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Tunable quantum gases in optical lattices: ground state molecules and strongly-interacting 1D systems

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I will report on several of our recent experiments with tunable quantum gases in optical lattices. In the first experiment, we produce ultracold and dense samples of rovibrational ground state (RGS) molecules near quantum degeneracy. We first associate dimer molecules out of a lattice-based Mott-insulator state loaded from an atomic BEC and then coherently transfer the molecules to the RGS by a multiphoton STIRAP process. With an overall efficiency of 50%, we prepare a molecular quantum gas state in which every second site of an optical lattice is occupied with a RGS molecule [1]. We expect that, with further optimization of the transfer procedure, a BEC of RGS molecules is possible.

In the second experiment, we prepare an exotic many-body, highly-correlated quantum state in 1D geometry known as the super-Tonks-Girardeau (sTG) gas. In contrast to the well-known case of the Tonks-Girardeau (TG) gas, interactions are strongly attractive for the sTG gas. The question thus arises whether this state exists at all and how it can be accessed, as attractive interactions give rise to a family of more deeply bound states. We observe a confinement-induced resonance that allows us to first enter deeply into the TG regime and from there to cross over into the sTG regime, which we find to be surprisingly stable. We analyze the crossover in terms of the collective mode frequencies of the 1D system and in terms of the energy and particle loss [2].

In the third experiment, we observe the superfluid-to-Mott-insulator (SF-MI) phase transition for a strongly-interacting 1D gas. For sufficiently strong interactions, the insulating state is induced by an arbitrarily week lattice, in striking contrast to the SF-MI transition observed for weakly-interacting 3D gases. We map out the phase diagram and find that our measurements agree well with a quantum-field description of the transition based on the well-known sine-Gordon model.

[1] Quantum gas of rovibronic ground-state molecules in an optical lattice, J.G. Danzl et al., to appear in Nature Physics, arXiv:0909.4700 (2009). [2] Realization of an Excited, Strongly Correlated Quantum Gas Phase, E. Haller et al., Science 325, 1224 (2009).

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