Extending rotational coherence of interacting polar molecules in a spin-decoupled magic trap

Superpositions of rotational states in polar molecules induce strong, long-range dipolar interactions. Here we extend the rotational coherence by nearly one order of magnitude to 8.7(6) ms in a dilute gas of polar $^{23}$Na$^{40}$K molecules in an optical trap. We demonstrate spin-decoupled magic trapping, which cancels first-order and reduces second-order differential light shifts. The latter is achieved with a dc electric field that decouples nuclear spin, rotation and trapping light field. We observe density-dependent coherence times, which can be explained by dipolar interactions in the bulk gas.Comment: 10 pages, 8 figure

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

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

Understanding and controlling the collisions of ultracold polar molecules

Ultrakalte polare Moleküle bieten eine Vielzahl von faszinierenden Möglichkeiten für physikalische Forschung. Die Vorschläge für ihre Nutzung reichen von Quantenchemie, wo sie detaillierte Untersuchung von Reaktionsprozessen erlauben, über Präzisionsspektroskopie zur Detektion von Supersymmetrie oder Dunkler Materie, bis hin zur Simulation stark wechselwirkender Quantenmaterie. Die hohe Anzahl von internen Freiheitsgraden der Moleküle, die all diese Anwendungen erst ermöglicht, bringt jedoch auch signifikante Schwierigkeiten mit sich. Entsprechend ist es eine nicht zu unterschätzende Aufgabe, ultrakalte Moleküle in Experimenten herzustellen und zu kontrollieren. Obwohl während der letzten Jahrzehnte große Anstrengungen unternommen wurden, experimentelle Techniken zu entwickeln und verbessern, ist die Kühlung von Molekülen noch immer schwierig und die tiefsten erreichten Temperaturen sind weit von dem entfernt, was mit atomaren Gasen möglich ist. Aus diesem Grund konnte bis heute die überwiegende Mehrzahl der vorhergesagten neuen Quantenphasen nicht realisiert werden. In den vergangenen zehn Jahren waren unverstandene und verlustbehaftete Stoßprozesse zwischen Molekülen das größte Hindernis. Sie machten evaporative Kühlung sehr schwierig und begrenzten die Lebensdauer stark wechselwirkender Gase mit hoher Dichte. In dieser Dissertation berichte ich über Fortschritte bei der experimentellen Kontrolle von fermionischen polaren Molekülen der Spezies ²³Na⁴⁰K, die es schließlich erlaubten, die Stoßprozesse zu regulieren und evaporative Kühlung zu demonstrieren. In Kapitel 1 gebe ich eine Einführung in die Molekülphysik. Ich erkläre dort die grundlegenden Eigenschaften zweiatomiger Moleküle und ihrer Quantenzustände sowie deren gruppentheoretische Beschreibung. Dies ermöglicht das Verständnis der Herausforderungen des Forschungsfeldes. Darauf folgt in Kapitel 2 eine chronologische Zusammenfassung der Entwicklungen dieses Feldes in den letzten Jahrzehnten. Diese soll nicht nur die erfolgreichen Pfade betrachten, sondern auch die Vielzahl an Sackgassen und Umwegen, ohne die der Erfolg nicht möglich gewesen wäre. Ab Kapitel 3 beschreibe ich neue Forschung. Kapitel 3 ist auf eine technische Verbesserung fokussiert: Es beschreibt ein neues Lasersystem für stimulated Raman adiabatic passage (STIRAP), das unser Team gebaut und getestet hat. Dieses Lasersystem hat die Effizienz der Molekülerzeugung verbessert und einen großen Beitrag zur Vereinfachung unserer Experimente geleistet. Kapitel 4 enthält eine Untersuchung des X¹Σ⁺ ↔ b³Π₀-Übergangs im ²³Na⁴⁰K-Molekül. Es handelt sich hierbei um einen nahezu dipolverbotenen Übergang mit entsprechend schmaler Linienbreite, der außerdem im Spektrum der Übergänge relativ isoliert liegt. Dies erlaubt es, nahe am Übergang sowohl anziehende als auch abstoßende optische Dipolfallen zu erzeugen, ohne dabei hohe Photonenstreuraten in Kauf nehmen zu müssen. Außerdem kann durch kleine Änderungen der Wellenlänge die Wirkung der Dipolfalle auf bestimmte Rotationszustände gesteuert werden, was sich für viele Experimente als nützlich erwiesen hat. Kapitel 5 beinhaltet eine Untersuchung von Stößen zwischen zwei Molekülen im internen Grundzustand bei extrem geringer Lichtintensität. Mithilfe einer repulsiven, zylinderförmigen Dipolfalle konnte unser Team zeigen, dass die Verlustrate bei Zweikörperstößen von ²³Na⁴⁰K nicht von der Lichtintensität abhängt. Diese Messung steht im Widerspruch mit theoretischen Vorhersagen sowie Beobachtungen an anderen Molekülspezies, wurde jedoch von Kollegen aus Hong Kong und Hannover bestätigt. Dieses Rätsel ist aktuell ungelöst. In Kapitel 6 beschreibe ich die erfolgreiche Demonstration einer Methode zur Unterdrückung der Zweikörperverluste. Mittels starker Mikrowellenfelder werden dabei Wechselwirkungen erzeugt, die bei ausreichend kurzem Abstand abstoßend wirken und Moleküle daran hindern, sich zu nahe zu kommen. Mit diesem Mikrowellenschild konnte unser Team evaporative Kühlung von polaren Molekülen durchführen und das derzeit kälteste molekulare Quantengas bei einer Temperatur von 21nK herstellen. Eine Reihe an Vorhersagen, die bis jetzt nicht experimentell realisiert werden konnten, sind damit nun erreichbar geworden. Daher gebe ich schließlich in Kapitel 7 einen Überblick über diese neuen Möglichkeiten. Dieses Kapitel enthält Abschätzungen darüber wie realistisch die Umsetzung ausgewählter Vorhersagen ist und welche experimentellen Herausforderungen zu erwarten sind.Ultracold polar molecules offer a multitude of fascinating possibilities for physical research. Proposals for their use reach from quantum chemistry, where they enable detailed studies of reaction processes, to precision spectroscopy for detecting supersymmetry or dark matter, to simulating of strongly interacting quantum matter. However, the large number of internal degrees of freedom, which enables these applications, also causes significant difficulties. Consequently, the task of preparing and controlling molecules in experiments is not to be underestimated. Though large efforts have been undertaken to develop and improve experimental techniques during recent decades, the cooling of molecules remains difficult, and the lowest reachable temperatures are still far higher than with atoms. Therefore, the majority of predicted new quantum phases could not be realised yet. During the last ten years, the main obstacle were lossy collision processes between molecules, which are still not understood. They have made evaporative cooling very difficult and have limited the lifetime of strongly interacting gases at high density. In this dissertation, I report on progress in the experimental control of fermionic polar molecules of ²³Na⁴⁰K, which allowed regulating the collisions and demonstrating evaporative cooling. In Chapter 1, I give an introduction to molecule physics. I explain the fundamental properties of diatomic molecules and their quantum states, as well as their group-theoretical description. This allows understanding of the challenges of the field. Chapter 2 follows with a chronological summary of the developments in ultracold-molecule research during recent decades. It focuses not only on the paths which turned out to be successful, but also sheds light on the many detours and dead ends without which success could not have been achieved. Beginning from Chapter 3, I describe original research. Chapter 3 is focused on a technical improvement: it describes a new laser system for stimulated Raman adiabatic passage (STIRAP), which our team built and tested. This laser system has improved the efficiency of molecule creation and has contributed greatly to simplifying our experiments. Chapter 4 contains an investigation of the X¹Σ⁺ ↔ b³Π₀ transition in the ²³Na⁴⁰K molecule. This transition is almost dipole-forbidden with a correspondingly small linewidth. Additionally, its position is relatively isolated in the transition spectrum. This allows the creation of both attractive and repulsive dipole traps near the transition. Furthermore, small wavelength changes can influence the effect of the dipole trap on specific rotational states, which has turned out to be useful for many experiments. Chapter 5 treats collisions between molecules in their internal ground state at extremely low light intensity. By using a repulsive, cylinder-shaped dipole trap, our team was able to show that the loss rate in two-body collisions of ²³Na⁴⁰K is independent of light intensity. This measurement is in strong contradiction with theoretical predictions and with observations on other molecule species, but has been confirmed by colleagues in Hong Kong and Hannover. As of now, this riddle remains unsolved. In Chapter 6, I describe the successful demonstration of a method for suppressing these two-body losses. With strong microwave fields, interactions can be induced which become repulsive at sufficiently small distance and thereby prevent colliding molecules from coming too close to each other. This microwave shielding has enabled our team to demonstrate evaporative cooling of polar molecules, and to create the currently coldest molecular quantum gas at a temperature of 21nK. With this, a number of predictions, which were previously out of experimental reach have now become feasible. Therefore, in Chapter 7, I give an overview of these new possibilities. This chapter contains estimations about how realistic certain proposals are and which experimental challenges are to be expected

Efficient conversion of closed-channel dominated Feshbach molecules of 23^{23}Na40^{40}K to their absolute ground state

We demonstrate the transfer of $^{23}$Na$^{40}$K molecules from a closed-channel dominated Feshbach-molecule state to the absolute ground state. The Feshbach molecules are initially created from a gas of sodium and potassium atoms via adiabatic ramping over a Feshbach resonance at 78.3$\,$G. The molecules are then transferred to the absolute ground state using stimulated Raman adiabatic passage with an intermediate state in the spin-orbit-coupled complex $|c^3 \Sigma^+, v=35, J=1 \rangle \sim |B^1\Pi, v=12, J=1\rangle$. Our measurements show that the pump transition dipole moment linearly increases with the closed-channel fraction. Thus, the pump-beam intensity can be two orders of magnitude lower than is necessary with open-channel dominated Feshbach molecules. We also demonstrate that the phase noise of the Raman lasers can be reduced by filter cavities, significantly improving the transfer efficiency.Comment: 8 pages, 7 figure

Suppression of Unitary Three-body Loss in a Degenerate Bose-Fermi Mixture

We study three-body loss in an ultracold mixture of a thermal Bose gas and a degenerate Fermi gas. We find that at unitarity, where the interspecies scattering length diverges, the usual inverse-square temperature scaling of the three-body loss found in non-degenerate systems is strongly modified and reduced with the increasing degeneracy of the Fermi gas. While the reduction of loss is qualitatively explained within the few-body scattering framework, a remaining suppression provides evidence for the long-range RKKY interactions mediated by fermions between bosons. Our model based on RKKY interactions quantitatively reproduces the data without free parameters, and predicts one order of magnitude reduction of the three-body loss coefficient in the deeply Fermi-degenerate regime

Long-lived fermionic Feshbach molecules with tunable pp-wave interactions

Ultracold fermionic Feshbach molecules are promising candidates for exploring quantum matter with strong $p$-wave interactions, however, their lifetimes were measured to be short. Here, we characterize the $p$-wave collisions of ultracold fermionic $^{23}\mathrm{Na}^{40}\mathrm{K}$ Feshbach molecules for different scattering lengths and temperatures. By increasing the binding energy of the molecules, the two-body loss coefficient reduces by three orders of magnitude leading to a second-long lifetime, 20 times longer than that of ground-state molecules. We exploit the scaling of elastic and inelastic collisions with the scattering length and temperature to identify a regime where the elastic collisions dominate over the inelastic ones allowing the molecular sample to thermalize. Our work provides a benchmark for four-body calculations of molecular collisions and is essential for producing a degenerate Fermi gas of Feshbach molecules

Evaporation of microwave-shielded polar molecules to quantum degeneracy

Ultracold polar molecules offer strong electric dipole moments and rich internal structure, which makes them ideal building blocks to explore exotic quantum matter, implement novel quantum information schemes, or test fundamental symmetries of nature. Realizing their full potential requires cooling interacting molecular gases deeply into the quantum degenerate regime. However, the complexity of molecules which makes their collisions intrinsically unstable at the short range, even for nonreactive molecules, has so far prevented the cooling to quantum degeneracy in three dimensions. Here, we demonstrate evaporative cooling of a three-dimensional gas of fermionic sodium-potassium molecules to well below the Fermi temperature using microwave shielding. The molecules are protected from reaching short range with a repulsive barrier engineered by coupling rotational states with a blue-detuned circularly polarized microwave. The microwave dressing induces strong tunable dipolar interactions between the molecules, leading to high elastic collision rates that can exceed the inelastic ones by at least a factor of 460. This large elastic-to-inelastic collision ratio allows us to cool the molecular gas down to 21 nanokelvin, corresponding to 0.36 times the Fermi temperature. Such unprecedentedly cold and dense samples of polar molecules open the path to the exploration of novel many-body phenomena, such as the long-sought topological p-wave superfluid states of ultracold matter.Comment: 11 pages, 7 figure