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.

We have created a dark quantum superposition state of a Rb Bose-Einstein condensate (BEC) and a degenerate gas of Rb2 ground state molecules in a specific ro-vibrational state using two-color photoassociation. As a signature for the decoupling of this coherent atom-molecule gas from the light field we observe a striking suppression of photoassociation 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. PACS numbers: 34.50.Rk, 32.80.Pj, 03.75.Nt, 42.50.Gy The phenomenon of coherent dark states is well known in quantum optics and is based on a superposition of long-lived system eigenstates which decouples from the light field. Since their discovery [1] dark states have found numerous applications. Prominent examples are electromagnetically induced transparency and lasing without inversion [2], sub-recoil laser cooling [3], and ultra-sensitive magnetometers [4]. A particular application is the coherent transfer of population between two long-lived states by a stimulated Raman adiabatic passage (STIRAP) [5].In the emerging field of ultracold molecules, the conversion of atomic into molecular BECs is a central issue. A series of recent experiments on the creation of molecular quantum gases rely on the application of Feshbach resonances [6]. This coupling mechanism, however, is restricted to the creation of molecules in the highest ro-vibrational level and is only practicable for a limited number of systems. As a more general method a stimulated optical Raman transition can directly produce deeply bound molecules as demonstrated a few years ago [7,8]. STIRAP was proposed as a promising way for a fast, efficient and robust process to convert a BEC of atoms into a molecular condensate [9,10,11,12,13,14]. The central prerequisite for this kind of STIRAP is a dark superposition state of a BEC of atoms and a BEC of molecules.In this Letter, we report the observation of such a collective multi-particle dark state in which atoms in a BEC are pairwise coupled coherently to ground state molecules. This dark atom-molecule BEC shows up in a striking suppression of photoassociative loss, as illustrated by the spectra in Fig. 1. In one-color photoassociation, the excitation of a molecular transition produces a resonant loss feature that reflects the optical transition linewidth, see Fig. 1(a). The presence of a second laser field coupling the electronically excited molecular state to a long-lived ground-state level can drastically reduce this loss, as shown in Fig. 1(b) and (c). In (b), for example, we observe a striking loss suppression by about a factor of 70 on resonance.Already the mere observation of an atom-molecule dark resonance in a BEC proves that a coherent, quantum degenerate gas of molecules has been formed. This follows from the facts that 1) the dark state is by definition a coherent superposition of atoms and molecules and 2) the atomic BEC is a coh...

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We demonstrate a novel method of inducing an optical Feshbach resonance based on a coherent free-bound stimulated Raman transition. In our experiment atoms in a 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 modelled theoretically.

There is an increasing interest in high flux sources of metastable species in many scientific communities, for example for lithography and quantum optics experiments. We present a simple dc discharge design, based on microstructured electrodes (MSE), for the production of truly thermal beams of metastable atoms. Even for inlet pressures above 1 atm the discharge runs stably, at relatively modest voltages. Time-of-flight data prove that the expansion is supersonic with speed ratios up to 8.5 and internal temperatures of less than 10 K. The MSE source works equally well for many different gases like He, Ne, Kr, Ar, H2, and N2. Its measured yield of ∼1014 metastable atoms s−1 sr−1 compares favorably with conventional discharge sources. In addition, its simple design holds good promise for cooling the source down to cryogenic temperatures.