An ultracold high-density sample
of rovibronic ground-state molecules
in an optical lattice

ultracold.atoms
Institut für Experimentalphysik,
University of Innsbruck, and
IQOQI
Austrian Academy of Sciences,
Innsbruck, Austria

Ground-state molecules in an optical lattice

Ultracold samples of molecules at high number densities trapped in optical lattices are ideal starting points for a series of fundamental studies in physics and chemistry, including few-body collisional physics, ultracold chemistry, precision measurements, quantum gas preparation and molecular Bose-Einstein condensation, and quantum information processing. For these, full control over all internal and external degrees of freedom of molecules at the level of single quantum states is required. However, high phase-space densities for molecular samples are not readily attainable as efficient cooling techniques such as laser cooling are lacking.

In this work [1], we produce molecular samples in the near quantum degenerate regime with full control over the internal and external degrees of freedom in an optical lattice. We exploit the fact that high phase-space densities are readily achievable for atoms and that atoms can efficiently and state-selectively be associated on a Feshbach resonance to weakly bound molecules, while maintaining the quantum gas character of the sample. We then transfer the molecules with over 50 % efficiency from the weakly bound level to a specific hyperfine sublevel of the lowest vibrational and rotational level of the singlet X 1Σg+ electronic ground-state potential, i. e. of the rovibronic ground state X 1Σg+ |v=0,J=0>. Here, v and J are the vibrational and rotational quantum numbers, respectively. The figure summarizes our preparation procedure.

 

rovibronic ground state molecules

 

We load a BEC of cesium atoms into an optical lattice and drive the superfluid-to-Mott insulator transition. During loading, we aim to create a sample where, in the central region of the lattice, each lattice site is occupied by exactly two atoms by adjusting the external confinement appropriately. The shell structure in the Mott-insulator (MI) arises from the external confinement and we thus aim to maximize the size of the two-atom Mott shell in the center. Due to the extremely high local density at each doubly occupied lattice site, Feshbach association (FA) takes place with near unit efficiency in the optical lattice, resulting in a sample where almost every lattice site is occupied by one weakly bound molecule in the motional ground state of the lattice.

Population transfer is mediated by coherent optical transfer involving a total of four laser transitions, linking the initial weakly bound level |1> to the rovibronic ground state |5> via two intermediate levels |2> and |4> in electronically excited states and one intermediate ground-state level |3> that we populated in our previous experiments [2]. We employ the Stimulated Raman Adiabatic Passage (STIRAP) technique. This four-photon scheme is preferred for Cs2 because of prohibitively low Franck-Condon factors in homonuclear molecules for a single two-photon Λ-type transition for the electronic potentials used here. Transfer is either done in a single 4-photon STIRAP step or in two consecutive 2-photon STIRAP transfers. The coherent population transfer takes away an energy of about kB x 6000 K whereas our molecular samples typically have expansion energies on the order of kB x 10 nK. Here, kB is Boltzmann's constant.

During the transfer, we maintain motional state control by operating the optical lattice at a wavelength that is close to a magic wavelength where the trapping potentials for the initial and the final molecular states are matched. The rovibronic ground state molecules thus remain trapped in the lowest vibrational level of the lattice well and each molecule is trapped at an individual lattice site. We observe a lifetime of 8 seconds of the lattice-trapped ensemble, making it an ideal starting point for precision measurements and quantum gas studies.

Our results present a crucial step towards Bose-Einstein condensation of ground-state molecules and, when suitably generalized to polar heteronuclear molecules, the realization of dipolar quantum-gas phases in optical lattices.

Our results have been published in: Nature Physics (advance online publication, 21 Feb 2010, DOI:10.1038/NPHYS1533)

 

Team

These experiments were carried out in the CsIII experiment with theoretical support from Jesus Aldegunde and Jeremy Hutson from Durham University (UK).

The Innsbruck team (standing, left to right) Elmar Haller, Johann G. Danzl, Russel Hart, Manfred J. Mark
(front, left to right) Mattias Gustavsson, Hanns-Christoph Nägerl

References

[1]  An ultracold high-density sample of rovibronic ground-state molecules in an optical lattice
Johann G. Danzl, Manfred J. Mark, Elmar Haller, Mattias Gustavsson, Russell Hart, Jesus Aldegunde, Jeremy M. Hutson, and Hanns-Christoph Nägerl
Nature Physics advance online publication, 21 Feb 2010, DOI:10.1038/NPHYS1533

[2]  Quantum gas of deeply bound ground state molecules
Johann G. Danzl, Elmar Haller, Mattias Gustavsson, Manfred J. Mark, Russell Hart, Nadia Bouloufa, Olivier Dulieu, Helmut Ritsch, and Hanns-Christoph Nägerl
Science 321, 1062 (2008)

 

Funding

The experiments are supported by the START-price of the Bundesministerium für Wissenschaft und Forschung (BMWF), the Austrian Science Fund (FWF) and by the European Science Foundation within the framework of the EuroQUASAR collective research project QuDeGPM and within the framework of the EuroQUAM collective research project QuDipMol.

last change: 2010-Feb-21 by JGD