be the ideal system for creating a quantum simulator of the lattice-spin system, which can describe the magnetism in solids. We have chosen the example of RbSr molecules for whose an experience runs in Amsterdam. We explored some alternatives based on the use of lasers for the formation of ultracold RbSr molecules.First, we considered the photoassociation whose the principle is coupling the initial scattering state with a rovibrational level of an excited electronic state. The following spontaneous emission step creates molecules in the electronic ground state. We also considered the problem of atom losses observed by experiments in Bangalore, when a focused photoassociation laser is applied. In the rest of the thesis, we explored coherent methods. Firstly, we showed a STIRAP sequence could create weakly bound molecules from isolated atomic pairs confined in a Mott insulator. Lastly, we explored some of these methods where the dynamic occurs only in the electronic ground state. The formation is induced by the use of a chirped pulse or a pi-pulse. We studied the factors of the transfer. Moreover, we discovered this method is related to a new kind of Feshbach resonances in the photon dressed picture, called Laser Assisted Self-Induced Feshbach Resonance (LASIFR). We showed LASIFR present the advantages of Magnetic and Optical Feshbach Resonances. They are a promising and powerful tool for the control of properties of quantum gas mixtures, like the interspecies scattering length.

D’immenses progrès ont été réalisés ces dernières années dans le champ de production de condensats de Bose-Einstein moléculaires pour l’obtention et le piégeage de molécules ultrafroides ioniques et neutres. Ces échantillons de molécules ultrafroides trouvent des applications dans des domaines très variés tels que les mesures pectroscopiques de haute précision ou bien encore le stockage de l’information quantique. La volonté d’optimiser les procédés de refroidissement et des techniques de piégeage a stimulé un grand nombre d’études théoriques. La plupart de ces études sont dédiées au refroidissement des molécules en collision avec un gaz tampon (3 He). La surface d’énergie potentielle de l’état fondamental du complexe de van der Waals He–MnH(X7Σ+) est présentée, suivie du calcul des états liés correspondant au complexe ainsi que l’étude de la relaxation Zeeman associée au système en fonction du champ magnétique appliqué. Dans le domaine de l’astrochimie, l’étude de la désexcitation rotationnelle de CH+ en collision avec l’hélium est également présentée. / Tremendous progress in experimental production and trapping of ultracold neutral and ionic molecules has been achieved over the past few years which even allowed the production of molecular Bose–Einstein condensates. These ultracold molecules samples have potential applications in many different fields, such as precision spectroscopic measurements or quantum information storage and processing. The optimization of the cooling processes and the trapping techniques also stimulated a great number of theoretical studies. Many of them are dedicated to inelastic scattering of molecules in collisions with 3He atoms (buffer gas cooling method). The potential energy surface of the ground state of the He–MnH(X7Σ+) van der Waals complex is presented, followed by the calculations of the bound states of this system and the Zeeman relaxation in function of the magnetic field. In the field of astrochemistry, a quantum mechanical investigation of rotational energy transfer in cold collisions of CH+ with 4He atoms is presented.

Nous étudions la dynamique non linéaire en champ moyen de la formation de molécules diatomiques par photo-association ou magnéto-association d’atomes ultra froids pour un système entièrement atomique dans l’état initial. Nous montrons que dans la limite d’une forte interaction non linéaire entre un système atome-molécule ultra froid et un champ électromagnétique quasi résonnant, le processus de formation du condensat moléculaire peut évoluer suivant deux scénarios en fonction des caractéristiques du champ : régime faiblement oscillatoire ou régime fortement oscillatoire. Dans le cas du régime faiblement oscillatoire, le nombre de molécules augmente sans oscillations prononcées des populations atomiques et moléculaires alors que de fortes oscillations de Rabi apparaissent dans le second cas. Nous présentons des solutions analytiques décrivant la dynamique temporelle du système dans ces deux cas. Nous étudions ensuite l’influence de la diffusion élastique entre particules sur la dynamique de formation cohérente de molécules sous l’action d’un champ extérieur représenté par le modèle de Landau-Zener. Nous déterminons une solution approchée qui décrit bien toute la dynamique temporelle de formation moléculaire dans ce cas général. / We study the nonlinear mean-field dynamics of diatomic molecule formation at coherent photo- and magneto-association of ultracold atoms focusing on the case when the system is initially in the all-atomic state. We show that in the limit of strongly nonlinear interaction between an ultra-cold atomic-molecular system and a quasi-resonant electromagnetic field, the molecule formation process, depending on the characteristics of the associating field, may evolve according two different scenarios, namely, weak- and strong-oscillatory regimes. In the first case the number of molecules increases without pronounced oscillations of atom-molecule populations, while in the second case high-amplitude Rabi-type oscillations arise. Assuming an arbitrary external field configuration, we construct analytical solutions to describe the system’s temporal dynamics in the both interaction regimes. Further, we investigate the influence of inter-particle elastic scattering on the dynamics of coherent molecule formation subject to an external field configuration of the resonance-crossing Landau-Zener model. We derive an approximate solution which for the first time describes the whole temporal dynamics of the molecule formation in this general case.

Le travail théorique présenté dans cette thèse concerne la formation de molécules ultra-froides bialcalines et le contrôle de leurs degrés de liberté externes et internes. Cette étude est motivée par les nombreuses expériences en cours visant à l'obtention d'un gaz quantique dégénéré de molécules dans leur état fondamental absolu. Le schéma de formation étudié repose sur le processus de transfert adiabatique stimulé (STIRAP) réalisé en présence d'un potentiel optique de piégeage (réseau optique) des atomes et des molécules.Nous avons déterminé les paramètres du réseau optique (intensité et fréquence du champ laser) qui permettent de piéger efficacement des dimères d'alcalins en évaluant la polarisabilité dynamique acquise par les molécules soumises à un champ externe. Ces calculs reposent en particulier sur la connaissance détaillée de la structure électronique des molécules. Nous avons identifié des plages de longueur d'ondes dites « magiques » où la polarisabilité est la même pour chaque niveau peuplé au cours du transfert adiabatique, permettant ainsi un transfert optimal. Ce formalisme nous a également permis d'obtenir les coefficients Van der Waals de l'interaction à longue portée nécessaires pour étudier les taux de collisions entre molécules.Nous avons réalisé une étude plus détaillée de la molécule RbCs. En étudiant précisément la probabilité de transition de la molécule vers un niveau excité, nous avons proposé un schéma STIRAP pour transférer des molécules de RbCs, initialement dans un niveau vibrationnel excité, vers leur état rovibrationnel fondamental.Ces travaux ont montré l'importance de la connaissance précise de la structure hyperfine de l'état électronique moléculaire excité pour réaliser un gaz dégénéré de molécules dans un état quantique bien défini. Un modèle asymptotique nous a permis d'obtenir une première estimation de la structure hyperfine des courbes d'énergies potentielles des premiers états moléculaires excités des molécules Cs2 et RbCs. / The theoretical work presented in this thesis is focused on the formation of ultracold bialcaline molecules and on the control of their external and internal degrees of freedom. This study is motivated by the increasing number of experiments aiming at obtaining a quantum degenerate gas of molecules in their absolute ground state. The formation scheme we worked on is based on the Stimulated Raman Adiabatic Passage (STIRAP) technique operated while molecules are trapped inside an optical lattice.We have determined the parameters of the optical lattice (intensity and wavelength of the laser) that allow for an efficient trapping of the alkali dimers by evaluating the dynamic polarizability of molecules in the presence of an external field. Such calculations require the accurate knowledge of the electronic structure of the molecules. We have identified the so-called ``magic'' wavelength for which all levels populated during the STIRAP sequence have the same polarizability, thus ensuring an optimal transfer. The same approach has also been used to compute the strength of the long-range interaction between polar bialkali molecules needed to evaluate collision rates.The particular case of the RbCs molecule has been investigated. We have selected a radiative transition allowing for an efficient STIRAP scheme yielding molecules in their rovibrational ground state. These works have raised the need for the precise knowledge of the hyperfine structure of the excited electronic molecular state involved in the STIRAP scheme. We have developed an asymptotic model to obtain a first estimate of the hyperfine structure for the potential curves of the lowest excited states of Cs2 and RbCs.