MIGROS · Microscopy of Interacting fermi-Gases : high-Resolution Imaging and Statistical properties

Microscopy of Interacting fermi-Gases: High-Resolution Imaging and Statistical Properties

Fermions are particles that are the constituents of matter, such as nucleons or electrons. The properties of a material in various conditions depend on the interplay of its internal structure such as the crystal structure, the interactions between the constituents and their fermionic nature. In the recent years, it has been possible to prepare gases of cold atoms cooled to quantum degeneracy, where their fermionic nature is manifest. A major challenge is to be able to use these cold fermionic atoms to reproduce the intricate behaviour of materials, for which the details of the internal structure is very complex and partially unknown. This research programme is known as quantum simulation, and is actively pursued by tens of research teams worldwide. In this project, we have investigated the behaviour of cold fermionic atoms in various conditions. We have used high-resolution microscopes to observe a small ensemble of atoms immersed in a large cloud, and observed fluctuations of the total spin contained in this region, providing access to the spin susceptibility, and demonstrating the impossibility of a classical description of them. We have then set up an apparatus allowing the observation of conduction of Fermions between two reservoirs connected by a mesoscopic channel. This system faithfully emulates the conduction of electrons in a nanostructure. This breakthrough opens the possibility to investigate transport properties in atomic Fermi gases with direct analogies with solid state physics and material physics. We have been able to observe ballistic conduction, and to observe the contact resistance predicted at the connection of a conductor with reservoirs, in a direct and unambiguous way. We have then used the unique possibility to tune interactions between atoms to investigate conduction of very strongly interacting fermions, which were known to display superfluidity. We have observed the very low resistance associated to the onset of superfluidity. Eventually, we have investigated the properties of disordered superfluids, produced by applying a controlled disorder onto a clean superfluid. We have observed the interplay of disorder and superfluidity and observed the breakdown of the latter for strong disorder. Our experiments open the way towards a systematic investigation of transport in cold atomic systems. Transport properties are very sensitive to the strong quantum correlations that emerge when interactions are strongly enhanced. At a quantum phase transition, they show universal properties that are connected to the deepest concepts of theoretical physics, such as the structure of space-time in the neighbourhood of a black hole (the so-called AdS-CFT correspondence). With the progresses demonstrated during the project, one can now realise not only simulated materials but simulated devices made of several parts, realising different functions and exchanging particles with each other. These cold atom devices allow to explore the behaviour of matter in a complementary regime as electronic devices, and may open new perspectives on quantum machines and quantum computers.

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