Transition from a polaronic condensate to a degenerate Fermi gas of heteronuclear molecules

The interplay of quantum statistics and interactions in atomic Bose--Fermi mixtures leads to a phase diagram markedly different from pure fermionic or bosonic systems. However, investigating this phase diagram remains challenging when bosons condense. Here, we observe evidence for a quantum phase transition from a polaronic to a molecular phase in a density-matched degenerate Bose--Fermi mixture. The condensate fraction, representing the order parameter of the transition, is depleted by interactions and the build-up of strong correlations results in the emergence of a molecular Fermi gas. By driving through the transition, we ultimately produce a quantum-degenerate sample of sodium-potassium molecules exhibiting a large molecule-frame dipole moment of 2.7 Debye. The observed phase transition represents a new phenomenon complementary to the paradigmatic BEC-BCS crossover observed in Fermi systems.Comment: 12 pages, 9 figure

Observation of confinement-induced resonances in a 3D lattice

We report on the observation of confinement-induced resonances for strong three-dimensional (3D) confinement in a lattice potential. Starting from a Mott-insulator state with predominantly single-site occupancy, we detect loss and heating features at specific values for the confinement length and the 3D scattering length. Two independent models, based on the coupling between the center-of-mass and the relative motion of the particles as mediated by the lattice, predict the resonance positions to a good approximation, suggesting a universal behavior. Our results extend confinement-induced resonances to any dimensionality and open up an alternative method for interaction tuning and controlled molecule formation under strong 3D confinement.Comment: 9 pages, 5 figure

Ultracold Dense Samples of Dipolar RbCs Molecules in the Rovibrational and Hyperfine Ground State

We produce ultracold dense trapped samples of Rb87Cs133 molecules in their rovibrational ground state, with full nuclear hyperfine state control, by stimulated Raman adiabatic passage (STIRAP) with efficiencies of 90%. We observe the onset of hyperfine-changing collisions when the magnetic field is ramped so that the molecules are no longer in the hyperfine ground state. A strong quadratic shift of the transition frequencies as a function of applied electric field shows the strongly dipolar character of the RbCs ground-state molecule. Our results open up the prospect of realizing stable bosonic dipolar quantum gases with ultracold molecules

Collisions of ultracold molecules in bright and dark optical dipole traps

Understanding collisions between ultracold molecules is crucial for making stable molecular quantum gases and harnessing their rich internal degrees of freedom for quantum engineering. Transient complexes can strongly influence collisional physics, but in the ultracold regime, key aspects of their behavior have remained unknown. To explain experimentally observed loss of ground-state molecules from optical dipole traps, it was recently proposed that molecular complexes can be lost due to photo-excitation. By trapping molecules in a repulsive box potential using laser light near a narrow molecular transition, we are able to test this hypothesis with light intensities three orders of magnitude lower than what is typical in red-detuned dipole traps. This allows us to investigate light-induced collisional loss in a gas of nonreactive fermionic $^{23}$Na$^{40}$K molecules. Even for the lowest intensities available in our experiment, our results are consistent with universal loss, meaning unit loss probability inside the short-range interaction potential. Our findings disagree by at least two orders of magnitude with latest theoretical predictions, showing that crucial aspects of molecular collisions are not yet understood, and provide a benchmark for the development of new theories.Comment: 13 pages, 11 figure

Ultracold field-linked tetratomic molecules

Ultracold polyatomic molecules offer intriguing new opportunities in cold chemistry, precision measurements, and quantum information processing, thanks to their rich internal structure. However, their increased complexity compared to diatomic molecules presents a formidable challenge to employ conventional cooling techniques. Here, we demonstrate a new approach to create ultracold polyatomic molecules by electroassociation in a degenerate Fermi gas of microwave-dressed polar molecules through a field-linked resonance. Starting from ground state NaK molecules, we create around $1.1\times 10^3$ tetratomic (NaK)$_2$ molecules, with a phase space density of $0.040(3)$ at a temperature of $134(3)\,\text{nK}$, more than $3000$ times colder than previously realized tetratomic molecules. We observe a maximum tetramer lifetime of $8(2)\,\text{ms}$ in free space without a notable change in the presence of an optical dipole trap, indicating these tetramers are collisionally stable. The measured binding energy and lifetime agree well with parameter-free calculations, which outlines pathways to further increase the lifetime of the tetramers. Moreover, we directly image the dissociated tetramers through microwave-field modulation to probe the anisotropy of their wave function in momentum space. Our result demonstrates a universal tool for assembling ultracold polyatomic molecules from smaller polar molecules, which is a crucial step towards Bose--Einstein condensation (BEC) of polyatomic molecules and towards a new crossover from a dipolar Bardeen-Cooper-Schrieffer (BCS) superfluid to a BEC of tetramers. Additionally, the long-lived FL state provides an ideal starting point for deterministic optical transfer to deeply bound tetramer states

Quantum engineering of a low-entropy sample of RbCs molecules in an optical lattice

Ultrakalte Moleküle mit elektrischen Dipolmomenten erfahren derzeit großes Interesse seitens theoretischer und experimenteller Quantenphysiker. Auf Grund der lange Lebensdauer stabiler Grundzustandsmoleküle in Kombination mit der starken, langreichweitigen Wechselwirkung der elektrischen Dipolmomente sind dipolare Moleküle ideal geeignet zur Erforschung von exotischen Quanten-Zuständen in optischen Gittern, wie zum Beispiel der Suprasolidität eines wechselwirkenden Vielteilchen-Systems. Weitere potentielle Anwendungen finden dipolare Moleküle in der Simulation von Spin-Systemen oder in der Realisierung von Quantencomputern. Die Umsetzung der meisten dieser Vorschläge basiert auf Ensembles von dipolaren Molekülen mit geringer Entropie in Gittern. Solche ultrakalten Molekül-Ensembles in optischen Gittern zu präparieren, stellt dabei jedoch eine große experimentelle Herausforderung dar. In der Vergangenheit gelang es nicht, Ensembles aus dipolaren Molekülen mit geringer Entropie in Gittern zu realisieren. Die Molekül-Ensembles wurden üblicherweise zunächst ohne Gitter erzeugt und in manchen Fällen im Anschluss in ein optisches Gitter geladen. Die erreichten Phasenraumdichten der Molekülensembles waren zu gering, um Systeme mit geringer Entropie zu erzeugen. In dieser Dissertation wird eine neue Methode vorgestellt, bei der Ensembles aus schwachgebundenen, heteronuklearen Molekülen direkt in einem optischen Gitter erzeugt werden. Dadurch lassen sich deutlich niedrigere Entropien erreichen, was einen idealen Ausgangspunkt für weiterführende Experimente mit dipolaren Molekülen in Gittern darstellt. Diese Arbeit ist wie folgt strukturiert: Zunächst wird die Erzeugung von 87Rb133Cs-Molkülen, den dipolaren Molekülen unserer Wahl, allgemein beschrieben. Ausgangspunkt sind hierfür ultrakalte Ensembles aus Rb- und Cs-Atomen. Die Atome werden mit Hilfe einer Feshbach-Resonanz zu schwachgebundenen RbCs-Molekülen zusammengeführt. Anschließend werden die schwachgebundenen Moleküle mit einem stimulierten, adiabatischen Raman-Übergang (STIRAP) in ihren rovibronischen und hyperfeinen Grundzustand überführt. Als nächstes wird unsere Methode präsentiert, mit der schwachgebundene RbCs-Moleküle direkt in einem dreidimensionalen (3D) optischen Gitter erzeugt werden. Wir beginnen mit räumlich getrennten Rb- und Cs-Bose-Einstein-Kondensaten (BECs). Die BECs werden in ein 3D optisches Gitter geladen. Das Gitter ist so konzipiert, dass das Cs-Ensemble in einem Einzelschalen-Mott-Isolator ausgefroren werden kann, während das Rb-Ensemble noch suprafluid ist. Das Rb-Ensemble wird anschließend auf den Cs-Mott-Isolator befördert. Schließlich wird die Tiefe des Gitters weiter erhöht, um auch das Rb Ensemble in einem Mott-Isolator-Zustand auszufrieren. Auf diese Weise lassen sich Rb-Cs-Paare auf individuellen Gitterplätzen präparieren. Diese Paare werden anschließend in RbCs-Feshbach-Moleküle überführt. Wir erzeugen mehr als 5000 RbCs-Moleküle mit einer Gitterbesetzung von mehr als 30%, was in etwa einer Entropie von 2 k_B pro Molekül entspricht. Zuletzt charakterisieren wir zwei wichtige Aspekte, die die Effizienz unserer Molekül-Produktion einschränken: Die kritische Transportgeschwindigkeit, bei der der suprafluide Transport zusammenbricht, und die Stabilität des Cs-Mott-Isolators. Die gemessene kritische Transportgeschwindigkeit wird mit den Messwerten anderer Gruppen verglichen und der Parameterraum, in dem ein superfluider Transport realisiert werden kann, wird dargestellt. Darüber hinaus diskutieren wir, ob sogenannte Einschlussresonanzen (engl.: confinement-induced resonances) für einen Zerfall des Einzelschalen-Cs-Mott-Isolators, den wir bei starker repulsiver Cs-Wechselwirkung beobachten, verantwortlich sein können.Ultracold molecules with electric dipole moments are currently of large interest to the community of experimental and theoretical quantum physicists. Due to the long lifetime of stable ground-state molecules in combination with the strong long-range interaction of the electronic dipole moments, dipolar molecules are ideal candidates to study exotic quantum phases in optical lattices, such as the many-body lattice super solid. Further potential applications of dipolar molecules are the simulation of spin systems, or to use them as a platform for quantum computation. Most of these proposals require a low-entropy sample of dipolar molecules in a lattice. However, it is a great experimental challenge to prepare such ultracold molecular samples in an optical lattice. In the past the realization of low-entropy samples of dipolar molecules in lattices was not feasible. The molecular samples were typically created in the absence of a lattice and in some cases the samples were loaded into an optical lattice subsequently. The resulting phase-space densities of the molecular samples were too low to form low-entropy samples. In this thesis a novel method is introduced that is based on the formation of weakly bound heteronuclear molecules directly in an optical lattice. This way, a significantly lower entropy can be achieved, which presents an ideal starting point to study dipolar molecules in a lattice. This thesis is structured as follows: Initially, the preparation of 87Rb133Cs ground-state molecules, the dipolar molecules of our choice, is introduced. We start from ultracold samples of Rb and Cs atoms. The atoms are associated to weakly bound RbCs molecules by means of a magnetic Feshbach resonance. Subsequently the weakly bound molecules are transferred to their rovibrational and hyperfine ground-state via stimulated Raman adiabatic passage (STIRAP). Next, we demonstrate our method to form the weakly bound RbCs molecules directly in a three-dimensional (3D) optical lattice. We start from spatially separated Rb and Cs Bose-Einstein condensates (BECs). The BECs are loaded into a 3D lattice. The lattice is designed in a way that the Cs sample is frozen out in a single-shell Mott insulator, so that the single Cs atoms are isolated on individual lattice sites, while the Rb sample is still superfluid. The Rb sample is subsequently transported onto the Cs Mott insulator. Finally the depth of the lattice is further increased to also freeze out the Rb atoms in a Mott insulator state. In this way, Rb-Cs pairs can be prepared on individual lattice sites. These pairs are subsequently converted into RbCs Feshbach molecules. We create samples of more than 5000 RbCs molecules with a lattice filling of more than 30%, corresponding to an entropy per molecule of about 2 k_B. Finally, we characterize two important aspects that limit our molecule production efficiency: The critical transport velocity at which the superfluid transport breaks down and the stability of the Cs Mott insulator. We compare the critical transport velocity that we measure with results from other groups and map out the parameter regime in which superfluid transport can be realized. Furthermore, we discuss whether confinement induced resonances might be the reason for a decay of the single-shell Cs Mott insulator that we observe at large repulsive Cs interaction.by Andreas SchindewolfKumulative Dissertation aus zwei ArtikelnZusammenfassung in deutscher SpracheUniversity of Innsbruck, Dissertation, 2018OeBB(VLID)288020