Bosonische und fermionische Quantengase in dreidimensionalen optischen Gittern

In den letzten Jahren haben sich atomare Quantengase in optischen Gittern zu einem faszinierenden und interdisziplinär bedeutsamen Forschungsfeld entwickelt. Die in den periodischen Potentialen gefangenen ultrakalten Atome stellen ein ideales Modellsystem dar, anhand dessen sich grundlegende Fragestellungen der modernen Festkörper- und Vielteilchenphysik untersuchen lassen. In der vorliegenden Arbeit werden neue Methoden zur Manipulation und Analyse von Quantenzuständen in optischen Gittern demonstriert. Insbesondere wird mittels der sogenannten Rauschkorrelationsanalyse die Ordnung der Atome im Gitter bestimmt und erstmals fermionisches Antibunching an freien neutralen Atomen nachgewiesen. Grundlage für die vorgestellten Experimente ist eine im Rahmen dieser Arbeit neu entwickelte Apparatur, mit der sich simultan entartete bosonische und fermionische Quantengase aus 87-Rubidium und 40-Kalium präparieren und in einem dreidimensionalen optischen Gitter untersuchen lassen. Die Apparatur zeichnet sich durch eine Serie technischer Innovationen aus: Eine neuartige Spulen- und Fallenkonfiguration eröffnet einen hervorragenden optischen Zugang zu den präparierten Ensemblen und ermöglicht es, starke homogene Magnetfelder bei einer geringen dissipierten Leistung zu erzeugen. Dies sind wichtige Voraussetzungen, um definierte Gitterpotentiale verwirklichen und die interatomaren Wechselwirkungen mittels Feshbach-Resonanzen beeinflussen zu können. Das optische Potential geht aus der Überlagerung einer gekreuzten Dipolfalle und eines blauverstimmten dreidimensionalen Gitters hervor. Eine solche Kombination erlaubt es, sehr tiefe und relativ homogene Gitterpotentiale zu erzeugen sowie den externen Einschluss unabhängig von der Gittertiefe zu variieren. Des Weiteren lassen sich über eine frei einstellbare Wellenlänge speziesabhängige Gitter realisieren. Die Vereinigung der hier aufgeführten Technologien liefert uns eine außergewöhnlich flexible Plattform für das Studium maßgeschneiderter Quantenzustände in periodischen Potentialen. Durch den unabhängigen externen Einschluss kann erstmals ein Fermigas allein über dessen Kompression zwischen einem metallischen und einem isolierenden Zustand hin- und hergeschaltet und – in ersten Ansätzen – die entsprechende Dynamik beobachtet werden. Die Ergebnisse werden mit numerischen Simulationen verglichen. Neben der Durchführung von Transportmessungen lässt sich hieraus ein neues Diagnoseverfahren ableiten, das es ermöglicht, Quantenphasen, wie den bosonischen oder fermionischen Mott-Isolator, anhand der charakteristischen Kompressibilität zu identifizieren. Als weiteres Diagnoseverfahren wird die Korrelationsanalyse von Flugzeitaufnahmen vorgestellt. Durch die Auswertung von Hanbury Brown und Twiss (HBT)-Korrelationen im Quantenrauschen der expandierenden Atomwolken lässt sich die mikroskopische Ordnung der Atome im Gitter nachweisen. Ausgangspunkt für die Messungen sind jeweils vollständig spinpolarisierte bosonische Mott-Isolatoren und fermionische Bandisolatoren. Trotz identischer Dichteverteilungen innerhalb des Gitters, weisen die Korrelationen von Bosonen und Fermionen entgegengesetzte Vorzeichen auf. Mit diesen Messungen gelingt es erstmals, fermionisches Antibunching an freien neutralen Atomen zu beobachten und innerhalb einer selben Apparatur mit dem bosonischen Bunching zu vergleichen. Neben dem Nachweis dieses fundamentalen Quanteneffektes lässt sich die Ordnung und die Temperatur der Fermionen im Gitter bis hinauf zur Fermi-Temperatur bestimmen. Damit erweist sich die Korrelationsanalyse als ein robustes Verfahren, mit dem sich in Zukunft noch weitaus komplexere Quantenphasen in optischen Gittern untersuchen lassen.In the past couple of years atomic quantum gases in optical lattices have evolved into a fascinating research field of increasing interdisciplinary importance. Ultracold atoms trapped in periodic potentials of light represent an ideal model system to study fundamental questions of modern solid state and many-body physics. In the present work new experimental methods are presented for analysing and manipulating quantum states in optical lattices. By analysing noise correlations we determine the atomic order in the lattice and for the first time demonstrate fermionic antibunching with free neutral atoms. Basis of these experiments is a newly developed apparatus allowing the simultaneous preparation of degenerate quantum gases of bosonic rubidium-87 and fermionic potassium-40 atoms and permitting the investigation of these gases in a versatile three-dimensional optical lattice. The apparatus features a series of technical innovations: New coil and trap configurations provide an excellent optical access to the prepared atomic ensembles and allow the generation of strong homogeneous magnetic fields at low dissipated powers. These are important prerequisites to produce well-defined lattice potentials and to precisely manipulate the atomic interactions via magnetic Feshbach resonances. The optical potential is generated by superimposing a crossed dipole trap and a blue-detuned three-dimensional optical lattice. This configuration enables the creation of both very deep and very homogeneous lattice potentials. Moreover, the external confinement can be controlled independently from the lattice depth, and the freely tunable wavelength gives access to species-specific lattice potentials. These features and technologies provide us with an exceptionally flexible platform for the study of precisely tailored many-body quantum states in periodic potentials. The independent external confinement for the first time allows switching a Fermi gas from a metallic to an insulating state, and vice versa, only by changing its compression. Preliminary measurements of the associated dynamics are presented and all results are compared with numerical simulations. These measurements open up a new avenue to detect bosonic and fermionic Mott insulators via their characteristic compressibility. As further diagnostic tool, noise correlation analysis of time of flight images is demonstrated. By evaluating Hanbury Brown and Twiss (HBT) correlations in the quantum noise of expanding atom clouds the microscopic atomic order in the lattice is revealed. Starting point for the measurements are fully spin-polarised bosonic Mott and fermionic band insulators. Despite identical in-trap distributions, the correlations of bosons and fermions show opposite signs. These measurements constitute the first proof for fermionic antibunching of free neutral atoms and allow the comparison of bosonic and fermionic HBT effects within the same apparatus for the very first time. Besides the demonstration of these fundamental quantum effects the method is used to determine the ordering and temperature of the fermions in the periodic potential. Hence noise correlation analysis is proven to be a robust tool for future investigations of even more complex many-body quantum states in optical lattices

Coherent transport of neutral atoms in spin-dependent optical lattice potentials

We demonstrate the controlled coherent transport and splitting of atomic wave packets in spin-dependent optical lattice potentials. Such experiments open intriguing possibilities for quantum state engineering of many body states. After first preparing localized atomic wave functions in an optical lattice through a Mott insulating phase, we place each atom in a superposition of two internal spin states. Then state selective optical potentials are used to split the wave function of a single atom and transport the corresponding wave packets in two opposite directions. Coherence between the wave packets of an atom delocalized over up to 7 lattice sites is demonstrated.Comment: 4 pages, 6 figure

Quantum Information Processing in Optical Lattices and Magnetic Microtraps

We review our experiments on quantum information processing with neutral atoms in optical lattices and magnetic microtraps. Atoms in an optical lattice in the Mott insulator regime serve as a large qubit register. A spin-dependent lattice is used to split and delocalize the atomic wave functions in a controlled and coherent way over a defined number of lattice sites. This is used to experimentally demonstrate a massively parallel quantum gate array, which allows the creation of a highly entangled many-body cluster state through coherent collisions between atoms on neighbouring lattice sites. In magnetic microtraps on an atom chip, we demonstrate coherent manipulation of atomic qubit states and measure coherence lifetimes exceeding one second at micron-distance from the chip surface. We show that microwave near-fields on the chip can be used to create state-dependent potentials for the implementation of a quantum controlled phase gate with these robust qubit states. For single atom detection and preparation, we have developed high finesse fiber Fabry-Perot cavities and integrated them on the atom chip. We present an experiment in which we detected a very small number of cold atoms magnetically trapped in the cavity using the atom chip

Controlled Collisions for Multiparticle Entanglement of Optically Trapped Atoms

Entanglement lies at the heart of quantum mechanics and in recent years has been identified as an essential resource for quantum information processing and computation. Creating highly entangled multi-particle states is therefore one of the most challenging goals of modern experimental quantum mechanics, touching fundamental questions as well as practical applications. Here we report on the experimental realization of controlled collisions between individual neighbouring neutral atoms trapped in the periodic potential of an optical lattice. These controlled interactions act as an array of quantum gates between neighbouring atoms in the lattice and their massively parallel operation allows the creation of highly entangled states in a single operational step, independent of the size of the system. In the experiment, we observe a coherent entangling-disentangling evolution in the many-body system depending on the phase shift acquired during the collision between neighbouring atoms. This dynamics is indicative of highly entangled many-body states that present novel opportunities for theory and experiment.Comment: 17 pages, including 5 figures, accepted for publication in Natur

Organizational Studies in Space: Stanislaw Lem and the Writing of Social Science Fiction

This paper seeks to introduce the oeuvre of the Polish science fiction author, Stanislaw Lem, whose work is argued to carry significance for students of organizational conduct. Singling out his most famous novel, Solaris, for particular attention, a critical interpretation is offered that selectively highlights Lem's epistemological and ontological pre-occupations concerning scientific inquiry and the human condition. These concerns are seen to resonate with contemporary issues in the field of organization studies. In particular, the rhetorical role of mimesis, viewed as a synthesis of rational and non-rational human motives, within Solaris is taken to inform a wide range of human conduct. The paper concludes by calling for a realist mode of organizational discourse that explores the dialectical relationship between what it characterizes as 'solar' and 'lunar' dimensions of human behaviour. A new challenge to organization studies will be not simply to learn from the substantive concerns of literary genres such as science fiction, but also to aspire after the narrative skills of their leading exponents