The development of quantum-gas microscopes has brought novel ways of probing quantum degenerate many-body systems at the single-atom level. Until now, most of these setups have focused on alkali atoms. Expanding quantum-gas microscopy to alkaline-earth elements will provide new tools, such as SU( N )-symmetric fermionic isotopes or ultranarrow optical transitions, to the field of quantum simulation. Here we demonstrate the site-resolved imaging of a 84 Sr bosonic quantum gas in a Hubbard-regime optical lattice. The quantum gas is confined by a two-dimensional in-plane lattice and a light-sheet potential, which operate at the strontium clock-magic wavelength of 813.4 nm. We realize fluorescence imaging using the broad 461-nm transition, which provides high spatial resolution. Simultaneously, we perform attractive Sisyphus cooling with the narrow 689-nm intercombination line. We reconstruct the atomic occupation from the fluorescence images, obtaining imaging fidelities above 94 % . Finally, we realize a 84 Sr superfluid in the Bose-Hubbard regime. We observe its interference pattern upon expansion, a probe of phase coherence, with single-atom resolution. Our strontium quantum-gas microscope provides a new platform to study dissipative Hubbard models, quantum optics in atomic arrays, and SU( N ) fermions at the microscopic level. Published by the American Physical Society 2024
The development of quantum-gas microscopes has brought novel ways of probing quantum degenerate many-body systems at the single-atom level. Until now, most of these setups have focused on alkali atoms. Expanding quantum-gas microscopy to alkaline-earth elements will provide new tools, such as SU( N )-symmetric fermionic isotopes or ultranarrow optical transitions, to the field of quantum simulation. Here we demonstrate the site-resolved imaging of a 84 Sr bosonic quantum gas in a Hubbard-regime optical lattice. The quantum gas is confined by a two-dimensional in-plane lattice and a light-sheet potential, which operate at the strontium clock-magic wavelength of 813.4 nm. We realize fluorescence imaging using the broad 461-nm transition, which provides high spatial resolution. Simultaneously, we perform attractive Sisyphus cooling with the narrow 689-nm intercombination line. We reconstruct the atomic occupation from the fluorescence images, obtaining imaging fidelities above 94 % . Finally, we realize a 84 Sr superfluid in the Bose-Hubbard regime. We observe its interference pattern upon expansion, a probe of phase coherence, with single-atom resolution. Our strontium quantum-gas microscope provides a new platform to study dissipative Hubbard models, quantum optics in atomic arrays, and SU( N ) fermions at the microscopic level. Published by the American Physical Society 2024
(English) The development of quantum-gas microscopes has revolutionized the field of quantum simulation with ultracold atoms. More specifically, their ability of direct observation and manipulation of degenerate quantum gases in optical lattices on a single particle level has brought novel ways of probing and engineering quantum degenerate many-body systems. So far, most of these setups have focused on alkali atoms. Combining quantum-gas microscopy with the properties of alkaline-earth atoms such as strontium gives rise to exciting research directions. In this thesis, we report on the design and construction of a strontium quantum-gas microscope. The findings in this thesis can be divided into three parts. In the first part, we focus on the accumulation of atoms in the science cell and develop a scheme to enhance the atom number in magneto-optical traps of strontium atoms operating on the 461-nm transition. This scheme resonantly populates a short-lived reservoir state, partially shielding the atomic cloud from losses in the cooling cycle. We demonstrate a factor of 2 enhancement in the atom number for the bosonic isotopes Sr-88 and Sr-84, and the fermionic isotope Sr-87, showing the efficient capture of these isotopes in our experiment. Our scheme can be readily implemented in the majority of strontium experiments, given that the shielding transition at 689 nm is commonly used for further cooling. In our case, the shielding scheme facilitates the generation of Bose-Einstein condensates. The second part of the thesis reports on the generation of degenerate quantum gases of Sr-84 with up to 200000 atoms. After summarizing the required cooling steps, we study the formation of Bose-Einstein condensates during evaporative cooling in our experiment. Analyzing the evolution of the horizontal and vertical size of our quantum-degenerate clouds in free fall leads to the characteristic asymmetric expansion, which we compare to theory for our experimental parameters. We also show the generation of smaller Bose-Einstein condensates of less than 20000 atoms with the help of a light-sheet potential. With this highly-anisotropic confinement we can consider our Bose-Einstein condensates two-dimensional for atom numbers of the order of 1000. In the third part we demonstrate site-resolved imaging of a Sr-84 bosonic quantum gas in a Hubbard-regime optical lattice potential. We confine the quantum gas by a two-dimensional optical lattice and the aforementioned light-sheet potential, both operating at strontium's clock-magic wavelength. A high-NA imaging objective enables single-atom and single-site resolved fluorescence imaging by scattering photons on strontium's broad 461-nm transition, while performing efficient attractive Sisyphus cooling of the atoms on a narrower transition at 689 nm. We reconstruct the atomic occupation of the lattice sites from the fluorescence images, obtaining imaging fidelities above 94%. Finally, we realize a Sr-84 superfluid in the Bose-Hubbard regime and observe its characteristic interference pattern after free expansion in the light sheet with single-atom resolution. Our strontium quantum-gas microscope provides a new platform to study dissipative Hubbard models and cooperative effects in atom-light interaction at the microscopic level. Moreover, the ability to capture also the fermionic isotope Sr-87 paves the way to generate degenerate Fermi gases with SU(N) symmetry and study SU(N) quantum magnetism. (Català) El desenvolupament dels microscopis de gasos quàntics ha revolucionat el camp de la simulació quàntica amb àtoms ultrafreds. Més concretament, la capacitat d'observar i manipular gasos quàntics degenerats en xarxes òptiques a nivell d'una sola partícula ha aportat noves maneres d'explorar i dissenyar sistemes quàntics de molts cossos. Fins ara, la majoria d'aquests sistemes s'han centrat en els àtoms alcalins. La combinació de la microscòpia de gasos quàntics amb les propietats dels àtoms alcalinoterris, com l'estronci, dona lloc a interessants línies de recerca. En aquesta tesi, informem sobre el disseny i construcció d'un microscopi de gasos quàntics d'estronci. Els resultats d'aquesta tesi es poden dividir en tres parts. A la primera part, ens centrem en l'acumulació d'àtoms a la cel·la científica i desenvolupem una estratègia per augmentar el nombre d'àtoms en trampes magneto-òptiques d'àtoms d'estronci que operen en la transició de 461 nm. Aquesta estratègia pobla ressonantment un estat reservori de curta vida, blindant parcialment el núvol atòmic de les pèrdues al cicle de refredament. Demostrem un augment en un factor 2 en el nombre d'àtoms per als isòtops bosònics Sr-88 i Sr-84, i l'isòtop fermiònic Sr-87, mostrant la captura eficient d'aquests isòtops al nostre experiment. La nostra tècnic es pot implementar fàcilment a la majoria dels experiments amb estronci, atès que la transició de blindatge s'utilitza habitualment per al refredament posterior. En el nostre cas, l'estratègia de blindatge facilita la generació de condensats de Bose-Einstein. La segona part de la tesi presenta la generació de gasos quàntics degenerats de fins a 200000 àtoms. Després de resumir els passos de refredament necessaris, estudiem la formació de condensats de Bose-Einstein durant el refredament evaporatiu al nostre experiment. L'anàlisi de l'evolució de la mida horitzontal i vertical dels nostres núvols quàntics en caiguda lliure condueix a la característica expansió asimètrica que comparem amb la teoria per als nostres paràmetres experimentals. També mostrem la generació de condensats de Bose-Einstein més petits de menys de 20000 àtoms amb l'ajuda d'un potencial de fulla de llum. Amb aquest confinament altament anisòtrop podem considerar els nostres condensats de Bose-Einstein bidimensionals per a números d'àtoms de l'ordre de 1000. A la tercera part demostrem l'obtenció d'imatges amb resolució d'un sol node d'un gas quàntic bosònic de Sr-84 en un potencial de xarxa òptica en règim de Hubbard. Confinem el gas quàntic mitjançant una xarxa òptica bidimensional i el potencial de fulla de llum abans esmentat, tots dos operant a la longitud d'ona màgica del rellotge d'estronci. Un objectiu d'imatge d'alta obertura numèrica permet obtenir imatges de fluorescència amb una resolució de resoltes en un sol àtom i un sol node mitjançant la dispersió de fotons a l'àmplia transició de 461 nm de l'estronci, alhora que es realitza un refredament eficaç de Sísif atractiu dels àtoms en una transició més estreta. Reconstruïm l'ocupació atòmica dels nodes de la xarxa a partir de les imatges de fluorescència, obtenint fidelitats d'imatge superiors al 94%. Finalment, realitzem un superfluid de Sr-84 al règim de Bose-Hubbard i observem el seu patró d'interferència característic després de l'expansió lliure al full de llum amb resolució d'un sol àtom. El nostre microscopi de gasos quàntics d'estronci proporciona una nova plataforma per estudiar models Hubbard dissipatius i efectes cooperatius en la interacció àtom-llum a nivell microscòpic. A més, la capacitat de capturar també l'isòtop fermiònic Sr-87 aplana el camí per generar gasos de Fermi degenerats amb simetria SU(N) i estudiar el magnetisme quàntic SU(N). (Español) El desarrollo de los microscopios para gases cuánticos ha revolucionado el campo de la simulación cuántica con átomos ultrafríos. Más concretamente, su capacidad de observar y manipular gases cuánticos degenerados en redes ópticas a nivel de una sola partícula ha aportado nuevas formas de explorar y diseñar sistemas cuánticos de muchos cuerpos. Hasta ahora, la mayoría de microscopios para gases cuánticos han utilizado átomos alcalinos. No obstante, la combinación de la microscopía cuántica de gases con las propiedades de los átomos alcalinotérreos, como el estroncio, abre nuevas e interesantes líneas de investigación. En esta tesis, detallamos el diseño y construcción de un microscopio para gases cuánticos de estroncio. Los resultados de esta tesis pueden dividirse en tres partes. En la primera parte, nos centramos en la acumulación de átomos en la celda de ciencia y desarrollamos una estrategia para aumentar el número de átomos en trampas magneto-ópticas de átomos de estroncio que utilizan la transición a 461 nm. Esta estrategia consiste en poblar resonantemente un estado reservorio de corto tiempo de vida, evitando parcialmente las pérdidas atómicas asociados al ciclo de enfriamiento. Demostramos un aumento en un factor 2 en el número de átomos para los isótopos bosónicos Sr-88 y Sr-84, y el isótopo fermiónico Sr-87, mostrando su captura eficiente en nuestro experimento. Nuestra estrategia puede ser fácilmente utilizado en la mayoría de experimentos con estroncio, dado que la transición empleada se utiliza habitualmente para el enfriamiento posterior. En nuestro caso, dicha estrategia facilita la generación de condensados de Bose-Einstein. La segunda parte de la tesis presenta la generación de gases cuánticos de hasta 200000 átomos. Tras resumir los pasos de enfriamiento necesarios, estudiamos la formación de condensados de Bose-Einstein durante el enfriamiento por evaporación en nuestro experimento. El análisis de la evolución del tamaño horizontal y vertical de las nubes atómicas en caída libre conduce a una expansión asimétrica característica. También mostramos la generación de condensados de Bose-Einstein más pequeños de menos de 20000 átomos con la ayuda de un potencial de tipo "hoja de luz". Con este confinamiento altamente anisótropo nuestros condensados pueden considerarse bidimensionales para números de átomos del orden de 1000. En la tercera parte demostramos la obtención de imágenes de un gas cuántico bosónico de Sr-84 confinado en una red óptica en régimen de Hubbard, con resolución a nivel del átomo único. Confinamos el gas cuántico mediante una red óptica bidimensional y el poten
Spin-resolved microscopy of 87Sr SU(N) Fermi-Hubbard systems
Quantum-gas microscopes provide direct access to the phases of the Hubbard model, bringing microscopic insight into the complex competition between interactions, SU(2) magnetism, and doping. Alkaline-earth(-like) fermions extend this spin-1/2 paradigm by realizing higher symmetries and giving access to SU(N) Hubbard models, with rich phase diagrams to be unveiled. Despite its fundamental interest, a microscopic exploration of SU(N) quantum systems has remained elusive. Here we report the realization of a quantum-gas microscope for fermionic 87Sr. Our imaging scheme, based on cooling and fluorescence on the narrow intercombination line at 689 nm, enables spin-resolved single-atom detection. By implementing a spin-selective optical pumping protocol, we determine the occupation of each of the 10 spin states in a single experimental realization, a crucial capability for probing site-resolved magnetic correlations. We benchmark our method by observing single-particle Larmor precession across the full spin-9/2 ground-state manifold and use numerical simulations of SU(N) Fermi-Hubbard chains, incorporating the measured detection fidelities, to demonstrate its capability to resolve many-body spin correlations. Our results establish 87Sr quantum-gas microscopy as a powerful approach to study exotic magnetism in the SU(N) Fermi-Hubbard model, and provide a new detection tool for studies in quantum simulation, computation, and metrology.
Imagine a new generation of computers that dramatically surpasses the most powerful supercomputers, being able to tackle the most challenging of computational problems. These problems arise, for example, in quantum physics where the computational power required to model a many-particle system increases exponentially with the number of its constituents, and as a consequence the calculations become prohibitively time-consuming. Nobel laureate Richard Feynman proposed this new type of computer in the early 1980s: ‘Let the computer itself be built of quantum mechanical elements which obey quantum mechanical laws.’ Such a quantum simulator would allow researchers to tackle challenging problems in condensed-matter physics. In particular many-body fermionic quantum systems are very difficult to compute due to the antisymmetric nature of the wave function, and the resulting sign problem for quantum Monte Carlo methods. This is why a quantum simulator will be an excellent test bed to investigate phenomena and properties of strongly correlated fermionic quantum systems. A powerful experimental platform to realize a quantum simulator is laser-cooled atoms [1] held in an artificial crystal of light, generated by superimposing laser beams. The atoms are kept in this optical lattice similar to marbles in the hollows of an egg carton, and they can mimic the behaviour of electrons in a real crystal. These systems interface atomic physics with condensed matter physics in a new way, as the tools of atomic physics allow for exquisite control of many experimental parameters, which are often inaccessible in solid state experiments. In-situ imaging techniques [2] using ‘quantum-gas microscopes’ constitute a new approach to the study of many-body quantum systems in an optical lattice with unprecedented resolution at the single-particle and single-site level [3,4]. This new method is complementary to earlier optical lattice experiments measuring distributions in momentum space after a time-of-flight expansion. A quantum-gas microscope uses an optical imaging system (Fig. 1a) for collecting the fluorescence light of ultracold atoms in an optical lattice with high spatial resolution comparable to the lattice spacing of ≈ 500 nm. (a) Atoms in optical lattice atoms are imaged with a high-resolution microscope objective [4]. (b) Single-site addressing is achieved by focussing a laser beam through the microscope onto selected atoms, bringing them into resonance with microwave radiation which produces spin flips [12]. (c) and (d) Single-atom-resolved fluorescence images of bosonic Mottinsulators [4,9]. (e) Deterministically prepared atom distribution using single-site addressing [12]. (f) Direct observation of single-atom quantum walks [15]. Fluorescence imaging of atoms in a quantum-gas microscope has made it possible to directly observe bosonic Mott insulators with single-atom resolution [4–6], giving access to in-situ measurements of temperature and entropy distributions. Subsequently, ground-breaking experiments in many-body quantum dynamics have been performed, such as the first direct observation of correlation spreading after quenches of the system to lower lattice depths [7]. Other experiments include the quantum simulation of an antiferromagnetic spin-1/2 chain [8], the realization of algorithmic cooling [9], the direct measurement of correlations across the superfluid-to-Mott-insulator transition [10], or the observation of the ‘Higgs’ amplitude mode [11]. The high-resolution imaging system is also capable of optically addressing individual atoms with sub-diffraction-limited resolution. For this purpose, a laser beam is focussed though the microscope objective onto selected lattice sites, bringing the atoms into resonance with microwave radiation which produces spin flips [12,13]. The preparation of arbitrary atom distributions in the lattice not only opens perspectives for neutral-atom quantum information processing, but also allows the deterministic creation of specific out-of-equilibrium situations, and their ensuing system dynamics can be studied with single-atom resolution. Examples include the creation of spin impurities and the study of their dynamics in a many-body system [14], the observation of quantum walks [15] and magnon-bound states [16], and the measurement of entanglement entropy [17]. The technique used for single-site-resolved detection of individual atoms in the optical lattice is fluorescence imaging. It requires laser cooling of the atoms to sub-Doppler temperatures while detecting the fluorescence photons emitted during this process. A challenge is to achieve a low particle loss rate and to prevent the atoms from hopping between lattice sites. In the case of the first quantum-gas microscopes, this has been implemented with bosonic rubidium-87 atoms, and also recently for ytterbium-174. However, it has proven more challenging for fermionic alkaline species such as potassium-40 and lithium-6, as their low mass and small excited state hyperfine splitting-prevented standard sub-Doppler cooling from working. As a result, it took several years to extend the quantum-gas microscope technique to fermionic species, and in 2015 several groups have demonstrated single-atom-resolved imaging of fermions [18]. In these setups, the direct observation of band insulators [19], fermionic Mott insulators [20,21] and even antiferromagnetic correlations [22] has been reported most recently. These developments provide fascinating opportunities to study many-body fermionic quantum systems with single-particle access. The fermionic nature of the electron is vital to understanding a range of phenomena within solid-state physics, such as electron pairing in superconductivity, and quantum magnetism. In this context, the quantum simulator will allow, e.g. for direct measurement of ordered quantum phases and out-of-equilibrium dynamics, with access to quantities such as spin--spin correlation functions and the growth of many-body entanglement. The high-resolution imaging systems could also be used to project tailored time-varying light fields onto the atoms, allowing the removal of high-entropy regions by locally modifying the confining potential to attain very low-entropy states. Attaining these low-entropy many-particle states is one of the key challenges in our field. In summary, quantum-gas microscopes offer unprecedented access to many-body quantum systems, and they will allow us to gain an even deeper insight into the intriguing world of quantum physics that governs the properties of strongly correlated materials. Using a well-controlled quantum system to simulate other quantum systems could help resolve some of the most challenging problems scientists face in condensed matter physics and materials science, but it will also provide new insights in quantum chemistry, high-energy physics and biology. SK acknowledges funding from the EU (ERC FERMILATT, SIQS, QuProCs) and SUPA.
This thesis describes the methods used to achieve single-atom and single-site resolved fluorescence imaging of a quantum gas of fermionic atoms held in an optical lattice. This kind of quantum gas microscopy has been previously applied to bosonic atoms in two experiments, and has allowed for extensive studies of many-body quantum systems. Here we detail methods to extend the techniques of quantum gas microscopy to fermionic species of atoms, opening the door to quantum simulation of electronic models of condensed matter physics. We prepare quantum degenerate gases of fermionic 40K through sympathetic cooling with a gas of \Rb. The gas prepared is trapped less than 1 mm away from a thin vacuum window, an experimental geometry which permits interrogation with a large Numerical Aperture microscope objective. The cloud is loaded into a 527 nm-period cubic optical lattice, ideal for quantum simulation of many-body models of fermionic particles such as the Hubbard model. The optical lattice is designed to allow for potential depths up to V/kB = 250 microKelvin, sufficient to strongly confine 40K atoms into individual lattice sites. EIT cooling --- a technique previously demonstrated in ion-trapping experiments --- is used to cool the fermionic atoms deeply towards the ground motional state of the lattice wells. We collect the photons scattered during continuous application of EIT cooling, while atoms are kept pinned in the lattice for many seconds. We strongly suppress all other sources of light with the use of a modulated cooling scheme and a spinning chopper wheel. Collecting the scattered light produced during 2 s of EIT cooling allows us to resolve single atoms in a fluorescence image. We demonstrate that the high resolution of our imaging apparatus also permits atom positions to be precisely identified on the length scale of the optical lattice. Thus we demonstrate the realisation of a quantum gas microscope for fermionic atoms, allowing for precise investigation of the ground states and dynamics of correlated quantum states of fermionic particles.
(English) Ultracold atoms have proven to be a valuable asset to study and understand complex quantum many-body systems in a well-controlled setting. In particular, quantum-gas microscopes provide unprecedented access to local observables and allow one to investigate those systems at the level of each individual particle, giving new insights on their behaviour. While so far most of these microscopes used alkali atoms, the distinct properties of alkaline-earth atoms, in particular strontium, combined with quantum-gas microscopy are expected to shed new light on a broad variety of many-body problems. This thesis describes the realization of single-site-resolved imaging for both bosonic and fermionic strontium atoms in a Hubbard-regime optical lattice, which unlocks the possibility to study novel types of Bose- and SU(N) Fermi-Hubbard systems. An essential step in ultracold-atom experiments is the preparation of a quantum degenerate cloud. In the first part of this thesis, we discuss the methods we have implemented in our apparatus to achieve this goal. We developed a new resonant-shielding method to double the atom number during the first cooling stage in a broad-linewidth blue magneto-optical trap. During the second cooling stage in a narrow-linewidth red magneto-optical trap, the hyperfine structure of the fermionic atoms adds additional challenges which are addressed by mixing the hyperfine states with an additional laser for efficient trapping and cooling. After the laser-cooling stages, we perform evaporative cooling in a far red-detuned optical potential before loading the atoms into a two-dimensional optical lattice. The lattice laser operates at the clock-magic wavelength of strontium (813.4nm) which will enable high-precision measurements in future experiments. To image the individual atoms in the optical lattice, we place a high-NA objective in close vicinity to the atoms. We demonstrate single-atom resolution for bosonic and fermionic strontium and successfully reconstruct the lattice occupation for both of them, reaching fidelities as high as 96%. For the bosonic Sr-84 atoms, we induce fluorescence on the blue broad-linewidth transition and simultaneously perform attractive Sisyphus cooling on the red narrow-linewidth transition. Moreover, combining this imaging method with momentum-space detection, we observe the matter-wave interference arising from the phase coherence of the Bose-Hubbard superfluid. For the fermionic Sr-87 atoms, we image with the red transition only, which allows us to obtain for the first time for a fermionic alkaline-earth atom both single-atom resolution and spin-resolved detection. This thesis has combined, for the first time, quantum-gas microscopy with ultracold strontium and its distinct spectral properties. This platform should enable a broad range of future studies. For the bosons, it unlocks investigation of the single-atom-resolved dissipative Bose-Hubbard systems and the exploration of collective atom-photon scattering in ordered atomic arrays. For the fermions, the spin-dependent single-atom detection provides the ideal setting for investigations of antiferromagnetic correlations in SU(N≤10) Fermi-Hubbard systems and the realization of exotic magnetic phases. (Català) Els àtoms ultrafreds han demostrat ser un recurs valuós per l’estudi i la comprensió de sistemes quàntics de molts cossos en un entorn controlat. En particular, els microscopis de gasos quàntics proporcionen un accés sense precedents a observables locals i permeten investigar aquests sistemes a nivell de cada partícula individual, oferint noves perspectives sobre el seu comportament. Malgrat que fins ara la majoria d’aquests microscopis han utilitzat àtoms alcalins, s’espera que les propietats distintives dels àtoms alcalinoterris, en particular l’estronci, combinades amb la microscòpia de gasos quàntics, aportin nova llum a una àmplia varietat de problemes de molts cossos. Aquesta tesi descriu la realització d’imatges amb resolució d’un sol punt de xarxa per a àtoms d’estronci bosònics i fermiònics en una xarxa òptica al règim de Hubbard. Aquest resultat desbloqueja la possibilitat d’estudiar nous tipus de sistemes de Bose-Hubbard i Fermi-Hubbard SU(N). Un pas essencial en els experiments d’àtoms ultrafreds és la preparació d’un núvol quàntic degenerat. A la primera part d’aquesta tesi, discutim els mètodes implementats al nostre aparell per aconseguir aquest objectiu. Hem desenvolupat un nou mètode de protecció en ressonància que duplica el nombre d’àtoms a la primera etapa de refredament, la trampa magneto-òptica blava de transició ampla. Durant la segona etapa de refredament en una trampa magneto-òptica vermella de línia estreta, l’estructura hiperfina dels àtoms fermiònics presenta nous reptes, que abordem barrejant els estats hiperfins amb un làser addicional per atrapar i refredar els àtoms eficientment. Després de les etapes de refredament làser, realitzem refredament per evaporació en una trampa dipolar abans de carregar els àtoms en una xarxa òptica bidimensional. El làser de la xarxa opera a la longitud d’ona màgica per a la transició rellotge de l’estronci (813.4nm), que permetrà mesures d’alta precisió en experiments futurs. Per prendre imatges dels àtoms individuals a la xarxa òptica, col·loquem un objectiu de microscopi d’alta obertura numèrica a poca distància dels àtoms. Hem demostrat la detecció d’àtoms individuals per a estronci bosònic i fermiònic, i hem reconstruït l’ocupació a la xarxa per a ambdós, assolint fidelitats de fins a un 96%. Per a l’isòtop bosònic de 84-Sr, hem induït fluorescència en la transició ampla blava i simultàniament hem realitzat refredament atractiu de Sísif en la transició estreta vermella. A més, mitjançant la combinació d’aquest mètode i la detecció en l’espai de moments, hem aconseguit observar la interferència de l’ona de matèria que sorgeix de la coherència de fase del superfluid Bose-Hubbard. Per als àtoms fermiònics de 87-Sr, només hem utilitzat la transició vermella per a la fluorescència, cosa que ens permet obtenir per primera vegada tant la resolució d’àtoms individuals com la detecció de l’estat d’espín en àtoms alcalinoterris. Aquesta tesi ha combinat, per primera vegada, la microscòpia de gasos quàntics amb l’estronci ultrafred i les seves propietats espectrals distintives. Aquesta plataforma habilita una àmplia varietat d’estudis en el futur. Per als bosons, desbloqueja la investigació de sistemes Bose-Hubbard dissipatius amb detecció d’àtoms individuals, així com l’exploració de la interacció col·lectiva d’àtoms i fotons en matrius ordenades d’àtoms. Per als fermions, la detecció individual d’àtoms i el seu estat d’espín proporciona l’entorn ideal per investigar les correlacions antiferromagnètiques en sistemes SU(N≤10) de Fermi-Hubbard i la realització de fases magnètiques exòtiques. (Español) Los átomos ultrafríos han demostrado ser un recurso valioso para el estudio y la comprensión de sistemas cuánticos de muchos cuerpos en un entorno controlado. En particular, los microscopios de gases cuánticos proporcionan un acceso sin precedentes a observables locales y permiten investigar esos sistemas a nivel de cada partícula individual, ofreciendo nuevas perspectivas sobre su comportamiento. Aunque hasta ahora la mayoría de estos microscopios han utilizado átomos alcalinos, se espera que las propiedades distintivas de los átomos alcalinotérreos, en particular el estroncio, combinadas con la microscopía de gases cuánticos, arrojen nueva luz sobre una amplia variedad de problemas de muchos cuerpos. Esta tesis describe la realización de imágenes con resolución de átomo único de la red para estroncio bosónico y fermiónico en una red óptica en régimen de Hubbard. Este resultado abre la posibilidad de estudiar nuevos tipos de sistemas de Bose-Hubbard y Fermi-Hubbard SU(N). Un paso esencial en los experimentos de átomos ultrafríos es el de preparar una nube cuántica degenerada. En la primera parte de esta tesis, discutimos los métodos implementados en nuestro aparato para lograr este objetivo. Hemos desarrollado un nuevo método de protección sobre una transición en que duplica el número de átomos en la primera etapa de enfriamiento, la trampa magneto-óptica azul sobre la transición ancha. Durante la segunda etapa de enfriamiento en una trampa magneto-óptica roja sobre la línea estrecha, la estructura hiperfina de los átomos fermiónicos presenta nuevos desafíos, que abordamos mezclando los estados hiperfinos con un láser adicional para lograr un atrapamiento y enfriamiento eficiente. Después de las etapas de enfriamiento láser, realizamos enfriamiento por evaporación en una trampa dipolar antes de cargar los átomos en una red óptica bidimensional. El láser de la red opera a la longitud de onda mágica para la transición reloj del estroncio (813.4nm), lo que permitirá mediciones de alta precisión en futures experimentos. Para tomar imágenes de los átomos individuales en la red óptica, colocamos un objetivo de microscopio de alta apertura numérica a poca distancia de los átomos. Demostramos la detección de átomos individuales para estroncio bosónico y fermiónico, y reconstruimos la ocupación en la red para ambos, alcanzando fidelidades de hasta un 96%. Para el isótopo bosónico de Sr-84, inducimos fluorescencia en la transición ancha azul y simultáneamente realizamos enfriamiento atractivo de Sísifo en la transición estrecha roja. Además, mediante la combinación este método y la detección en el espacio de momentos, conseguimos observar la interferencia de la onda de materia que surge de la coherencia de fase del superfluido de Bose-Hubbard. Para los átomos fermiónicos de Sr-87, utilizamos solo la transición roja para la fluorescencia, lo que nos permite obtener por primera vez tanto resolución de átomos individuales como detección del estado de espín en átomos a
We demonstrate site-resolved imaging of individual bosonic atoms in a Hubbard-regime two-dimensional optical lattice with a short lattice constant of 266 nm. To suppress the heating by probe light with the 1 S 0 – 1 P 1 transition of the wavelength λ = 399 nm for high-resolution imaging and preserve atoms at the same lattice sites during the fluorescence imaging, we simultaneously cool atoms by additionally applying narrow-line optical molasses with the 1 S 0 – 3 P 1 transition of the wavelength λ = 556 nm. We achieve a low temperature of , corresponding to a mean oscillation quantum number along the horizontal axes of 0.22(4) during the imaging process. We detect, on average, 200 fluorescence photons from a single atom within a 400 ms exposure time, and estimate a detection fidelity of 87(2)%. The realization of a quantum gas microscope with enough fidelity for Yb atoms in a Hubbard-regime optical lattice opens up the possibilities for studying various kinds of quantum many-body systems such as Bose and Fermi gases, and their mixtures, and also long-range-interacting systems such as Rydberg states.