Browsing by Author "Pfau, Tilman (Prof. Dr.)"

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Open Access
Aufbau einer Messapparatur zur Laserkühlung und hochauflösende Rydberg-Spektroskopie an 87 Rb-Atomen
(2006) Grabowski, Axel; Pfau, Tilman (Prof. Dr.)
Die Entwicklung der Methoden zur Laserkühlung von Atomen hat in den letzten 15 Jahren vielfältige wissenschaftliche Fortschritte erzielt. Ein sich dabei in den letzten Jahren neu entwickelndes Feld ist die Kombination der Laserkühlung mit der zustandsselektiven Anregung von Atomen in Rydberg-Zustände. Ein solches System wird als "gefrorenes Rydberg-Gas" bezeichnet. Dieser Name reflektiert die Tatsache, dass die Experimente mit diesen gekühlten Atomen auf einer Zeitskala ablaufen, auf der die thermische Bewegung zu vernachlässigen ist und nur die Wechselwirkung der Atome untereinander relevant ist. Gelingt es, solche kalten Rydberg-Atome in periodischen Potentialen zu speichern, so ergeben sich Möglichkeiten, dieses System für die Quanteninformationsverarbeitung (QIV) zu nutzen. Zur Untersuchung solcher Systeme wurde im Rahmen dieser Arbeit ein neuer experimenteller Aufbau konzipiert und realisiert. Die dazu aufgebaute Apparatur wurde charakterisiert, indem Untersuchungen zur Laserkühlung von aus einem Dispenser emittierten 87Rb-Atomen in einer magneto-optischen Falle (MOT) durchgeführt wurden. Die hierbei gekühlten Atome wurden anschließend in eine Magnetfalle in Drahtfallengeometrie transferiert und konnten dort magnetisch gespeichert werden. Zur Speicherung von ultrakalten Atomensembles in periodischen Strukturen wurden die Möglichkeiten untersucht, aus periodisch angeordneten stromtragenden Leitern ein Gitter von zwei Magnetfallentypen (Quadrupol- und Ioffe-Pritchard-Fallen) zu konstruieren. Hierzu wurden aufbauend auf einzelnen Segmenten die Möglichkeiten der Konstruktion von Gittern untersucht. Diese Gitter erlauben es, drei verschiedene Grundkonfigurationen von Fallen mit unterschiedlichen Eigenschaften aufzubauen, die theoretisch erläutert werden. Dabei wird auch eine experimentelle Realisierung solcher Gitter mit Quadrupol-Fallen vorgestellt. Hierbei wurde ein Gitter von 4 MOTs experimentell realisiert und untersucht. Im letzten Teil der Arbeit werden Experimente zur 2-Photonen Rydberg-Anregung und hochauflösenden Rydberg-Spektroskopie von 87Rb-Atomen vorgestellt. Hierbei wird zunächst das verwendete Anregungs- und Detektionsschema der Atome erläutert, gefolgt von Untersuchungen zur Stabilität und Leistungsfähigkeit des Lasersytems zur Rydberg-Anregung. Hierzu wurden hochauflösende Spektren in der Umgebung der beiden 41D-Feinstrukturzustände von 87Rb in Abhängigkeit vom elektrischen Feld aufgenommen und die feldabhängige Aufspaltung der Zustände untersucht. Zur Realisierung der erläuterten Schemata zur QIV ist es nötig, einzelne oder Ensembles von Atomen gezielt orts- und zustandsselektiv in Rydberg-Zustände anregen zu können. Erste Demonstrationsexperimente hierzu werden präsentiert. Abschließend wird über die Messung der Rabifrequenz durch Untersuchung der Autler-Townes-Aufspaltung des an das Lichtfeld gekoppelten 5S1/2->5P3/2-Übergangs berichtet.
Open Access
Coherent Rydberg excitations and photon correlations in dense thermal vapor cells
(2025) Mäusezahl, Max; Pfau, Tilman (Prof. Dr.)
This thesis describes the second generation of experiments towards a single-photon source concept based on a thermal rubidium vapor in a Rydberg-blockaded microcell. First, the situation is described in the density matrix formalism, and the requirements for the experimental system are derived. Key results of dipolar interactions and the influence of optical cavities are studied to contextualize the major challenges of the concept. In the first experimental part the fabrication of vapor cells and specifically our wedge vapor cell design is studied. Based on the daily experimental operation over several years the design is further improved, and the operational limits are marked out. A novel set of vapor cells with internal optical cavity is fabricated and used in a pilot experiment as a path towards a collectively enhanced, superradiant ensemble. The single-photon source concept optical setup is explained along its full development towards a well characterized and stable platform for all experiments with ns laser pulses that provide sufficient power for GHz Rabi frequencies. Spectroscopic experiments involving a strong light-induced atomic desorption (LIAD) pulse serve as benchmark for the performance of the inverted four-wave mixing (FWM) excitation scheme, and several open questions beyond our current understanding are identified. Finally, the single-photon source concept is tested for its photon statistics, which necessitates a careful analysis of statistical significance in the realm of low total correlation counts.
Open Access
Collapse of dipolar Bose-Einstein condensates for different trap geometries
(2010) Metz, Jonas; Pfau, Tilman (Prof. Dr.)
We experimentally investigate how the collapse dynamics of a Cr-52 Bose-Einstein condensate depends on the external harmonic trap geometry. When the collapse is initiated by reducing the s-wave scattering length below its critical value, a complex dynamics is observed, involving a d-wave symmetric explosion. We find good agreement between our experiments and simulations of the Gross-Pitaevskii equation including 3-body losses. In order to probe the phase-coherence of collapsed condensates we induce the collapse in several condensates simultaneously and let them interfere.
Open Access
Continuous wave Doppler-free spectroscopy on the 𝐀 𝟐𝚺+ ← 𝐗 𝟐𝚷𝟑/𝟐 transition in thermal nitric oxide
(2022) Kaspar, Patrick; Pfau, Tilman (Prof. Dr.)
The framework of this thesis is given by the development of a laboratory prototype for a new kind of gas sensing scheme to detect smallest quantities of nitric oxide in a large background of other gases. This thesis presents the underlying concepts of the sensing principle and some of the technical aspects developed for its investigation. In addition, the technique of Doppler-free saturated absorption spectroscopy, which is a standard technique in atomic physics, was applied to thermal nitric oxide molecules. It enabled direct resolution of the hyperfine structure and the determination of the corresponding hyperfine constants for the involved excited state. The results show the capabilities of the application of this technique to thermal molecules and in addition prove that saturation on the investigated transition is possible. This is an important result in terms of the overall goal the development of the trace gas sensor prototype.
Open Access
D-state Rydberg electrons interacting with ultracold atoms
(2014) Krupp, Alexander Thorsten; Pfau, Tilman (Prof. Dr.)
This thesis was established in the field of ultracold atoms where the interaction of highly excited D-state electrons with rubidium atoms was examined. This work is divided into two main parts: In the first part we study D-state Rydberg molecules resulting from the binding of a D-state Rydberg electron to a ground state rubidium atom. We show that we can address specific rovibrational molecular states by changing our laser detuning and thus create perfectly aligned axial or antialigned toroidal molecules, in good agreement with our theoretical calculations. Furthermore the influence of the electric field on the Rydberg molecules was investigated, creating novel states which show a different angular dependence and alignment. In the second part of this thesis we excite single D-state Rydberg electrons in a Bose-Einstein condensate. We study the lifetime of these Rydberg electrons, the change of the shape of our condensate and the atom losses in the condensate due to this process. Moreover, we observe quadrupolar shape oscillations of the whole condensate created by the consecutive excitation of Rydberg atoms and compare all results to previous S-state measurements. In the outlook we propose a wide range of further experiments including the proposal of imaging a single electron wavefunction by the imprint of its orbit into the Bose-Einstein condensate.
Open Access
Dynamic processes in Rydberg-atom-ion systems
(2024) Berngruber, Moritz; Pfau, Tilman (Prof. Dr.)
In this thesis an ion microscope is used to study ion-Rydberg interactions in a cold quantum gas of rubidium atoms in real space. This allowed the observation of charged ultralong-range Rydberg molecules consisting of an ion and a Rydberg atom. The molecule is based on a charge-induced flipping dipole bond and shows a bond length of several micrometers. Due to the versatile nature of the ion microscope, the molecules could not only be studied by means of conventional spectroscopy and mass spectrometry, but could also be directly observed in real space images. Thus, the spatial alignment, caused by the laser polarization during the photoassociation, could be detected in situ. Furthermore, the response of the molecule to an external electric field was investigated, opening up the possibility to not only associate the molecule in an aligned, but also an orientated way. Owing to the exceptionally long bond length of these molecules, dynamical processes are slowed down drastically, which provides the possibility to observe the molecular vibrational motion directly. Finally, the onset of collision dynamics between an ion and a Rydberg atom in a purely attractive potential was analyzed. At sufficiently small distances, the charge-induced Stark shift leads to a large number of avoided crossings between an atomic Rydberg state and the adjacent hydrogenic manifold. These crossings open up additional fast collision channels whose population can be described by a Landau-Zener hopping process. For a certain energy regime this results in a counter-intuitive behavior, as initially slow particles predominantly occupy steep collision channels and therefore experience a drastic speed up in the collisional process.
Open Access
Electrical detection of Rydberg interactions in nitric oxide at room temperature
(2023) Munkes, Fabian; Pfau, Tilman (Prof. Dr.)
In this work I will present measurements of Rydberg states in nitric oxide (NO) at room temperature. The detection of the Rydberg states is realized by measuring the current of free charges resulting from collisions of the excited molecules. All measurements are performed using continuous-wave (cw) lasers in a sub-Doppler configuration, which together with a stabilization setup yield a frequency error of only 2𝜋 × 2.5 MHz. The full width at half maximum (FWHM) of a typical Rydberg state is only about 2𝜋 × 130 MHz. We take a look at the necessary theory of diatomic molecules first. Afterward, a thorough walkthrough of the experimental setup is given. The heart of our setup is a custom-designed measurement cell, which features readout electronics based on a transimpedance amplifier (TIA). As such I will also give an overview on the basics of operational amplifiers (OpAmps). When all prerequisites are introduced, we will take an in-depth look on the Stark effect in Rydberg states. To our knowledge, the presented resolution is unmatched, and may enable us to give a more precise value to the g–quantum defect in NO in the future. In a final experimental section I show the collisional broadening and shift of Rydberg states of NO due to an increasing background gas density. Such measurements have a long history in alkalis, yet to our knowledge, no such measurements in NO exist. The overall experiment is performed in the context of a trace-gas sensor for NO in a medical application. This work gives suitable density and electric field ranges for such a sensor.
Open Access
Erzeugung eines Bose-Einstein-Kondensats in einer stark anisotropen Magnetfalle
(2003) Schoser, Jürgen; Pfau, Tilman (Prof. Dr.)
Im Rahmen dieser Arbeit wurde eine Apparatur zur Erzeugung eines Bose-Einstein-Kondensats mit Rubidium-Atomen in einer stark anisotropen Fallengeometrie konzipiert und im Labor realisiert. Die Bose-Einstein-Kondensation von verdünnten atomaren Gasen wird durch verschiedene Einfang- und Kühlmethoden erreicht. Abweichend von bisherigen Experimenten wurden hier bei den einzelnen experimentellen Stufen zum Teil neue Wege begangen: Ausgehend von der Dampfphase bei Raumtemperatur wird ein intensiver Strahl kalter Atome mittels zweidimensionaler magnetooptischer Kühlung erzeugt. Dieser ermöglicht es, eine großvolumige magnetooptische Atomfalle mit einer hohen Atomzahl zu laden. Mit der hohen optischen Dichte geht zwar eine geringe Kühleffizienz einher, was jedoch durch einen speziellen Kühlschritt, eine verstimmte magnetooptische Fallen-Phase, behoben wird, um Temperaturen im Bereich der Polarisationsgradientenkühlung zu erreichen. Das so präparierte Atomensemble wird in einer Magnetfalle durch Verdampfungskühlung in ein Bose-Einstein-Kondensat überführt. Hierbei wirkt sich besonders die anisotrope Fallengeometrie auf die Effizienz des letzten Kühlschritts aus. Die hier realisierte Apparatur erlaubt es, in das quasi-eindimensionale Regime entarteter Quantengase mit einer gut detektierbaren Atomzahl vorzudringen. Ein analytisches Modell rundet die theoretische Beschreibung der zweidimensionalen magnetooptischen Kühlung ab. Darüberhinaus werden erste Experimente von Bose-Einstein-Kondensaten in optischen Gittern vorgestellt und der Einfluss von interatomarer Wechselwirkung aufgrund von s-Wellen-Streuung auf die Materiewellenbeugung an optischen Stehwellen diskutiert.
Open Access
Interacting Rydberg atoms : coherent control at Förster resonances and polar homonuclear molecules
(2012) Nipper, Johannes Maximilian; Pfau, Tilman (Prof. Dr.)
Interactions between single atoms are fundamental to physics and to control them is an ultimate goal. The exaggerated properties of Rydberg atoms offer to met the technical challenges to isolate and control single interaction channels in ultracold gases. Here, I present experiments on two subjects related to interactions of Rydberg atoms in dense ultracold clouds. One subject concerns coherence in strongly interacting ensembles of atoms, where the interaction between Rydberg atoms is induced via Stark-tuned Förster resonances. Pulsed experiments, following the idea of Ramsey experiments, are used for high resolution spectroscopy of the Förster defect and phase sensitive detection. Coherent oscillations between pair states and an interaction-induced phase shift of Rydberg atoms are measured. These experiments are accompanied by calculations of the interaction strength and by simulations using the concept of a pair state interferometer. The simulations nicely reproduce the experimental findings and support the observation that the ensemble of atoms in the presence of interactions can be described and controlled coherently. The second subject of this thesis is the measurement of a permanent dipole moment in a homonuclear diatomic molecule that arises by the interaction between a Rydberg atom and a ground state atom. Usually parity symmetry prohibits a permanent dipole moment in diatomic molecules, but here the strong asymmetry between the constituents of the ultralong-range Rydberg molecule allows breaking parity symmetry. These molecules consist of one ground state atom bound inside the Rydberg electron wavefunction of a highly excited atom. Calculations predict dipole moments on the order of 1 Debye. Experimental proof is reported on the measurement of a linear Stark effect of these molecules, in excellent agreement with the calculations.
Open Access
Ions and electrons interacting with ultracold atoms : novel approaches based on Rydberg excitations
(2018) Kleinbach, Kathrin Sophie; Pfau, Tilman (Prof. Dr.)
In this thesis, the interaction of ions and electrons with ultracold atoms was investigated at the example of Rubidium-87. Thereto, a Rydberg atom was excited in a dense ultracold cloud. Both the interaction of the ionic core of the Rydberg atom and the Rydberg electron itself with neighboring neutral atoms was studied. Two main achievements were reported: First, photo-association of hybrid Trilobite Rydberg molecules was demonstrated. This is a special type of homonuclear molecules with a large electric dipole moment, which is bound by the electron-atom interaction. Second, it was shown for the first time that the ion-atom interaction between the Rydberg ionic core and the neutral atom can be accessed experimentally. The ion-atom interaction was observed at submicrokelvin temperature, once the nearest neighbor spacing in BEC is small and the electron-atom interaction is suppressed.
Open Access
Kohärente Rydberg-Spektroskopie in einer Rubidium Mikrozelle
(2012) Kübler, Harald; Pfau, Tilman (Prof. Dr.)
Diese Arbeit befasst sich mit den Wechselwirkungen der Rydbergatome mit den Glasoberflächen. Sind diese zu stark zerfallen die Rydbergatome oder die Kohärenzen zu schnell und damit sind auch die gespeicherten Informationen verloren. Zur spektroskopischen Untersuchung der Atome wurde in dieser Arbeit eine Glaszelle entwickelt und mit Rubidium befüllt. Es wurde eine Referenzspektroskopie für die Zweiphotonenanregung in den Rydbergzustand entwickelt, aufgebaut und charakterisiert. Die Rydbergatome in der Glaszelle wurden mit einer EIT-Spektroskopie untersucht [MJA07]. Während die meisten Zustände starke Wechselwirkungen mit der Wand zeigen, wurde in dieser Arbeit ein Zustand identifiziert, der bis zu Atom-Wandabständen unter einem Mikrometer nur sehr geringe Wechselwirkungseffekte zeigt. In Kapitel 2 werden die grundlegenden Eigenschaften von Rydbergatomen vorgestellt. Ausgehend vom einfachsten Fall des Wasserstoffatoms wird auf die energetische Lage der Rydbergzustände bei Rubidium und Cäsium eingegangen und die Quantendefekttheorie vorgestellt. Abschließend wird das Verhalten in elektrischen Feldern (Stark Effekt) beschrieben. Kapitel 3 liefert die theoretischen Grundlagen für die EIT-Spektroskopie. Zuerst wird anhand eines Zweiniveausystems mittels Dichtematrixformalismus die Absorption von Licht beschrieben. Es wird auf Verbreiterungen in thermischen Atomen eingegangen und das System dann durch ein drittes Niveau und einen zweiten Laser (Coupling-Laser) erweitert. Hier spielt es für die Absorptionsspektren nun eine Rolle, welcher der beiden Laser gescannt wird. Beachtet man noch den Doppler-Effekt in thermischen Atomen, so erhält man sogar unterschiedliche Spektren für unterschiedliche Verhältnisse der Laserwellenlängen der beiden Laser. In Kapitel 4 werden die Wechselwirkungen zwischen einem Atom und einer sich in der Nähe befindenden Wand beschrieben. Kapitel 5 beschreibt die allgemeinen Teile des Aufbaus. In Kapitel 6 wird auf die Frequenzreferenz für die Zweiphotonenanregung eingegangen. Nach der Beschreibung des Aufbaus werden die Auswirkungen des Dopplereffekts beschrieben. Die Zuverlässigkeit der Referenz wird daraufhin bei der Verwendung als Stabilisierung experimentell überprüft. Bei den folgenden Simulationen für die Scanreferenz werden zwei Fälle für verschiedene Wellenlängenverhältnisse der beteiligten Laser unterschieden. Eine Kombination entspricht einem möglichen Anregungsweg in Rubidium, die andere mit umgekehrtem Wellenlängenverhältnis einem in Cäsium. Anschließend wird die Übereinstimmung mit einem gemessenen Referenzspektrum gezeigt. Kapitel 7 beinhaltet den Aufbau für die Mikrozellen und die daran durchgeführten Messungen. Neben einer Abschätzung der zu erwartenden Linienbreiten ohne die Wandwechselwirkung werden Vergleichsmessungen zwischen verschiedenen Rydbergzuständen gezeigt. Es wird versucht, diese Spektren durch elektrische Felder und Polaritonen (Anregungen in den Wänden) zu erklären. In Kapitel 8 werden die Beobachtungen nochmals zusammengefasst, interpretiert und ein Ausblick auf weitere mögliche Schritte gegeben.
Open Access
Large bandwidth excitation of Rydberg atoms in thermal vapor : fast dynamics and strong interaction effects
(2016) Urvoy, Alban; Pfau, Tilman (Prof. Dr.)
Open Access
Laser cooling of a magnetically guided ultra cold atom beam
(2014) Aghajani-Talesh, Anoush; Pfau, Tilman (Prof. Dr.)
This thesis examines two complimentary methods for the laser cooling of a magnetically guided ultra-cold atom beam. If combined, these methods could serve as a starting point for high-through put and possibly even continuous production of Bose-Einstein condensates. First, a mechanism is outlined to harvest ultra cold atoms from a magnetically guided atom beam into an optical dipole trap. A continuous loading scheme is described that dissipates the directed kinetic energy of a captured atom via deceleration by a magnetic potential barrier followed by optical pumping to the energetically lowest Zeeman sublevel. The application of this scheme to the transfer of ultra cold chromium atoms from a magnetically guided atom beam into a deep optical dipole trap is investigated via numerical simulations of the loading process. Based on the results of the theoretical studies the feasibility and the efficiency of our loading scheme, including the realisation of a suitable magnetic field configuration, are analysed. Second, experiments were conducted on the transverse laser cooling of a magnetically guided beam of ultra cold chromium atoms. Radial compression by a tapering of the guide is employed to adiabatically heat the beam. Inside the tapered section heat is extracted from the atom beam by a two-dimensional optical molasses perpendicular to it, resulting in a significant increase of atomic phase space density. A magnetic offset field is applied to prevent optical pumping to untrapped states. Our results demonstrate that by a suitable choice of the magnetic offset field, the cooling beam intensity and detuning, atom losses and longitudinal heating can be avoided. Final temperatures below 65 µK have been achieved, corresponding to an increase of phase space density in the guided beam by more than a factor of 30.
Open Access
Laser cooling of barium monofluoride
(2024) Rockenhäuser, Marian; Pfau, Tilman (Prof. Dr.)
In this work, the first implementation of direct laser cooling of barium monofluoride (BaF) molecules using sub-Doppler forces is presented. This species is a promising candidate for parity violation measurements, the search for the electron’s permanent electric dipole moment and ultracold chemistry. However, due to its large mass, comparatively narrow linewidth and potential branching losses through an intermediate electronic state, this molecular species is notoriously difficult to cool. To achieve laser cooling, first, spectroscopic measurements of the relevant optical transitions were performed. This allowed for an improvement of the molecular constants by one order of magnitude. Next, Doppler-free spectroscopy was conducted on the cooling transition, which revealed a resolved hyperfine splitting in the excited state. Previous molecular laser cooling experiments employed sinusoidal sideband modulation to address such hyperfine structure states. Here, serrodyne modulation was used to create optimized optical spectra, resulting in significantly improved laser cooling. Finally, a Raman cycling scheme was implemented to achieve background-free imaging of the resulting cold molecular beam. In conclusion, an intense and transversally cold molecular beam of 138BaF was prepared, which paves the way for precision tests of fundamental symmetries using BaF.
Open Access
Laser cooling of molecules for precision measurements of parity violation
(2025) Kogel, Felix; Pfau, Tilman (Prof. Dr.)
Understanding the fundamental laws of physics has been an intrinsic motivation for the scientific community for many generations. The Standard Model of particle physics (SM), while extraordinarily successful in describing a wide range of phenomena, does not answer all fundamental questions. The weak interaction, especially its parity-violating nature, remains a promising area to search for new physics. Precision measurements using laser-cooled heavy molecules offer a tabletop-scale approach to probe poorly characterized properties of nuclear-spin-dependent parity violation (NSD-PV) connected to this force, and could complement findings from high-energy particle collider facilities. However, the complex hyperfine structure of suitable molecular species substantially complicates the creation of cold samples since conventional laser cooling techniques, which have emerged over the last decade, remain constrained to the most abundant isotopologues of simpler and mostly lighter molecules. This thesis presents a novel strategy for designing optimized optical spectra and implementing them via serrodyne waveforms to selectively laser cool heavy, low abundant barium monofluoride (BaF) molecules, whose additional nuclear spin leads to a level structure significantly exceeding the complexity of other laser-cooled species. A general prerequisite to any type of laser interaction is the thorough characterization of the molecular level structure. For this purpose, high-resolution spectroscopy is performed and transition spectra are modeled to disentangle the hyperfine and rovibrational spectra of the five most abundant isotopologues, from 138BaF to 134BaF, enabling a King plot analysis of the isotope shifts. Laser cooling in complex multi-level systems is modeled numerically using a dedicated simulation software package. Combined with spectroscopic input, this approach enables the identification of optimized laser sideband configurations for various BaF isotopologues. To realize the simulated laser forces, an experimental setup is designed that includes the generation of these sideband configurations via serrodynes. This enables the observation of strong and efficient Sisyphus cooling forces for the most abundant isotopologue 138BaF. High-fidelity detection and manipulation of select low-abundance isotopologues in the same molecular beam is first shown for 136BaF as a test system. Finally, these principles are combined to demonstrate optical cycling and one-dimensional transverse Sisyphus laser cooling for 137BaF which features an exceptionally complex hyperfine structure. These results enable efficient state preparation, detection, and intense collimated molecular beams of 137BaF molecules, which will significantly enhance the sensitivity to NSD-PV effects in the future. Moreover, the demonstrated techniques are also broadly applicable to other previously inaccessible molecules with complex hyperfine structure, thereby paving the way for their use in other sensitive tests of physics beyond the SM.
Open Access
Nonlinear optics in dense atomic vapors
(2022) Christaller, Florian; Pfau, Tilman (Prof. Dr.)
In this thesis, two building blocks for our next generation single-photon source, based on thermal rubidium Rydberg atoms, have been investigated. One of these is the coherent excitation of atoms with lasers in a four-wave mixing process on the nanosecond timescale. The second effect is the light-induced atomic desorption, where the atomic density is increased by an off-resonant laser pulse in a micrometer sized vapor cell on the nanosecond timescale.
Open Access
Observation of Feshbach resonances in an ultracold gas of 52Cr
(2006) Werner, Jörg; Pfau, Tilman (Prof. Dr.)
The first realization of Bose-Einstein condensation (BEC) in dilute atomic gases in the year 1995 has pioneered the exploration of an exciting new form of matter: the macroscopic quantum state. In many of the more recent experiments the interatomic interaction is responsible for the astonishing variety of observed phenomena. The exact knowledge of the interaction potentials between two atoms is one of the prerequisites for these kind of experiments. A very precise method for determining the interaction potentials exploits the existence of magnetically induced Feshbach resonances. For certain values of the applied external magnetic field the interaction can be arbitrarily tuned. From the positions of these Feshbach resonances one can deduce many details of the acting interaction potentials. The topic of this thesis is the first observation of fourteen magnetically induced Feshbach resonances in collisions between optically trapped ultracold chromium atoms. The search for Feshbach resonances was performed for magnetic fields between 4 and 600 G. A calibration of the magnetic field was done for each observed resonance slightly above and below the resonance using rf-spectroscopy. This allowed us to determine the positions of the resonances with an accuracy of below 100 mG. Up to now, not much was known about the interaction potentials between two chromium atoms. Due to a close collaboration with Andrea Simoni and Eite Tiesinga of the National Institute of Standards and Technology, Gaithersburg, USA we succeeded in identifying thirteen of the fourteen experimentally observed resonances and to assign the relevant quantum numbers. The eleven strongest Feshbach resonances build a complete set of all possible resonances up to second-order in the magnetic dipole-dipole coupling for the deca-triplett s-wave entrance channel. The two remaining identified resonances are resonances with a d-wave entrance channel and are thus much weaker. From the experimental resonance positions and knowing the corresponding quantum numbers allowed us to calculate a set of parameters describing the Born-Oppenheimer potentials with unprecedent precision. The average difference between theoretical and experimental resonance positions is only 0.6 G. Neglecting the spin-spin dipole interaction in our calculations leads to an average deviation of about 10 G. This is a clear sign, that the dipole-dipole interaction plays an important role in collisions between ultracold chromium atoms. The precise knowledge of the molecular potentials opens up new vistas. Based on the measured molecular potentials we can predict the collisional properties in other entrance channels and even for other chromium isotopes. Maybe these new results will lead to a better understanding of the bonding mechanisms of the chromium dimer, which is not yet fully understood in detail. Concerning further experiments with ultracold chromium atoms, the possibility to vary the interaction strength by using a Feshbach resonance is of interest. This will allow to change the strength of the isotropic contact interaction in relation to the anisotropic dipole-dipole interaction. The observed Feshbach resonances can even be used to create ultracold molecules.
Open Access
Optical pumping of a dense quantum gas at its limits : continuous Sisyphus cooling and demagnetization cooling towards degeneracy
(2013) Volchkov, Valentin V.; Pfau, Tilman (Prof. Dr.)
In this thesis, I study optical pumping as a powerful cooling tool for trapped ultra-cold atoms in a highly collisional regime. First application of optical pumping is a continuous loading scheme used to transfer atoms from a guided beam into a hybrid trap. Further, I introduce a Sisyphus cooling scheme based on radio-frequency transitions and optical pumping, operating simultaneously to the accumulation of atoms in the trap. The combined scheme of continuous loading and Sisyphus cooling is demonstrated for a large range of initial conditions of the guided atoms. Thereby, I show that collisional thermalization occurs in a steady-state for almost arbitrary initial conditions, provided that the first dissipative step is able to prevent the atom from leaving the trap during its first passage. On the one hand, this scheme could be applied to a wide range of atomic or molecular beams. On the other hand, phase-space density of 4*10^-4 is reached in a continuous operation mode with chromium atoms. In the second part, I investigate demagnetization cooling based on dipolar relaxation collisions driving the thermalization of the internal (spin) and the external (motional) degrees of freedom. In the case of a gas, one has the advantage that the spin degree of freedom can be cooled very efficiently using optical pumping. It is shown, that demagnetization cooling of a gas is more efficient than evaporation cooling in terms of phase-space density gain versus loss of atoms. This allows reaching a temperature of 6uK at a phase-space density of 0.03. It is observed, that both, continuous Sisyphus cooling and demagnetization cooling are limited by a density dependent loss mechanism. I present circumstantial evidence for excited-state collisions as the dominant limiting process. Finally, I discuss possible extensions to the current experimental procedures, possibly allowing reaching quantum degeneracy by optical means only.
Open Access
Realization of alkaline-earth circular Rydberg qubits in optical tweezer arrays
(2024) Hölzl, Christian; Pfau, Tilman (Prof. Dr.)
Individually controlled neutral atoms in arrays of optical tweezers have enabled rapid advances in the development of quantum computers and simulators over the last years. The finite lifetime of highly excited Rydberg atoms, which provide the interactions between the individual atoms, fundamentally limits the coherence times and gate fidelities of these machines. Circular Rydberg states are a promising candidate to overcome those limitations since their maximum angular momentum allows them to be protected from decay. In this work, I will report on the design and setup of a room-temperature quantum simulator based on circular Rydberg states of strontium atoms trapped in an optical tweezer array. With this setup, the tweezer-trapped circular Rydberg state qubits are realized in an alkaline-earth atom for the first time. A sophisticated electrode structure stabilizes the qubit states against black-body-radiation and enables the creation of the longest lived Rydberg states ever observed in a room-temperature setup.
Open Access
A Rydberg interferometer : from coherent formation of ultralong-range Rydberg molecules to state tomography of Rydberg atoms
(2011) Butscher, Björn; Pfau, Tilman (Prof. Dr.)
In the course of this thesis the techniques of coherent state preparation and interrogation introduced by Stern, Rabi, Ramsey and others are extended to large samples of ultracold Rydberg atoms and molecules. The key technique to realize the coherent control is a Ramsey-like excitation scheme. To achieve this, a Rydberg interferometer is set up and successfully applied in the investigation of the recently discovered ultralong-range Rydberg molecules as well as for the exploration of the dynamics of atomic Rydberg states. Therby, the coherent control of Rydberg molecules is proven. This is a major step towards time resolved investigation of the dynamics of the exotic molecule and opens new opportunities for a more sophisticated understanding of the underlying binding mechanisms. The first experimental application of the interferometer has been given for ultralong-range Rydberg molecules. Following the work of Ramsey, an experimental sequence with two excitation pulses split in time was set up. Because the Rydberg molecules are coherently photoassociated in the experiment, an interference pattern known as Ramsey fringes is created. The loss of visibility of the Ramsey fringe pattern could be described in a two-level model and from these calculations, the lifetime of the molecules was determined interferometrically. Thus aside from being an elegant and unambiguous proof for the coherence of the photoassociation, the occurrence of Ramsey fringes also allows one to see the experiment as an atom-molecule interferometer. These experiments have paved the way towards experimental exploration of the dynamics of these exotic molecule. For another open question in the field of ultralong-range Rydberg molecules - their lifetime - the findings in this thesis brought clarity. From the density-dependent investigations it could be found that the lifetime of the vibrationally excited molecular state is systematically shorter than that of the molecular ground state. These findings confirm the theory that the excited molecular states are bound by quantum reflection: The shortened lifetime can be ascribed to the inward penetration of the bound atomic pair due to imperfect quantum reflection leading to the dissociation of the molecule. In the further course of this thesis, the idea of a Ramsey setup for Rydberg molecules has been extend to Rydberg atoms. With the introduction of a control parameter to tune the relative phase shift between the ground-state arm and the Rydberg arm, the so far empty interferometer has been extended to a full working interferometer. The measurements on the 43s-state in electric fields demonstrated the usability of the interferometer in a simple and well controllable system and the interferometric studies on the 46d-state in crossed external fields can be regarded as the first investigative application of the interferometer. The key point of the investigation is the link between the two observables of the Rydberg interferometer, phase shift and visibility, and the complex value describing the Rydberg state of interest. It was found that the phase shift of the Rydberg state mostly follows the adiabatic energy curves in the electric field calculated for these experimental parameters. However, in the region where avoided crossings du to crossed electric and magnetic field occur, the phase shift behaves substantially different. While the phase rises fast by pi near one avoided crossing, a 2pi jump occurs at another avoided crossing. Since the phase can only be determined in the interval between 0 and 2pi experimentally, the jump cannot be observed as function of the electric field. This limitation has been overcome by taking the phase shift as function of time for different electric fields, which allows to reconstruct the phase at the given field-on time independently. From these measurements, the occurrence of the 2pi jump could be validated. To explore the reasons underlying the behavior of the phase in the electric field, the dynamics of the system has been modeled theoretically. The experimental findings for the phase shift as well as for the visibility are thoroughly reproduced by theory. Especially the characteristic behavior of phase shift and visibility near the avoided crossings is also seen in the calculations. With the help of this modeling one can see the fast pi rise in analogy to the phase of a driven harmonic oscillator that changes from in-phase when the driving frequency is below the resonance of the oscillator and out-of-phase above. The 2pi jump, however, emerges from the complex dynamics of the several levels involved and can not be easily broken down to a two-level picture. Altogether, these findings confirm that the Rydberg interferometer can track the dynamics of the selected Rydberg state and enabled the tomography of the Rydberg state. Moreover, these measurements show the potential of the Rydberg interferometer as tool to explore the dynamic of quantum systems.