July 2011

Abstracts of the QSIT Lunch Seminar, July 7, 2011

Criticality with trapped cold atoms

Lode Pollet, Institute for theoretical Physics, ETH Zurich

Identifying different phases in the models realized with cold atomic gases such as the Bose-Hubbard model is nowadays well under control, but can we study the phase transitions between them? Do the trapping potentials and the finite size of the atomic cloud not form impossible barriers to study critical behavior? I will show how those effects do not prevent the study of criticality, but impose generic limits  on the accurate determination of the critical point for second order phase transitions, from which optimal protocols to extract the critical point can be devised.  I will also give an overview of recent experiments aiming at the study of criticality in the superfluid to Mott-insulator transition in the Bose-Hubbard model, by using single-site in-stu density measurements or by time-of-flight interference images.

Quantum-state controlled ultracold molecular ions and their applications

Xin Tong, Quantum Coherence Lab, University of Basel

The generation and study of translationally cold (T << 1K) molecules and molecular ions represents one of the most recent and exciting new developments in physical chemistry. Molecular ions can be cooled and Coulomb crystallised by exchanging kinetic energy with laser cooled atomic ions by means of the Coulomb interaction (“sympathetic cooling”) thus forming bicomponent or “molecular” Coulomb crystals. However sympathetically cooled molecular ions are only translationally cold, their internal degrees of freedom are generally in thermal equilibrium with the environment because of the coupling to the ambient black-body radiation (BBR) field.

Using N2+ as a test system, we recently developed a new method to produce internal quantum-state selected, translationally cold (mK) molecular ions in a quadrupole ion trap. Our approach is based on the initial generation of the N2+ ions in a desired quantum state using state-selective threshold-photoionization followed by sympathetic cooling of their translation motion. We discuss the performance of the new method and highlight upcoming applications of sympathetically-cooled, state-selected ions.

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