Ultracold quantum gases in three-dimensional optical lattice potentials

In this thesis I report on experiments that enter a new regime in the many body physics of ultracold atomic gases. A Bose-Einstein condensate is loaded into a three-dimensional optical lattice potential formed by a standing wave laser light field. In this novel quantum system we have been able to both realize a quantum phase transition from a superfluid to a Mott insulator, and to observe the collapse and revival of a macroscopic matter wave field. Quantum phase transitions are driven by quantum fluctuations and occur, even at zero temperature, as the relative strength of two competing energy terms in the underlying Hamiltonian is varied across a critical value. In the first part of this work I report on the observation of such a quantum phase transition in a Bose-Einstein condensate with repulsive interactions, held in a three-dimensional optical lattice potential. In the superfluid ground state, each atom is spread-out over the entire lattice, whereas in the Mott insulating state, exact numbers of atoms are localized at individual lattice sites. We observed the reversible transition between those states and detected the gap in the excitation spectrum of the Mott insulator. A Bose-Einstein condensate is usually described by a macroscopic matter wave field. However, a quantized field underlies such a "classical" matter wave field of a Bose-Einstein condensate. The striking behavior of ultracold matter due to the field quantization and the nonlinear interactions between the atoms is the focus of the second part of this work. The matter wave field of a Bose-Einstein condensate is observed to undergo a series of collapses and revivals as time evolves. Furthermore, we show that the collisions between individual pairs of atoms lead to a fully coherent collisional phase shift in the corresponding many-particle state, which is a crucial cornerstone of proposed novel quantum computation schemes with neutral atoms. With these experiments we enter a new field of physics with ultracold quantum gases. In this strongly correlated regime, interactions between atoms dominate the behavior of the many-body system such that it can no longer be described by the usual theories for weakly interacting Bose gases. This novel quantum system offers the unique possibility to experimentally address fundamental questions of modern solid state physics, atomic physics, quantum optics, and quantum information.In dieser Promotionsarbeit werden Experimente vorgestellt, in denen es gelungen ist, in ein neues Regime der Vielteilchenphysik eines atomaren Quantengases vorzudringen. Ein Bose-Einstein-Kondensat wird in ein dreidimensionales optisches Gitterpotential geladen, das durch interferierende Laserstrahlen gebildet wird. Mit diesem neuartigen Quantensystem konnte ein Quanten-Phasenübergang zwischen einer Superflüssigkeit und einem Mott Isolator realisiert und das Kollabieren und Wiederaufleben eines makroskopischen Materiewellenfeldes beobachtet werden. Quanten-Phasenübergänge werden durch Quantenfluktuationen getrieben und können daher selbst am absoluten Temperaturnullpunkt auftreten, an dem alle thermischen Fluktuationen ausgefroren sind. Im ersten Teil dieser Arbeit berichte ich über die Beobachtung eines solchen Quanten-Phasenübergangs in einem Bose-Einstein Kondensat mit repulsiver Wechselwirkung, das in einem dreidimensionalen optischen Gitterpotential gespeichert ist. Im superfluiden Grundzustand ist jedes Atom über das gesamte Gitter delokalisiert. Im Mott Isolator Zustand hingegen ist auf jedem Gitterplatz eine konstante Zahl von Atomen lokalisiert. Wir konnten den reversiblen Übergang zwischen diesen beiden Zuständen beobachten und die Lücke im Anregungsspektrum des Mott Isolators nachweisen. Ein Bose-Einstein Kondensat wird üblicherweise durch ein makroskopisches Materiewellenfeld beschrieben. Diesem "klassischen" Feld liegt bei genauerer Betrachtung jedoch ein quantisiertes Materiewellenfeld zu Grunde. Thema des zweiten Teils dieser Arbeit ist die erstaunliche Dynamik, die ultrakalte Materie aufgrund dieser Quantisierung und der nichtlinearen Wechselwirkung der Atome erfährt. Im Experiment konnten wir ein periodisches Kollabieren und Wiederaufleben des makroskopischen Materiewellenfeldes beobachten. Wir konnten zeigen, daß die Kollisionen zwischen jeweils zwei Atomen lediglich zu einer völlig kohärenten Kollisionsphase im jeweiligen Vielteilchenzustand führen. Die kohärente Kollisionphase ist eine wesentliche Grundlage für verschiedene Vorschläge zur Realisierung eines Quantencomputers. Mit diesen Experimenten ist es gelungen, in ein neues Gebiet der Physik der ultrakalten Quantengase vorzudringen. Das stark korrelierte System wird durch die Wechselwirkung zwischen den Atomen dominiert und kann daher nicht mehr durch die gängigen Theorien des schwach wechselwirkenden Bosegases beschrieben werden. Durch dieses neuartige Quantensystem eröffnet sich die einzigartige Möglichkeit, in einem ultrakalten atomaren Gas fundamentale Fragen der modernen Festkörperphysik, Atomphysik, Quantenoptik und Quanteninformation zu studieren

Fermi Condensates

Ultracold atomic gases have proven to be remarkable model systems for exploring quantum mechanical phenomena. Experimental work on gases of fermionic atoms in particular has seen large recent progress including the attainment of so-called Fermi condensates. In this article we will discuss this recent development and the unique control over interparticle interactions that made it possible.Comment: Proceedings of ICAP-2004 (Rio de Janeiro). Review of Potassium experiment at JILA, Boulder, C

A model of the effect of collisions on QCD plasma instabilities

We study the effect of including a BGK collisional kernel on the collective modes of a QCD plasma which has a hard-particle distribution function which is anisotropic in momentum space. We calculate dispersion relations for both the stable and unstable modes and show that the addition of hard particle collisions slows the rate of growth of QCD plasma unstable modes. We also show that for any anisotropy there is an upper limit on the collisional frequency beyond which no instabilities exist. Estimating a realistic value for the collisional frequency for alpha_s ~ 0.2 - 0.4 we find that for the large-anisotropy case which is relevant for the initial state of matter generated by free streaming in heavy-ion collisions that the collisional frequency is below this critical value.Comment: 15 pages, 12 figure

Collapse and Revival of the Matter Wave Field of a Bose-Einstein Condensate

At the heart of a Bose-Einstein condensate lies its description as a single giant matter wave. Such a Bose-Einstein condensate represents the most "classical" form of a matter wave, just as an optical laser emits the most classical form of an electromagnetic wave. Beneath this giant matter wave, however, the discrete atoms represent a crucial granularity, i.e. a quantization of this matter wave field. Here we show experimentally that this quantization together with the cold collisions between atoms lead to a series of collapses and revivals of the coherent matter wave field of a Bose-Einstein condensate. We observe such collapses and revivals directly in the dynamical evolution of a multiple matter wave interference pattern, and thereby demonstrate a striking new behaviour of macroscopic quantum matter

A Stopped delta-matter source in heavy ion collisions at 10-GeV/N?

We predict the formation of highly dense baryon-rich resonance matter in Au+Au collisions at AGS energies. The final pion yields show observable signs for resonance matter. The Delta1232 resonance is predicted to be the dominant source for pions of small transverse momenta. Rescattering e ects consecutive excitation and deexcitation of Delta's lead to a long apparent life- time (> 10 fm/c) and rather large volumina (several 100 fm3) of the Delta-matter state. Heavier baryon resonances prove to be crucial for reaction dynamics and particle production at AGS