p波相互作用するフェルミ原子気体の衝突特性

電気通信大学201

Towards Quantum Gas Microscopy of Ultracold Molecules

Ultracold atoms in optical lattices allow the study of large ensembles of strongly interacting quantum particles in an isolated environment. They are widely used as analogue quantum simulators to study phenomena from many areas of physics in a setting with high tunability and versatile detection methods. Many experiments now detect and control the atoms with single-site resolution in what are known as quantum gas microscopes. By extending these techniques to ultracold molecules it will be possible to extend the range of interparticle interactions and increase the diversity of types of quantum systems which can be studied in optical lattices. This thesis reports on the construction of a new apparatus which is designed to realise a quantum gas microscope for ultracold 87Rb 133Cs molecules. The apparatus consists of two trapping regions to provide sufficient optical access for the high numerical aperture lens and three-dimensional optical lattice required for quantum gas microscopy. By using degenerate Raman sideband cooling and fast moving-lattice optical transport followed by evaporation in an optical dipole trap we are able to cool both species to quantum degeneracy while maintaining a relatively fast repetition rate. Using a three-dimensional optical lattice we observe the superfluid to Mott insulator transition in 133Cs, demonstrating the ability to reach the strongly correlated regime. The final section of this thesis reports on preliminary experiments on fluorescence imaging of 133Cs atoms pinned to lattice sites, which will pave the way for the implementation of quantum gas microscopy of molecules

Assembling Single RbCs Molecules with Optical Tweezers

Optical tweezer arrays are useful tools for manipulating single atoms and molecules. An exciting avenue for research with optical tweezers is using the interactions between polar molecules for quantum computation or quantum simulation. Molecules can be assembled in an optical tweezer array starting from pairs of atoms. The atoms must be initialised in the relative motional ground state of a common trap. This work outlines the design of a Raman sideband cooling protocol which is implemented to prepare an 87-Rubidium atom in the motional ground state of an 817 nm tweezer, and a 133-Caesium atom in the motional ground state of a 938 nm tweezer. The protocol circumvents strong heating and dephasing associated with the trap by operating at lower trap depths and cooling from outside the Lamb-Dicke regime. By analysing several sources of heating, we design and implement a merging sequence that transfers the Rb atom and the Cs atom to a common trap with minimal motional excitation. Subsequently, we perform a detailed characterisation of AC Stark shifts caused by the tweezer light, and identify several situations in which the confinement of the atom pair influences their interactions. Then, we demonstrate the preparation of a molecular bound state after an adiabatic ramp across a magnetic Feshbach resonance. Measurements of molecular loss rates provide evidence that the atoms are in fact associated during the merging sequence, before the magnetic field ramp. By preparing a weakly-bound molecule in an optical tweezer, we carry out important steps towards assembling an array of ultracold RbCs molecules in their rovibrational ground states

Single-Atom Resolved Imaging and Manipulation in an Atomic Mott Insulator

This thesis reports on new experimental techniques for the study of strongly correlated states of ultracold atoms in optical lattices. We used a high numerical aperture imaging system to probe 87Rb atoms in a two-dimensional lattice with single-site resolution. Fluorescence imaging allows to detect single atoms with a large signal to noise ratio and to reconstruct the atom distribution on the lattice. We applied this new technique to a two-dimensional Mott insulator and directly observed number squeezing and the emerging shell structure. A comparison of the radial density and variance distributions to theory provides a precise in situ temperature and entropy measurement from single images. We find entropies around the critical value for quantum magnetism. In a second series of experiments, we demonstrated two-dimensional single-site spin control in the optical lattice. The differential light shift of a tightly focused laser beam shifts selected atoms into resonance with a microwave field driving a spin flip. In this way, we reach sub-diffraction limited spatial resolution well below the lattice spacing. Starting from a Mott insulator with unity filling we were able to create arbitrary spin patterns. We used this ability to prepare atom distributions to study one-dimensional single-particle tunneling dynamics in a lattice. By discriminating the dynamics of the ground state and of the first excited band, we find that our addressing scheme leaves most atoms in the vibrational ground state. Moreover, we studied coherent light scattering from the atoms in the optical lattice and found diffraction maxima in the far-field. We showed that an antiferromagnetic order leads to additional diffraction peaks which can be used to detect this order also when single-site resolution is not available. The new techniques described in this thesis open the path to a wide range of novel applications from quantum dynamics of spin impurities, entropy transport, implementation of novel cooling schemes, and engineering of quantum many-body phases to quantum information processing.In dieser Arbeit werden neue experimentelle Techniken für die Untersuchung von stark korrelierten Zuständen von ultrakalten Atomen in optischen Gittern vorgestellt. Wir untersuchen 87Rb Atome in einem zwei-dimensionalen Gitter und erreichen dabei eine Auflösung der einzelnen Gitterplätze mit Hilfe eines hochauflösenden Abbildungssystems. Fluoreszenzabbildung erlaubt es, einzelne Atome mit großem Signal-zu-Rausch-Verhältnis zu detektieren und die Verteilung der Atome auf dem Gitter zu rekonstruieren. Wir wenden diese neue Technik auf einen zwei-dimensionalen Mott-Isolator an and beobachten direkt das number squeezing und die Schalenstrukur. Ein Vergleich der radialen Dichte- und Varianzverteilung mit der Theorie ermöglicht eine präzise Temperatur- und Entropiemessung an einzelnen Bildern und wir finden Entropien um den kritischen Wert für Quantenmagnetismus. In einer zweiten Reihe von Experimenten zeigen wir, dass wir gezielt einzelne atomare Spinzustände im Gitter manipulieren können ohne die benachbarten Atome zu beeinflussen. Wir benutzen den differentiellen light shift eines stark fokussierten Laserstrahls, um einzelne Atome in Resonanz mit einem Mikrowellenfeld zu bringen, das den Spin umklappt. Auf diese Weise erreichen wir eine Ortsauflösung unter der Beugungsgrenze. Wir beginnen mit einem Mott-Isolator mit einem Atom pro Gitterplatz und können darin beliebige Spinmuster erzeugen. Diese neuen Möglichkeiten zur Präparation atomarer Verteilungen nutzen wir, um die eindimensionale Einteilchen-Tunneldynamik in einem Gitter zu untersuchen. Wir unterscheiden die Dynamik von Atomen im Grundzustand und im ersten angeregten Band und zeigen so, dass unser Adressierschema die meisten Atome im Grundzustand lässt. Darüber hinaus untersuchen wir kohärente Lichtstreuung an den Atomen im Gitter und finden Beugungsmaxima im Fernfeld. Wir zeigen, dass eine antiferromagnetische Ordnung der Atome zu zusätzlichen Beugungsmaxima führt, die man auch ohne unsere hohe Auflösung zum Nachweis dieser Ordnung nutzen könnte. Die neuen Techniken, die in dieser Arbeit vorgestellt werden, öffnen den Weg für viele neue Anwendungen von der Quantendynamik von Spin-Defekten, Entropietransport, der Umsetzung neuer Kühlschemata sowie der Realisierung von Quanten-Vielteilchenphasen bis hin zur Quanteninformationsverarbeitung

Construction of a caesium quantum gas microscope

In dieser Arbeit beschreibe ich den Aufbau eines neues Quantengasmikroskops. Das Ziel des Experiments ist die Simulation von topologischen Vielteilchensystemen in optischen Gittern. Als Atomspezies wurde aufgrund einer leicht zugänglichen Feshbach Resonanz bei niedrigem magnetischem Feld und seiner großen Feinstrukturaufspaltung Cäsium gewählt. Die Feshbach Resonanz erlaubt die Änderung der Wechselwirkung zwischen den Atomen. Die große Feinstrukturaufspaltung ermöglicht es die Atome in ein anti-magisches Gitter zu laden ohne in der experimentellen Versuchsdauer durch die Photonstreurate limitiert zu sein. Durch Ramanübergänge zwischen unterschiedlichen Hyperfeinzuständen von Cäsium kann dann ein künstliches Magnetfeld simuliert werden, eine essenzielle Methode für die Realisierung von Chernisolatoren. Die hohe numerische Apertur die für ein Quantengasmikroskop benötigt wird beschränkt den optischen Zugang zu den Atomen. Dies erschwert den Aufbau der optischen Laserstrahlen zum Kühlen und Manipulieren der Atome. Wir verwenden optischen Transport mithilfe eines laufenden optischen Gitters um die Atome nach einer Vorkühlphase in einen anderen Teil der Vakuumkammer, einer Glaszelle, zu schieben. Dies erlaubt es, die Laserstrahlen die fürs Vorkühlen benötigt werden unabhängig vom Mikroskopobjektiv auf die Atome auszurichten. Das Transportgitter wird durch die Interferenz zwischen einem Gauß-förmigen Laserstrahl und einem Bessel-förmigen Laserstrahl erzeugt. Der Besselstrahl, ein nahezu beugungsfreier Laserstrahl, erlaubt es die Atome über eine \SI{43}{cm} lange Transportdistanz gegen Gravitation zu halten. Wir transportieren \SI{3e6}{Atome} von der MOT Kammer in die Glasszelle in weniger als \SI{26}{ms}, ohne dabei die Temperatur zu erhöhen. Die Transporteffizienz ist etwa 75\% und durch Gravitation und Atomverluste am Anfang des Transports limitiert. Sobald die Atome in der Glaszelle ankommen, werden sie in eine gekreuzte Dipolfalle umgeladen. Wir evaporieren indem die Fallentiefe reduziert und die Falle gekippt wird. Nach der Kondensation wird das BEC in eine einzelne Ebene eines vertikalen Gitters und darauf folgend in ein horizontales Gitter geladen. Um die Atome durch das Mikroskopobjektiv abzubilden wird Fluoreszenzlicht verwendet. Während der Fluoreszenzabbildung werden die Atome durch optische Molasse gekühlt und die optischen Gitter auf etwa 120\,µK vertieft damit die Atome in 1\,s um die 25.000 Fluoreszenzphotonen streuen können ohne im Gitter zu tunneln. Der in dieser Arbeit beschriebene experimentelle Aufbau wird es uns ermöglichen den Einfluss von Wechselwirkungen auf topologische Phasen mit Einteilchenauflösung zu untersuchen. Dies erlaubt es, Annahmen über die mikroskopische Dynamik in diesen Phasen zu testen und unser Verständnis zu vertiefen.In this work I describe the setup of a new quantum gas microscope. The goal of the experiment is the simulation of topological many-body systems in lattices. Caesium was picked as atomic species, because of its easily accessible Feshbach resonance at low magnetic fields and its large fine-structure splitting. The Feshbach resonance allows changing the interaction between atoms. The large fine-structure splitting enables loading the atoms into an anti-magic lattice without limiting the experiment duration via scattering of lattice photons. Using Raman transitions between different hyperfine states of caesium, an artificial magnetic field can be simulated, an essential method for the realization of Chern insulators. The high numerical aperture necessary for a quantum gas microscope limits the optical access to the atoms. This complicates the setup of the optical laser beams for cooling and manipulating the atoms. We use optical transport based on a running wave optical lattice to transfer the atoms after pre-cooling into a different section of the vacuum system, a glass cell. This allows alignment of the pre-cooling laser beams independent of the microscope objective. The transport lattice is created via interference between a Gaussian laser beam and a Bessel beam. The Bessel beam, a diffractionless laser beam, enables us to hold the atoms against gravity over the transport distance of \SI{43}{cm}. We transport \SI{3e6}{atoms} from the MOT chamber to the glass cell in less than \SI{26}{ms} without any temperature increase. The transport efficiency is around 75\%, limited by gravity and loss at the start of transport. After the atoms have arrived in the glass cell they are transferred into a crossed dipole trap. We evaporate the atoms by reducing the trap depth and tilting the trap. After condensation we trap the BEC in a single plane of a vertical lattice. The BEC is subsequently loaded into a 2D horizontal lattice. Fluorescence light is used to image the atoms through the microscope objective. During fluorescence imaging, the atoms are cooled using an optical molasses and the optical lattice depth is increased to around 120\,µK to allow the atoms to scatter up to 25.000 fluorescence photons in 1\,s without tunneling in the lattice. The experimental setup detailed in this thesis will allow us to study the effects of interactions on topological phases of matter with single particle resolution. This paves the way to testing our assumptions and extending understanding of the microscopic dynamics in these phases

Measuring the Gap and Investigating Non-equilibrium in the BEC-BCS Crossover

This thesis presents a new Bose-Fermi mixture quantum gas experiment that has been used to measure the gap over the BEC-BCS crossover and investigate the non-equilibrium dynamics of a superfluid in response to a quench of the interaction strength. Bosonic 23Na sympathetically cools fermionic 6Li in an optically plugged magnetic trap before transferring the atoms to an optical dipole trap. The broad Feshbach resonance of 6Li is then used to tune the scattering length and by entering the strongly interacting regime, very efficient evaporation can be performed. Bose-Einstein condensation of molecules with over 5×1066Li atoms per spin state has been observed and temperatures T/TF=0.07±0.02 have been achieved. The broad Feshbach resonance of 6Li can then be used to bring the atoms into the BCS regime, where long range Cooper pairs of opposite spin and momentum form a superfluid state, or into the universality regime, where the scattering length diverges and the system obeys universal laws. A technique was developed whereby the population of one component of the superfluid was continuously modulated with a specific frequency. Theoretical studies show that this excitation couples to the amplitude/Higgs mode of the superfluid order parameter, which should have a resonance frequency at twice the gap value. By measuring the response of the condensate fraction at various modulation frequencies, a measure of the gap in the BEC-BCS crossover could be extracted. The measured gap value was found to be in agreement with the mean-field theory calculations and quantum Monte Carlo simulations. Extending the same method, it was possible to rapidly change one of the components of a superfluid to a different third component. The inversion was performed in less than 50μm, faster than the dynamical gap time and quasiparticle relaxation time. This provides an excellent realization of the fast quenches of the interaction strength that have been intensely investigated theoretically. By quenching a strongly interacting superfluid to much weaker interactions, the decay of the order parameter was studied. During these non-equilibrium dynamics, evidence of a revival of the order parameter has been observed at longer time scales for weak quenches. Additionally, a weakly interacting normal mixture above the critical temperature can be quenched to interaction strengths where, in equilibrium, a superfluid should be present. The emergence of the order parameter was measured as a function of time and was found to be faster for weak quenches into the BCS regime than for larger quenches into unitarity