Probing universal few-body physics with ultracold Cs atoms

In the last few years, ultracold atomic gases with tunable interactions have shown their unique potential for experimental investigations of phenomena related to universal few-body physics. The Efimov effect stands as a paradigm for few-body physics, and the new test bed provided by ultracold atoms has led to the emerging research field of few-body physics with ultracold atoms. The present proposal is dedicated to universal few-body phenomena in ultracold Cs gases. This atomic species has been the first one to reveal several elementary three- and four-body phenomena. However, all previous experiments on Cs have been performed in a region of low magnetic fields (<100G), where the range of interaction tunability is limited. Following a major technical upgrade of the Cs apparatus in Innsbruck, the necessary laboratory infrastructure is now at our disposal to apply high magnetic fields of up to 1200 G. This opens up completely new possibilities for interaction tuning via two broad Feshbach resonances. The new window of interaction tuning will allow us to probe few-body phenomena in novel regimes, to connect them to previous observations, and thus to answer important questions of general relevance in the field. The project is organized along two main lines: " Few-body spectroscopy in an atomic cesium gas will be performed by measuring three-body (and four-body) recombination with the main goal to pinpoint the unknown positions of Efimov resonances and of related recombination minima. This will probe the hypothesis of a universal connection between these phenomena, and will address the open question of a possible variation of the so-called "three-body parameter." The central question is whether knowledge of the s-wave scattering length alone is enough to connect different few-body features or whether short-range interactions lead to a more complicated dependencies. " Probing few-body phenomena with weakly bound cesium dimers. Samples of ultracold dimers and atom-dimer mixtures offer complementary approaches to access few-body phenomena. Experiments on atom-dimer resonances will thus complement the studies on three-body phenomena in atomic gases. In addition, experiments on dimer- dimer interactions will target universal four-body phenomena. An important goal is to observe resonant dimer- dimer interaction phenomena as predicted in theoretical work. Universal four-body resonance phenomena have been inaccessible so far. For both research lines, cesium atoms have important advantages as compared to other species. For magnetic interaction tuning, Cs offers three broad Feshbach resonances in the same spin channel. Moreover, the rich molecular structure is particularly well suited to prepare ultracold samples of dimers and atom-dimer mixtures.

In 1970, the Russian physicist Vitaly Efimov predicted the existence of three-body quantum states with bizarre properties. According to his theory, there should be an infinite series of quantum states with increasing size, and binding three bodies together should be possible even under conditions when the system is too weak to support two-body bound states. At that time many scientists did not believe in Efimovs predictions, but now they represent the paradigm of a new research field in the synergy region between quantum many-body physics, molecular and nuclear physics, and ultracold physics. Experimentally, Efimovs mysterious states were for the first time observed in 2006, i.e. 36 years after the prediction. The experiments at the University of Innsbruck are based on extremely cold gases of cesium atoms prepared in laser traps at temperatures only 10 billionths of a degree above absolute zero. When Efimov states are formed at very particular strengths of the magnetic field, particles can escape from the trap. This leads to characteristic loss resonances, then serving as fingerprints of Efimov states.The completed research project was dedicated to problems of fundamental importance, unsolved in previous work. In particular, we investigated the question what determines the size of the first state in Efimovs infinite series and identified the strength of the van-der-Waals interaction between atom pairs as the main factor. This question had been controversially discussed in the literature before, and the experimental results stimulated new theoretical investigations and finally led to a deeper understanding of the problem. Another important breakthrough was the precise measurement of the so-called Efimov period, which characterizes the size ratio of two subsequent three-body states. The theoretical expectation of 22.7 could be essentially confirmed by the experimental result of 21.0 (with an uncertainty of 1.3), but the result also hints on to a possible deviation towards smaller values in real atomic systems.The research project gave essential contributions to a fundamental understanding of Efimov three-body quantum states, and thus also provides a conceptual link between the relatively simple physics of two-body systems to more complex quantum many-body states.