Solid State Theory Seminar at LMU: Zooming in on Ultracold Few-Fermion Systems

February 14, 2020
Philipp Preiss, Heidelberg University, Germany
LMU München
Theresienstr. 37, IV, Room A 450
80333 Munich
Friday, February 14, at 9 a.m.

The emergence of collective modes from single-particle excitations is one of the most striking features of strongly interacting systems. Understanding such excitations is an ongoing challenge in nuclear physics, correlated electron systems, and high-energy physics. Ultracold atoms in optical potentials provide a unique setting to precisely study the appearance of collective excitations in a tunable laboratory environment. Here we experimentally observe the “birth" of a collective mode in a few-body system of ultracold fermions. Using optical tweezers, we deterministically prepare few fermions in the ground state of a two-dimensional trap. This system exhibits a shell structure of stable “magic" numbers of 2,6,12... particles. We perform many-body spectroscopy through a modulation of the interaction strength and find both single-particle and two-particle excitations. The latter consist of pairwise excitations akin to Cooper pairs and can be identified as the precursor of the Higgs mode in a two-dimensional Fermi gas. In the future, we will probe such mesoscopic Fermi systems with single-particle detection. We recently demonstrated spin-resolved fluorescence imaging of individual atoms in free space, which will allow us to detect the momenta of every particle in the system in time-of-light. We expect to directly see the formation of Cooper pairs and the momentum space signature of the BEC-BCS crossover.

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