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Quantum Electronics and Laser Science Conference

Stimulated Raman atom-molecule resonance in a Bose-Einstein condensate

R. H. Wynar, R. S. Freeland, D. J. Han, Daniel J. Heinzen

Published Jan 20, 2003• 1 citation
Publisher

Abstract

Summary form only given. We have observed a stimulated Raman resonance between free and bound molecular states of atoms in a Bose-condensed gas. In our experiment, we first produce a Bose-condensed cloud containing about 5/spl times/10/sup 5/ /sup 87/Rb 5/sup 2/S/sub 1/2/ (f=1,m/sub f/=-1) atoms in a magnetic TOP trap, with a typical condensate fraction of about 20%. We then illuminate the atoms with a /spl tau/=10 ms duration pulse of light from two laser beams 1 and 2 of frequency /spl nu//sub 1/ and /spl nu//sub 2/, and intensities I/sub 1/=120 W/cm/sup 2/ and I/sub 2/=2.2 W/cm/sup 2/, respectively. We measure the fraction of atoms N(/spl tau/)/N/sub 0/ remaining after the 10 ms pulse with laser-induced fluorescence.

DOI
10.1109/qels.1999.807467

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Summary form only given. We have observed a stimulated Raman resonance between free and bound molecular states of atoms in a Bose-condensed gas. In our experiment, we first produce a Bose-condensed cloud containing about 5/spl times/10/sup 5/ /sup 87/Rb 5/sup 2/S/sub 1/2/ (f=1,m/sub f/=-1) atoms in a magnetic TOP trap, with a typical condensate fraction of about 20%. We then illuminate the atoms with a /spl tau/=10 ms duration pulse of light from two laser beams 1 and 2 of frequency /spl nu//sub 1/ and /spl nu//sub 2/, and intensities I/sub 1/=120 W/cm/sup 2/ and I/sub 2/=2.2 W/cm/sup 2/, respectively. We measure the fraction of atoms N(/spl tau/)/N/sub 0/ remaining after the 10 ms pulse with laser-induced fluorescence.
Summary form only given. We have created state-selected /sup 87/Rb/sub 2/ molecules at rest in a dilute Bose-Einstein condensate of /sup 87/Rb atoms with coherent free-bound stimulated Raman transitions. The transition lineshape exhibits a resonance lineshape with an extremely narrow width as small as 1.5 kHz. As a result, we are able to measure the molecular binding energy with an unprecedented precision of 1.2 kHz. We find that this lineshape is sensitive to elastic interactions between the molecules and the atomic Bose-condensate, and as a result, we are able to measure an atom-molecule scattering length for the first time. In addition, we are able to set an upper bound on the rate constant for inelastic collisions between the molecules and the atoms. This is the first measurement of molecule-condensate interactions, and is an important step towards the application of this method to the formation of a molecular Bose-Einstein condensate.
Summary form only given. Coherent conversion of an atomic to a molecular Bose-Einstein condensate (BEC) is a first step towards 'superchemistry', which is the atom optics analog of frequency conversion in nonlinear optics. A number of studies of this have shown that direct conversion via Raman photo-association appears feasible, based on stimulated free-bound and bound-bound transitions in the presence of two laser fields E/sup (1)/=E/sub 0//sup (1)/ cos(/spl omega//sup (1)/t) and E/sup (2)/=E/sub 0//sup (2)/ cos(/spl omega//sup (2)/t). Here pairs of atoms from the two-atom continuum are transferred-via an excited bound molecular state-to a bound molecular state of lower energy. Raman photo-association allows coupling to a single molecular state, which can be selected by the Raman laser frequencies /spl omega//sup (1)/ and /spl omega//sup (2)/. However, practical calculations using available lasers and transitions indicate that coherent transfer is often limited by spontaneous emission from the intermediate molecular excited electronic state. A possible route towards minimizing losses and decoherence from spontaneous emission is to use the stimulated Raman adiabatic passage (STIRAP) method (Drummond et al, Phys. Rev. Lett. vol. 81, p. 3055, 1998) in which a counter-intuitive pulse sequence is used where the first input pulse couples the molecular levels, even when there are no molecules present.
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 87Rb^{87}\mathrm{Rb}87Rb 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.
This thesis reports on the first observation of optical tuning of interactions in an ultracold atomic gas. To control the elastic interactions, we use optically induced scattering resonances. They are realized by coupling pairs of atoms to a bound molecular state with the help of laser light. In comparison to the well-known magnetic Feshbach resonances, these optical Feshbach resonances allow a very fast switching of interactions in a spatially resolved way. This is because laser light can be switched on and off much faster than magnetic fields and because the light intensity can be spatially structured. Our experiments are performed with a Bose-Einstein condensate (BEC) of Rb atoms, which we produce in our new BEC setup. This apparatus has been constructed in the framework of this thesis and offers excellent optical access to the ultracold atom cloud. To investigate the tunability of atomic interactions with single-frequency laser light, we optically couple pairs of free atoms to an electronically excited molecular state. The coupling is achieved with a laser that is tuned close to the respective free-bound photoassociation transition. By changing the laser detuning we are able to adjust the atomic scattering length to any value between 10 a0 and 190 a0, where a0 denotes the Bohr radius. The atomic scattering length is a single quantity that fully describes ultracold elastic interactions. Since laser induced atom losses restrict the duration of our experiments, Bragg spectroscopy is employed as a fast method to measure the scattering length. We also demonstrate that optical Feshbach resonances can be realized with a two-color Raman coupling scheme. To implement this scheme, a second laser is added that couples the excited molecular state to a weakly bound molecular level in a ground state potential. Tuning the second laser results in a modification of the atomic scattering length similar to the one observed with a single laser. In addition, this scheme allows to control the width of the resonance via the detuning of the first laser. For the one-color and two-color coupling schemes, we investigate how the elastic scattering properties depend on the different available parameters, i.e. the laser detunings and intensities. Our experimental findings can successfully be described with an available theoretical model, which treats ultracold collisions in the presence of laser light. Optical Feshbach resonances have the possibility of becoming a valuable tool for fast switching of interactions. This is of special interest in quantum computation schemes with neutral atoms, where interactions between different atomic qubits need to be controlled.
We study the dynamics of the atomic and molecular Bose-Einstein condensates (BECs) of 87Rb^{87}\text{R}\text{b}87Rb in a spherically symmetric trap coupled by stimulated Raman photoassociation process. Considering the higher order nonlinearity in the atom-atom interaction we analyze the dynamics of the system using coupled modified Gross-Pitaevskii (MGP) equations and compare it with mean-field coupled Gross-Pitaevskii (GP) dynamics. Considerable differences in the dynamics are obtained in these two approaches at large scattering length, i.e., for large values of peak-gas parameter ${x}_{pk}\ensuremath{\ge}{10}^{\ensuremath{-}3}$. We show how the dynamics of the coupled system is affected when the atom-molecule and molecule-molecule interactions are considered together with the atom-atom interaction and also when the strengths of these three interactions are increased. The effect of detuning on the efficiency of conversion of atomic fractions into molecules is demonstrated and the feasibility of maximum molecular BEC formation by varying the Raman detuning parameter at different values of time is explored. Thus by varying the Raman detuning and the scattering length for atom-atom interaction one can control the dynamics of the coupled atomic-molecular BEC system. We have also solved coupled Gross-Pitaevskii equations for atomic to molecular condensate formation through magnetic Feshbach resonance in a BEC of 85Rb^{85}\text{R}\text{b}85Rb. We found similar features for oscillations between atomic and molecular condensates noted in previous theoretical study and obtained fairly good agreement with the evolution of total atomic condensate observed experimentally.