G. Thalhammer

Tuning the Scattering Length with an Optically Induced Feshbach Resonance

We demonstrate optical tuning of the scattering length in a Bose-Einstein condensate as predicted by Fedichev et al. [Phys. Rev. Lett. 77, 2913 (1996)]. In our experiment, atoms in a 87Rb condensate are exposed to laser light which is tuned close to the transition frequency to an excited molecular state. By controlling the power and detuning of the laser beam we can change the atomic scattering length over a wide range. In view of laser-driven atomic losses, we use Bragg spectroscopy as a fast method to measure the scattering length of the atoms.

Repulsively bound atom pairs in an optical lattice.

Throughout physics, stable composite objects are usually formed by way of attractive forces, which allow the constituents to lower their energy by binding together. Repulsive forces separate particles in free space. However, in a structured environment such as a periodic potential and in the absence of dissipation, stable composite objects can exist even for repulsive interactions. Here we report the observation of such an exotic bound state, which comprises a pair of ultracold rubidium atoms in an optical lattice. Consistent with our theoretical analysis, these repulsively bound pairs exhibit long lifetimes, even under conditions when they collide with one another. Signatures of the pairs are also recognized in the characteristic momentum distribution and through spectroscopic measurements. There is no analogue in traditional condensed matter systems of such repulsively bound pairs, owing to the presence of strong decay channels. Our results exemplify the strong correspondence between the optical lattice physics of ultracold bosonic atoms and the Bose–Hubbard model—a link that is vital for future applications of these systems to the study of strongly correlated condensed matter and to quantum information.

Coherent optical transfer of Feshbach molecules to a lower vibrational state.

Using the technique of stimulated Raman adiabatic passage (STIRAP) we have coherently transferred ultracold (87)Rb(2) Feshbach molecules into a more deeply bound vibrational quantum level. Our measurements indicate a high transfer efficiency of up to 87%. Because the molecules are held in an optical lattice with not more than a single molecule per lattice site, inelastic collisions between the molecules are suppressed and we observe long molecular lifetimes of about 1 s. Using STIRAP we have created quantum superpositions of the two molecular states and tested their coherence interferometrically. These results represent an important step towards Bose-Einstein condensation of molecules in the vibrational ground state.

Atom-Molecule Dark States in a Bose-Einstein Condensate

We have created a dark quantum superposition state of a Rb Bose-Einstein condensate and a degenerate gas of Rb2 ground-state molecules in a specific rovibrational state using two-color photo-association. As a signature for the decoupling of this coherent atom-molecule gas from the light field, we observe a striking suppression of photo-association loss. In our experiment the maximal molecule population in the dark state is limited to about 100 Rb2 molecules due to laser induced decay. The experimental findings can be well described by a simple three mode model.

Long-Lived Feshbach Molecules in a Three-Dimensional Optical Lattice

We have created and trapped a pure sample of Feshbach molecules in a three-dimensional optical lattice. Compared to previous experiments without a lattice, we find dramatic improvements such as long lifetimes of up to 700 ms and a near unit efficiency for converting tightly confined atom pairs into molecules. The lattice shields the trapped molecules from collisions and, thus, overcomes the problem of inelastic decay by vibrational quenching. Furthermore, we have developed an advanced purification scheme that removes residual atoms, resulting in a lattice in which individual sites are either empty or filled with a single molecule in the vibrational ground state of the lattice.

Long distance transport of ultracold atoms using a 1D optical lattice

We study the horizontal transport of ultracold atoms over macroscopic distances of up to 20 cm with a moving 1D optical lattice. By using an optical Bessel beam to form the optical lattice, we can achieve nearly homogeneous trapping conditions over the full transport length, which is crucial in order to hold the atoms against gravity for such a wide range. Fast transport velocities of up to 6 m s−1 (corresponding to about 1100 photon recoils) and accelerations of up to 2600 m s−2 are reached. Even at high velocities the momentum of the atoms is precisely defined with an uncertainty of less than one photon recoil. This allows for construction of an atom catapult with high kinetic energy resolution, which might have applications in novel collision experiments.

Inducing an optical Feshbach resonance via stimulated Raman coupling

We demonstrate a method of inducing an optical Feshbach resonance based on a coherent free-bound stimulated Raman transition. In our experiment atoms in a {sup 87}Rb Bose-Einstein condensate are exposed to two phase-locked Raman laser beams which couple pairs of colliding atoms to a molecular ground state. By controlling the power and relative detuning of the two laser beams, we can change the atomic scattering length considerably. The dependence of scattering length on these parameters is studied experimentally and modeled theoretically.

Repulsively Bound Atom Pairs: Overview, Simulations and Links

We review the basic physics of repulsively bound atom pairs in an optical lattice, which were recently observed in the laboratory, including the theory and the experimental implementation. We also briefly discuss related many‐body numerical simulations, in which time‐dependent Density Matrix Renormalisation Group (DMRG) methods are used to model the many‐body physics of a collection of interacting pairs, and give a comparison of the single‐particle quasimomentum distribution measured in the experiment and results from these simulations. We then give a short discussion of how these repulsively bound pairs relate to bound states in some other physical systems.

New experiments with BEC in an optical lattice

Summary form only given. Our new Rb BEC apparatus features a magnetic transport scheme, good optical access and maximum flexibility. This allows for new experiments which investigate the interaction of a BEC in various environments. Our first experiments will study BEC in a 1D optical lattice. We construct and characterize a coherent accelerator for a condensate which consists of a far detuned standing light wave. We load the BEC adiabatically into the lattice ground state and accelerate the lattice by frequency chirping the lattice laser beams. In simple words the accelerated lattice drags the condensate with it. Coherent acceleration to velocities corresponding to 10 photon recoils has been demonstrated recently in a similar setup. We test whether this scheme can be extended to transport BEC over distances of tens of centimeters. In order to represent a valuable new tool the accelerator has to be able to coherently transfer about 1000 photon recoils (recoil velocity of /sup 87/Rb = 5.85 mm/s) to the atoms in less than 100 ms. We test the efficiency of the transport and study atomic loss, heating and phase corrugation of the condensate wavefunction. In the near future we will extend our work to 3D optical lattices where we will use a Mott insulator state to study molecule formation out of atom number states.