The CQOM-Colloquium talks will be presented as part of the opening of the Center for Quantum Optics and Quantum Matter. Coffee and cake will be served in between both talks.

CQOM TALK

Nils Byg Jørgensen IFA, Aarhus University

TITLE: Observation of Attractive and Repulsive Polarons in a Bose-Einstein Condensate ABSTRACT: The problem of a mobile impurity particle in a bosonic medium plays a fundamental role in physics ranging from organic electronics to the Standard Model. We use radio frequency spectroscopy of ultracold bosonic 39K atoms to experimentally demonstrate the existence of a well-defined quasiparticle state of an impurity interacting with a Bose-Einstein condensate. We study the energy spectrum for both repulsive and attractive interactions, including the strong coupling regime. The spectral response consists of a well-defined quasiparticle peak at weak coupling, while for increasing interaction strength, the spectrum is strongly broadened and becomes a many-body continuum of exited states. A theoretical model which incorporates three-body correlations reproduces the spectrum to excellent agreement without including decay effects, which validates the existence a long lived polaron. Our results open up exiting prospects for studying mobile impurities in a bosonic environment and strongly interacting Bose systems in general.

INVITED TALK

Richard Schmidt ITAMP, Harvard-Smithsonian Center for Astrophysics Harvard University

TITLE: Many-body physics with quantum impurities in cold atoms and beyond ABSTRACT: When an impurity is immersed into an environment, it changes its properties due to its interactions with the surrounding medium. The impurity is dressed by many-body excitations and forms a quasiparticle, the polaron. Depending on the character of the environment and the form of interactions, different types of polarons are created. In this talk, I will review recent experimental and theoretical progress on studying the many-body physics of polarons in ultracold atomic systems [1], and discuss related polaronic phenomena encountered in two-dimensional semiconductors [2] and the study of rotating molecules in superfluid Helium [3]. In the second part of the talk I will then focus on impurities interacting with bosonic quantum gases. Specifically, I will discuss progress on the theoretical description of Rydberg excitations coupled to Bose-Einstein condensates. In such systems the interaction between the Rydberg atom and the Bose gas is mediated by the Rydberg electron. This gives rise to a new polaronic dressing mechanisms, where instead of collective excitations, molecules of gigantic size dress the Rydberg impurity. We develop a functional determinant approach [4] to describe the dynamics of such Rydberg systems which incorporates atomic and many-body theory. Using this approach we predict the appearance of a superpolaronic state which has recently been observed in experiments [5,6], and in this talk I will discuss its relation to Feshbach Bose polarons observed in ultracold atom experiments [7].

[1] R. Schmidt, M. Knap, D. A. Ivanov, J.-S. You, M. Cetina, and E. Demler, arXiv:1702.08587 (2017); to appear in Rep. Prog. Phys.

[2] M. Sidler et al., Nature Physics 13, 255 (2017)

[3] R. Schmidt, and M. Lemeshko, Phys. Rev. Lett. 114, 203001 (2015); M. Lemeshko and R. Schmidt, arXiv:1703.06753 (2017).

[4] R. Schmidt, H. Sadeghpour, and E. Demler, Phys. Rev. Lett. 116, 105302 (2016).

[5] F. Camargo et al., arXiv:1706.03717 (2017).

[6] R. Schmidt et al., arXiv:1709.01838 (2017).

[7] N. Jørgensen et al., Phys. Rev. Lett. 117, 055302 (2016).