J. Deiglmayr

Formation of Ultracold Polar Molecules in the Rovibrational Ground State

Ultracold LiCs molecules in the absolute ground state X1Sigma+, v = 0, J = 0 are formed via a single photoassociation step starting from laser-cooled atoms. The selective production of v = 0, J = 2 molecules with a 50-fold higher rate is also demonstrated. The rotational and vibrational state of the ground state molecules is determined in a setup combining depletion spectroscopy with resonant-enhanced multiphoton ionization time-of-flight spectroscopy. Using the determined production rate of up to 5 x 10(3) molecules/s, we describe a simple scheme which can provide large samples of externally and internally cold dipolar molecules.

Rabi oscillations and excitation trapping in the coherent excitation of a mesoscopic frozen Rydberg gas.

We demonstrate the coherent excitation of a mesoscopic ensemble of about 100 ultracold atoms to Rydberg states by driving Rabi oscillations from the atomic ground state. We employ a dedicated beam shaping and optical pumping scheme to compensate for the small transition matrix element. We study the excitation in a weakly interacting regime and in the regime of strong interactions. When increasing the interaction strength by pair state resonances, we observe an increased excitation rate through coupling to high angular momentum states. This effect is in contrast to the proposed and previously observed interaction-induced suppression of excitation, the so-called dipole blockade.

Rabi oscillations between ground and Rydberg states and van der Waals blockade in a mesoscopic frozen Rydberg gas

We present a detailed analysis of our recent observation of synchronous Rabi oscillations between the electronic ground state and Rydberg states in a mesoscopic ensemble containing roughly 100 ultracold atoms (Reetz-Lamour et al submitted, Preprint 0711.4321). The mesoscopic cloud is selected out of a sample of laser-cooled Rb atoms by optical pumping. The atoms are coupled to a Rydberg state with principal quantum number around 30 by a two-photon scheme employing flat-top laser beams. The influence of residual spatial intensity fluctuations as well as sources of decoherence such as redistribution to other states, radiative lifetime and laser bandwidth are analysed. The results open up new possibilities for the investigation of coherent many-body phenomena in dipolar Rydberg gases. As an example we demonstrate the van der Waals blockade, a variant of the dipole blockade, for a mesoscopic atom sample.

Core repulsion effects in alkali trimers

The present paper is related to a talk presented during the Symposium on Coherent Control and Ultracold Chemistry held during the Sixth Congress of the International Society for Theoretical Chemical Physics (ISTCP-VI, July 2008). The talk was entitled Electronic structure properties of alkali dimers and trimers. Prospects for alignment of ultracold molecules. Here we report on the electrostatic repulsion forces of the ionic cores at short separation, involved when the potential energy surfaces of alkali trimers are calculated with a quantum chemistry approach based on effective large-core potentials for ionic core description. We demonstrate that such forces in the triatomic molecule can be obtained as the sum of three pairwise terms. We illustrate our results on the lowest electronic states of Cs

INTERACTIONS IN AN ULTRACOLD GAS OF RYDBERG ATOMS

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Dipolar effects and collisions in an ultracold gas of LiCs molecules

, which are computed for the first time within a full configuration interaction based on a large Gaussian basis set. As a preliminary section, we also propose a brief introduction about the importance of alkali trimer systems in the context of cold and ultracold molecules.

Ultracold Polar Molecules in the Rovibrational Ground State

This thesis presents observations of ultralong range interactions in a frozen Rydberg gas. The observed signatures are interactioninduced line broadenings and a suppression of resonant Rydberg excitation. The latter effect can be interpreted as the onset of a local dipole blockade and can be exploited to perform quantum information processing with mesoscopic ensembles. The dominating interaction between a pair of Rydberg atoms separated as far as 100 000 Bohr radii is the van der Waals interaction. The strength of this interaction is calculated quantitatively in a perturbative approach. We describe in detail our experimental apparatus which employs narrow-bandwidth continuouswave (cw) laser excitation in a two-photon excitation process to produce a frozen Rydberg gas from magneto-optically trapped Rb atoms. As a particular feature of the apparatus we have implemented a novel scheme for the coherent addition of laser intensities and we have realized 3D degenerate Raman sideband cooling resulting in atom cloud temperatures in the sub-μK range. Systematic studies of the Rydberg spectra as a function of excitation-laser intensity, density of excitable atoms and excitation time are presented. Zusammenfassung: In dieser Doktorarbeit werden die langreichweitigen Wechselwirkungen in einem ultrakalten Gas aus Rydbergatomen untersucht. Die beobachteten Signaturen sind wechselwirkungsverursachte Linienverbreiterungen und eine Anregungsunterdrückung bei resonanter Rydberganregung. Der letztere Effekt kann als Beginn einer lokalen Dipolblockade interpretiert werden, welche zur Realisierung von Quanteninformationsverarbeitung mit mesoskopischen Gesamtheiten ausgenutzt werden kann. Die dominierende Wechselwirkung zwischen zwei Rydbergatomen in einem Abstand von 100 000 Bohrradien ist die van-der-Waals-Wechselwirkung. Die Stärke der Wechselwirkung wurde quantitativ in einem störungstheoretischen Ansatz berechnet. Der experimentelle Aufbau wird detailliert beschrieben. Wir setzen schmalbandige kontinuierliche Laseranregung in einem Zweiphotonenprozess zur Rydberganregung aus magneto-optisch gefangenen Rb Atomen ein. In unserem Aufbau kommt ein neuartiges Verfahren zur kohärenten Addition von Laserintensitäten zum Einsatz. Des Weiteren wurde 3D entartete Ramanseitenbandkühlung implementiert und Atomwolkentemperaturen im unteren μK Bereich erreicht. Systematische Studien der Rydbergspektren als Funktion der Anregungslaserintensitäten, der Dichte der anregbaren Atomen und der Anregungszeit wurden durchgeführt.

Coherent excitation of Rydberg atoms in an ultracold gas

We review our recent experiments on the formation and optical trapping of LiCs molecules, which are photoassociated from a mixed ultracold gas of Li and Cs atoms. The electric dipole moment of the atoms in the lowest vibrational states is measured, yielding the predicted value of 5.5 Debye, and vibrational redistribution due to spontaneous emission and blackbody radiation is observed. We also show preliminary results on inelastic collision between optically trapped LiCs molecules and Cs atoms.

Dynamics of resonant energy transfer in a cold Rydberg gas

Recently, there has been important progress in the formation of ultracold polar molecules in the rovibrational ground state, thus opening intriguing perspectives for the investigation of strongly correlated quantum systems under the influence of long-range dipolar forces. After an introduction into the field of ultracold polar molecules, I will present our recent experiments on the formation of ultracold LiCs molecules in the absolute ground state XΣ, v’’=0, J’’=0 via a single photo-association (PA) step starting from laser-cooled atoms [1].