Vera Bendkowsky

Evidence for coherent collective Rydberg excitation in the strong blockade regime

We report on strong van der Waals blockade in two-photon Rydberg excitation of ultracold magnetically trapped 87Rb atoms. The excitation dynamics was investigated for a large range of densities and laser intensities and shows a full saturation and a strong suppression with respect to single-atom behavior. The observed scaling of the initial increase with density and laser intensity provides evidence for coherent collective excitation. This coherent collective behavior, that was observed for up to several thousand atoms per blockade volume, is generic for all mesoscopic systems which are able to carry only one single quantum of excitation.

Observation of ultralong-range Rydberg molecules

Rydberg atoms have an electron in a state with a very high principal quantum number, and as a result can exhibit unusually long-range interactions. One example is the bonding of two such atoms by multipole forces to form Rydberg–Rydberg molecules with very large internuclear distances. Notably, bonding interactions can also arise from the low-energy scattering of a Rydberg electron with negative scattering length from a ground-state atom. In this case, the scattering-induced attractive interaction binds the ground-state atom to the Rydberg atom at a well-localized position within the Rydberg electron wavefunction and thereby yields giant molecules that can have internuclear separations of several thousand Bohr radii. Here we report the spectroscopic characterization of such exotic molecular states formed by rubidium Rydberg atoms that are in the spherically symmetric s state and have principal quantum numbers, n, between 34 and 40. We find that the spectra of the vibrational ground state and of the first excited state of the Rydberg molecule, the rubidium dimer Rb(5s)–Rb(ns), agree well with simple model predictions. The data allow us to extract the s-wave scattering length for scattering between the Rydberg electron and the ground-state atom, Rb(5s), in the low-energy regime (kinetic energy, <100 meV), and to determine the lifetimes and the polarizabilities of the Rydberg molecules. Given our successful characterization of s-wave bound Rydberg states, we anticipate that p-wave bound states, trimer states and bound states involving a Rydberg electron with large angular momentum—so-called trilobite molecules—will also be realized and directly probed in the near future.

Rydberg trimers and excited dimers bound by internal quantum reflection.

In a combined experimental and theoretical effort we report on two novel types of ultracold long-range Rydberg molecules. First, we demonstrate the creation of triatomic molecules of one Rydberg atom and two ground-state atoms in a single-step photoassociation. Second, we assign a series of excited dimer states that are bound by a so far unexplored mechanism based on internal quantum reflection at a steep potential drop. The properties of the Rydberg molecules identified in this work qualify them as prototypes for a new type of chemistry at ultracold temperatures.

Rydberg Excitation of Bose-Einstein Condensates

Rydberg atoms provide a wide range of possibilities to tailor interactions in a quantum gas. Here, we report on Rydberg excitation of Bose-Einstein condensed 87Rb atoms. The Rydberg fraction was investigated for various excitation times and temperatures above and below the condensation temperature. The excitation is locally blocked by the van der Waals interaction between Rydberg atoms to a density-dependent limit. Therefore, the abrupt change of the thermal atomic density distribution to the characteristic bimodal distribution upon condensation could be observed in the Rydberg fraction. The observed features are reproduced by a simulation based on local collective Rydberg excitations.

A Homonuclear Molecule with a Permanent Electric Dipole Moment

Two rubidium atoms, one in its ground state and the other with a highly excited electron, form a metastable polar molecule. Permanent electric dipole moments in molecules require a breaking of parity symmetry. Conventionally, this symmetry breaking relies on the presence of heteronuclear constituents. We report the observation of a permanent electric dipole moment in a homonuclear molecule in which the binding is based on asymmetric electronic excitation between the atoms. These exotic molecules consist of a ground-state rubidium (Rb) atom bound inside a second Rb atom electronically excited to a high-lying Rydberg state. Detailed calculations predict appreciable dipole moments on the order of 1 Debye, in excellent agreement with the observations.

Universal scaling in a strongly interacting Rydberg gas

We study a gas of ultracold atoms resonantly driven into a strongly interacting Rydberg state. The long-distance behavior of the spatially frozen effective pseudospin system is determined by a set of dimensionless parameters, and we find that the experimental data exhibit algebraic scaling laws for the excitation dynamics and the saturation of Rydberg excitation. Mean-field calculations as well as numerical simulations provide an excellent agreement with the experimental finding and are evidence for universality in a strongly interacting frozen Rydberg gas.

Lifetimes of ultralong-range Rydberg molecules in vibrational ground and excited states

Since their first experimental observation, ultralong-range Rydberg molecules consisting of a highly excited Rydberg atom and a ground state atom [1, 2] have attracted the interest in the field of ultracold chemistry [3, 4]. Especially the intriguing properties such as size, polarizability and type of binding they inherit from the Rydberg atom are of interest. An open question in the field is the reduced lifetime of the molecules compared to the corresponding atomic Rydberg states [2]. In this paper we present an experimental study on the lifetimes of the 3Σ(5s − 35s) molecule in its vibrational ground state and in an excited state. We show that the lifetimes depend on the density of ground state atoms and that this can be described in the frame of a classical scattering between the molecules and ground state atoms. We also find that the excited molecular state has an even more reduced lifetime compared to the ground state which can be attributed to an inward penetration of the bound atomic pair due to imperfect quantum reflection that takes place in the special shape of the molecular potential [5].

High Resolution Rydberg Spectroscopy of Ultracold Rubidium Atoms

Submitted for the DAMOP06 Meeting of The American Physical Society High-Resolution Rydberg-spectroscopy on ultracold Rubidium atoms TILMAN PFAU, 5. Physikalisches Institut, University of Stuttgart, ROLF HEIDEMANN, AXEL GRABOWSKI, VERA BENDKOWSKY, EVA KUHNLE, JUERGEN STUHLER — Rydberg atoms can have huge static electric dipole moments. We are working on the investigation of electric dipole-dipole interaction between them as well as the interaction between Rydberg atoms and ground-state atoms of a BEC. The starting point of our Rydberg spectroscopy measurements is a cloud of magneto-optically trapped 87Rb-atoms. Using two narrow band, frequency stabilized cw laser systems, we perform two-photon excitation via the 5P3/2-level of ground state (5S1/2) Rb atoms to high lying Rydberg states with linewidths below 1 MHz. For the coherent control of the excitation, we need high spectral resolution and precise control of the Rabi-frequencies on both of the transitions. To demonstrate the spectral resolution and stability of our system, we investigated the Stark splitting of the two 41D-finestructure-states by measuring the number of Rydberg atoms as a function of the excitation frequency for different electric fields. We found it to be in excellent agreement with our calculations using perturbation theory on the Rubidium-wavefunctions. The Rabi-frequencies were measured by observation of the Autler-Townes-splitting while driving one of the transitions strongly and probing the other one. A line broadening mechnism due to ionic background charges is discussed. Tilman Pfau 5. Physikalisches Institut, University of Stuttgart Date submitted: 24 Jan 2006 Electronic form version 1.4

Excitation of Rydberg Atoms in a Bose-Einstein Condensate

The authors performed experiments on the coherent dynamics of Rydberg excitation in thermal clouds of 87Rb for a wide range of atomic densities, single-atom Rabi-frequencies and excitation times in a specialized vacuum chamber. We further observed a clear signature of the phase transition to a Bose-Einstein condensate in the fraction of excited Rydberg atoms when cooling the thermal cloud below Tc. In our experiments the sample size is bigger than the so called blockade radius, which is defined as the interatomic distance where the interaction energy becomes equal to the power broadened linewidth of the excitation.