Ultracold Molecules Research Projects

We are setting up an experiment to produce a Bose Einstein condensate of molecules (MBEC) via the association of ultracold atoms. Molecular BECs increase the complexity and richness of phenomena that can be investigated. In collaboration with the ACQAO theory group at the University of Queensland, we propose to use the MBEC to study the dissociation of the quantum degenerate molecules into correlated (entangled) atom pairs [1], and to investigate the coherent interaction between the MBEC and a quantum degenerate quantum atomic gas and dynamical processes such as Bose enhanced molecule formation.

The systems initially chosen for study were molecular gases obtained from bosonic atoms (⁸⁷Rb, ¹³³Cs). However, investigations during the last year by several groups [2, 3, 4] have demonstrated that it is possible to produce a very stable MBEC obtained from fermionic ⁶Li atoms, which exhibit lifetimes of some tens of seconds, compared with typically 100 ms in the case of quantum degenerate molecular gases obtained from bosonic ²³Na, ⁸⁷Rb or ¹³³Cs atoms. The huge enhancement of the lifetime for MBECs based on fermionic 6Li atoms is a manifestation of Pauli blocking and represents a major breakthrough in the field of quantum degenerate molecular gases. In April 2004, a decision was made to switch from bosonic ¹³³Cs atoms to fermionic ⁶Li atoms.

In our experimental scheme a σ- Zeeman slower is used to produce a continuous high-flux beam of ⁶Li atoms at speeds low enough to load a magneto-optical trap (MOT). The atoms are then transferred to a far-off-resonance optical dipole trap (FORT), which is used to trap and evaporate the atoms and molecules. The FORT is produced by beams from a 20 W single-frequency ELS Yb:YAG laser at 1030 nm. The scattering length of the atoms is controlled via Feshbach resonances in magnetic fields up to 1.5 kG. Evaporation in the optical dipole trap is performed at magnetic field strengths that enhance three-body recombination to form ⁶Li₂ dimers, similar to the scheme used in [2].

Theoretical research on processes for generating MBECs from atomic condensates is being carried out. One process of interest combines the Feshbach resonance with STIRAP (Stimulated Raman Adiabatic Passage) leading to a MBEC in the ground vibrational state, and hence closer to absolute zero in temperature. Ideally, such conversion processes are coherent, but various decoherent processes such as spontaneous emission need to be taken into account to be realistic. Also, quantum fluctuations around the condensate wavefunctions, occurring when the number of atoms or molecules is small, would be important. The unified quantum Monte Carlo formalism developed by the ACQAO theory group at the University of Queensland [5] could be suitable for treating these issues.

In a related experiment we are setting up to produce ultracold (T < 1 mK) Rb₂ molecules by the photoassociation (‘light-assisted’ collisions) of laser-cooled rubidium atoms. The photoassociated molecules are captured in a FORT trap, based on a tightly focussed infrared beam from a 50 W CO₂ laser. Such an optical trap has a large detuning from all relevant atomic and molecular transitions, and thus long storage times should be possible.

1. K.V. Kheruntsyan and P.D. Drummond, Phys. Rev. A 66, 031602R (2002).
2. S. Jochim, M. Bartenstein, A. Altmeyer, G. Hendl, S. Reidl, C. Chin, J. Denschlag and R. Grimm, Science 302, 2101 (2003).
3. M.W. Zwierlein et al., Phys. Rev. Lett. 91, 250401 (2003).
4. T. Bourdel et al., Phys. Rev. Lett. 93, 050401 (2003).
5. J.F. Corney and P.D. Drummond, Phys. Rev. A 68, 063822 (2003); cond-matt/0417712.