title: Observation of Attractive and Repulsive Polarons in a Bose-Einstein Condensate.
abstract: The problem of an impurity particle moving through a bosonic medium plays a fundamental role in physics. However, the canonical scenario of a mobile impurity immersed in a Bose-Einstein condensate (BEC) has not yet been realized. Here, we use radio frequency spectroscopy of ultracold bosonic ^{39}K atoms to experimentally demonstrate the existence of a well-defined quasiparticle state of an impurity interacting with a BEC. We measure the energy of the impurity both for attractive and repulsive interactions, and find excellent agreement with theories that incorporate three-body correlations, both in the weak-coupling limits and across unitarity. 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 dominated by the many-body continuum of excited states. Crucially, no significant effects of three-body decay are observed. Our results open up exciting prospects for studying mobile impurities in a bosonic environment and strongly interacting Bose systems in general.
authors: Institut for Fysik og Astronomi, Aarhus Universitet, 8000 Aarhus C, Denmark.
authors: Nils B
authors: NB
authors: Jørgensen
authors: Institut for Fysik og Astronomi, Aarhus Universitet, 8000 Aarhus C, Denmark.
authors: Lars
authors: L
authors: Wacker
authors: Institut for Fysik og Astronomi, Aarhus Universitet, 8000 Aarhus C, Denmark.
authors: Kristoffer T
authors: KT
authors: Skalmstang
authors: School of Physics & Astronomy, Monash University, Victoria 3800, Australia.
authors: Meera M
authors: MM
authors: Parish
authors: School of Physics & Astronomy, Monash University, Victoria 3800, Australia.
authors: Jesper
authors: J
authors: Levinsen
authors: Institut for Fysik og Astronomi, Aarhus Universitet, 8000 Aarhus C, Denmark.
authors: Rasmus S
authors: RS
authors: Christensen
authors: Institut for Fysik og Astronomi, Aarhus Universitet, 8000 Aarhus C, Denmark.
authors: Georg M
authors: GM
authors: Bruun
authors: Institut for Fysik og Astronomi, Aarhus Universitet, 8000 Aarhus C, Denmark.
authors: Jan J
authors: JJ
authors: Arlt