Quantum impurity problems

Quantum impurities in ultracold atomic systems

The problem of an impurity coupled to a bath lies at the heart of numerous quantum many-body phenomena. From a theoretical point of view quantum impurity problems provide a minimal framework for gaining insight on the properties of multi-component systems, both in- and out-of-equilibrium. What's more, some of the most interesting properties in solid-state systems arise from the interaction between electrons and impurity degrees of freedom. Depending on their nature, impurities can couple to the bath in different ways, leading to a rich, genuinely strong correlation phenomenology dominated by impurity-bath interactions.

Ultracold atoms provide an interesting playground -- and complementary to traditional solid-state materials -- for the investigation of impurity problems. Their high degree of controllability allows to prepare on-demand impurity model systems, where parameters such as impurity concentration and mobility can be controlled accurately. Moreover, the typical time scales involved in the ensuing many-body dynamics, related to the Fermi energy in fermionic systems, are much slower for atomic systems then for electronic ones. This makes ultracold atoms a suitable platform for tackling open questions concerning the out-of-equilibrium dynamics of impurity problems, which can be investigated e.g. with interferometric probes.

The peculiar level structure of alkaline-earth-like atoms -- such as ytterbium -- makes them optimal candidates for the exploration of impurity physics in the strongly interacting regime. In particular, the metastable clock state allows to implement two-orbital systems, where atoms in the ground and clock states can be selectively addressed. The clock transition provides an ultra-precise spectroscopic probe which can be leveraged for high-resolution interferometric measurements. Combining these features with the local control provided by optical tweezers, we investigate the physics of individually controlled quantum impurities (spinless or spinful) embedded in a fermionic medium.

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