Study of quantum many body effects in ultracold trapped bose gases

Robert Chiaramonte

L. Rosales-Zárate, Rocío Jáuregui

In contrast to conventional ideal Bose and Fermi degenerate gases, atoms in experimental ultracold gases interact among themselves and are trapped by an external potential. In this work, we consider two particles of mass m trapped by a 3D harmonic potential of frequency ω, and interacting through an isotropic short range potential of intensity V0 and range b/2, with Eigenfunctions and eigenenergies are obtained and compared with those resulting (a) from an effective contact interaction and (b) in the absence of the trapping potential. Concepts like zero-resonance and binding energy, usually introduced for problems that allow acontinuum spectra, are discussed for trapped particles.

Ming-Guang Hu

An ultracold Bose-Fermi gas mixture of 40K and 87Rb atoms has tunable interspecies interactions and therefore provides a fantastic platform for exploring not only few-body physics such as Feshbach molecule formation and Efimov trimers, but also many-body physics including Bose polarons, quantum Hall physics and so on. In this thesis, I present experimental evidence of Bose polarons in cold atoms obtained using radio-frequency spectroscopy to measure the excitation spectrum of fermionic 40K impurities resonantly interacting with a BEC of 87Rb atoms. These Bose polarons originate from the dressing of an impurity coupled to its environment, which is an important paradigm in quantum many-body physics. I also present initial work that launches an exciting new direction for our experiment, which is exploring rotating quantum gases. Goals for this work include studying both rotating Bose and Fermi superfluids with tunable interactions as well as working toward rapidly rotating quantum gases in the quantum Hall regime. For these goals, a new all-optical trap for rotating gases was designed, implemented, and tested using a 87Rb Bose-Einstein condensate.

Kevin E. Strecker, Guthrie B. Partridge, Andrew G. Truscott +1 more

V. E. Colussi

Advances in trapping and cooling of ultracold gases over the last several decades have made it possible to test many formerly outstanding predictions from disparate branches of physics. This thesis touches on three historical problems that have found new life recently in the context of ultracold Bose gases of alkali atoms. The first problem revolves around an outstanding prediction from Boltzmann over a century and half old that the breathing mode of a isotropically trapped classical gas should oscillate indefinitely. I analyze recent experimental results [Nat. Phys. 11, 1009 (2015)], and attribute observed damping sources to trap imperfections. The second question is about the analogue of first and second sound modes from liquid helium in trapped dilute gases. I present the results of a joint theoretical/experimental investigation of the breathing mode of a finite temperature Bose-Einstein condensate (BEC), attributing a striking collapse revival behavior of the resultant oscillation to in-phase and out-of-phase normal modes of the thermal cloud and condensate. The third problem is that of the formation of Borromean ring-like three-body bound states, referred to as Efimov trimers, in strongly-interacting few-body systems. I extend the predicted spectrum of Efimov states into the realm of many degenerate internal levels, and investigate the difficult three-body elastic scattering problem.\nThese questions are part of the broader theme of this thesis: How can our understanding of few-body physics in the ultracold limit be translated into statements about the bulk behavior of an ultracold gas? For weakly-interacting Bose gases, this translation is well-known: the many-body properties of the gas are well-described by the tracking just the one and two particle correlations. I analyze a generalization of this procedure to higher order correlations, the general connection between few-body physics and correlations in a dilute gas, and results for the emergence of Efimov physics in the magnetic phase of the strongly-interacting Bose gas.

Mark Lee, Janne Ruostekoski