M. Reetz-Lamour

Suppression of Excitation and Spectral Broadening Induced by Interactions in a Cold Gas of Rydberg Atoms

We report on the observation of ultralong range interactions in a gas of cold rubidium Rydberg atoms. The van der Waals interaction between a pair of Rydberg atoms separated as far as 100,000 Bohr radii features two important effects: spectral broadening of the resonance lines and suppression of excitation with increasing density. The density dependence of these effects is investigated in detail for the S- and P-Rydberg states with principal quantum numbers n approximately 60 and n approximately 80 excited by narrow-band continuous-wave laser light. The density-dependent suppression of excitation can be interpreted as the onset of an interaction-induced local blockade.

Mechanical Effect of van der Waals Interactions Observed in Real Time in an Ultracold Rydberg Gas

We present time-resolved spectroscopic measurements of Rydberg-Rydberg interactions between two Rydberg atoms in an ultracold gas, revealing the pair dynamics induced by long-range van der Waals interactions between the atoms. By detuning the excitation laser, a specific pair distribution is prepared. Penning ionization on a microsecond time scale serves as a probe for the pair dynamics under the influence of the attractive long-range forces. Comparison with a Monte Carlo model not only explains all spectroscopic features but also gives quantitative information about the interaction potentials. The results imply that the interaction-induced ionization rate can be influenced by the excitation laser. Surprisingly, interaction-induced ionization is also observed for Rydberg states with purely repulsive interactions.

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.

Survival Probabilities in Coherent Exciton Transfer with Trapping

In the quest for signatures of coherent transport we consider exciton trapping in the continuous-time quantum walk framework. The survival probability displays different decay domains, related to distinct regions of the spectrum of the Hamiltonian. For linear systems and at intermediate times the decay obeys a power law, in contrast with the corresponding exponential decay found in incoherent continuous-time random walk situations. To differentiate between the coherent and incoherent mechanisms, we present an experimental protocol based on a frozen Rydberg gas structured by optical dipole traps.

Modeling many-particle mechanical effects of an interacting Rydberg gas

In a recent work [Phys. Rev. Lett. 98, 023004 (2007)], we investigated the influence of the attractive van der Waals interaction on the pair distribution and Penning ionization dynamics of ultracold Rydberg gases. Here we extend this description to atoms initially prepared in Rydberg states exhibiting repulsive interaction. We present calculations based on a Monte Carlo algorithm to simulate the dynamics of many atoms under the influence of both repulsive and attractive long-range interatomic forces. Redistribution to nearby states induced by blackbody radiation is taken into account, changing the effective interaction potentials. The model agrees with experimental observations, where the ionization rate is found to increase when the excitation laser is blue detuned from the atomic resonance.

Planar multipole ion trap

We report on the realization of a chip-based multipole ion trap manufactured using microelectromechanical systems technology, requiring minimal manual alignment of the electrodes. It provides ion confinement in an almost field-free volume between two planes of radiofrequency electrodes, deposited on glass substrates, which allows for optical access to the trap. An analytical model of the effective trapping potential is presented and compared with numerical calculations. Stable trapping of argon ions is achieved, and a lifetime of 16 s is measured. Electrostatic charging of the chip surfaces is studied and found to agree with a numerical estimate.

Spectroscopy of an ultracold Rydberg gas and signatures of Rydberg-Rydberg interactions

We report on experiments on Rydberg–Rydberg interaction-induced effects in a gas of 87Rb Rydberg atoms. A compact setup for two-photon continuous-wave excitation of high-lying Rydberg states out of an ultracold atomic gas is presented. The performance of the apparatus is characterized by high-resolution spectroscopy of Rydberg states. Signatures of interaction-induced effects are identified by qualitatively analysing the dependence of Rydberg excitation spectra on the intensity and the duration of the second-step laser excitation.

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.

INTERACTIONS IN AN ULTRACOLD GAS OF RYDBERG ATOMS

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.

Phase-coherent addition of laser beams with identical spectral properties

We coherently add two beams from separate diode lasers with identical frequencies to obtain a single coherent beam (degree of coherence ≃1) with well-defined polarization. The frequencies of the two sources are simultaneously injection locked to a third laser oscillator. The laser beams are merged at a polarizing beam splitter cube with an active stabilization of the relative phase against acoustic noise and thermal drifts. Different schemes to control the relative phase are investigated. The system can easily be extended to coherently add a large number of laser sources.