Quantum matter and materials:
Hanns-Christoph Naegerl (Institut fur Experimentalphysik, Universitat Innsbruck)
Ultracold atomic gases for the investigation of strongly-correlated 1D many-body systems
I will report on two of our recent experiments with tunable atomic quantum gases in optical lattices. In the first 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 should lead to instability. We observe a confinement-induced resonance, which 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 [1].
In the second experiment, we study the properties of a strongly-correlated 1D gas beyond the well-known context of the Mott-Hubbard model. We observe the superfluid-to-Mott-insulator (SF-MI) phase transition, which, for sufficiently strong interactions, but not yet in the TG regime, the insulating state is induced by an arbitrarily week lattice. This happens 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 quantum sine-Gordon model {2].
[1] Realization of an Excited, Strongly Correlated Quantum Gas Phase, E. Haller et al., Science 325, 1224 (2009).
[2] Observation of the commensurate-incommensurate quantum phase transition for a Luttinger liquid of strongly interacting bosons, E. Haller et al., manuscript in preparation.
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Henk Stoof (
Ultracold Fermi mixtures
With ultracold quantum gases it is possible to study in exquisite detail fundamental questions about quantum many-body systems. In this talk we give an overview of the understanding that has been obtained in the last few years of strongly interacting Fermi mixtures, if we vary both the temperature and the density ratio or polarization of the two components of the mixture. This understanding is not only of importance for atomic physics, but also for condensed-matter physics, nuclear physics, and (astro)particle physics, where such strongly interacting Fermi mixtures also exist. In all these cases exotic forms of superconductivity, which deviate strongly from the conventional superconductivity in metals, play an important role.
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Ground state properties of the BEC-impurity polaron
1. TQC, Universiteit Antwerpen,
2. Lyman Laboratory of Physics,
We show that if you apply the Bogoliubov approximation on the Hamiltonian of an impurity in a Bose-Einstein condensate it is possible to cast the resulting Hamiltonian in the generic form of Frölich's polaron Hamiltonian. As compared to the archetypical polaron the role of the electron is replaced by the impurity and the role of the phonons by the Bogoliubov excitations. With the specific dispersion and interaction amplitude for the system we derive an expression for the polaronic coupling strength as a function of the scattering lengths, the trap size and the number of bosons. This relation identifies several approaches to reach the polaronic strong coupling regime that so far hasn't been experimentally reachable in other polaron systems. We then make use of Feynman's path-integral approach to calculate the polaronic shift in the free energy and the effective mass for arbitrary coupling strength. This is done in a formalism that allows us to include the temperature dependence. We find some similarities with the acoustic polaron and indications of a transition between a free polaron and a self-trapped state. The current theory is applied to the specific system of a lithium impurity in a sodium condensate. See reference [1] for the article that arose from this research.
This research was supported by FWO-Vlaanderen and we would also like to thank M. K. Oberthaler, E. Timmermans and J. H. Denschlag for the enlightening discussions about the subject.
[1] J. Tempere, W. Casteels, M. K. Oberthaler, S. Knoop, E. Timmermans and J. T. Devreese - Feynman path-integral treatment of the BEC-impurity polaron – Phys. Rev. B 80, 184504-1, 2009.
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J. J. Renema1, A. Louchet-Chauvet1, D. Oblak1, J. Appel1, P. J. Windpassinger1, U. Hoff1, N. Kjærgaard1, E. S. Polzik1
Entanglement-assisted atomic clock beyond the projection noise limit
1. Quantop, Niels Bohr Institute,
In a proof-of-principle experiment, we show how the performance of an atomic clock can be enhanced by spin squeezing. Using a quantum non-demolition measurement, we create entanglement in a cloud of 105 cold Cesium atoms [1]. By using a modified Ramsey clock sequence, this entanglement is used to enhance the sensitivity of an atomic clock. The experiments show an improvement of 1.1 dB in the clock's signal-to-noise ratio, as compared the atomic projection noise limit [2].
This research was supported by EU grants COMPAS, Q-ESSENCE, HIDEAS, and QAP.
[1] J. Appel et al. Proc. Natl. Acad. Sci. U.S.A. 27 (106) 10960, 2009
[2] A. Louchet-Chauvet et al. ArXiv:0912.3895, 2009
