## Ultracold fermionic superfluids

Ultracold fermionic quantum gases are ideal model systems to investigate complex phenomena of quantum physics – such as superconductivity in solids – in a well-controlled environment. We aim for a better understanding of how various superfluid quantum states behave in external and dynamically-controlled environments. Particularly, we want to understand how controlled impurities allow manipulating the properties of a superfluid system.

## Experiment

In our Lab, ultracold quantum gases of fermionic lithium-6 atoms are prepared at temperatures close to absolute zero. This is achieved by applying several subsequent cooling mechanisms utilizing laser beams and magnetic fields. The quantum gases are prepared in ultra-high vacuum, isolating the atoms from the vicinity. In total, the preparation of a quantum gas takes a few seconds.

Due to the ultracold temperatures of less than a millionth Kelvin above absolute zero, quantum effects, which would otherwise be dominated by thermal effects, arise.

The interaction between the atoms is manipulated by applying strong magnetic fields and can be changed between attraction and repulsion. This allows to study the continuous transition of a superfluid as in a superconductor in the Bardeen-Cooper-Schrieffer (BCS) regime to a molecular Bose-Einstein condensate (mBEC). In between lies the unitary regime with strongest interactions.

Outside of laboratories, a system that can be prepared as clean and as precisely does not exist. Imperfections like defects in solids, thermal fluctuations or general disorder have an enormous impact on the properties of a material such as its conductivity. We investigate these phenomena by employing light fields that are scattered on diffusing glass plates, creating spatially-randomized potential landscapes. We can create static or temporally-varying disorder potentials.

## Quantum Engine

Heat engines convert thermal energy into mechanical work both in the classical and quantum regimes. However, quantum theory offers genuine nonclassical forms of energy, different from heat, which so far have not been exploited in cyclic engines to produce useful work. We here experimentally realize a novel quantum many-body engine fuelled by the energy difference between fermionic and bosonic ensembles of ultracold particles that follows from the Pauli exclusion principle. We employ a harmonically trapped superfluid gas of 6Li atoms close to a magnetic Feshbach resonance which allows us to effectively change the quantum statistics from Bose-Einstein to Fermi-Dirac. We replace the traditional heating and cooling strokes of a quantum Otto cycle by tuning the gas between a Bose- Einstein condensate of bosonic molecules and a unitary Fermi gas (and back) through a magnetic field. The quantum nature of such a Pauli engine is revealed by contrasting it to a classical thermal engine and to a purely interaction-driven device. We obtain a work output of several million vibrational quanta per cycle with an efficiency of up to 25 %. Our findings establish quantum statistics as a useful thermodynamic resource for work production, shifting the paradigm of energy-conversion devices to a new class of emergent quantum engines.

## Ultracold Bose Gases in Dynamic Disorder with Tunable Correlation Time

We study experimentally the dissipative dynamics of ultracold bosonic gases in a dynamic disorder potential with tunable correlation time. First, we measure the heating rate of thermal clouds exposed to the dynamic potential and present a model of the heating process, revealing the microscopic origin of dissipation from a thermal, trapped cloud of bosons. Second, for Bose-Einstein condensates, we measure the particle loss rate induced by the dynamic environment. Depending on the correlation time, the losses are either dominated by heating of residual thermal particles or the creation of excitations in the superfluid, a notion we substantiate with a rate model. Our results illuminate the interplay between superfluidity and time-dependent disorder and on more general grounds establish ultracold atoms as a platform for studying spatiotemporal noise and time-dependent disorder.

## Observing the loss and revival of long-range phase coherence through disorder quenches

Relaxation of quantum systems is a central problem in nonequilibrium physics. In contrast to classical systems, the underlying quantum dynamics results not only from atomic interactions but also from the long-range coherence of the many-body wave function. Experimentally, nonequilibrium states of quantum fluids are usually created using moving objects or laser potentials, directly perturbing and detecting the system’s density. However, the fate of long-range phase coherence for hydrodynamic motion of disordered quantum systems is less explored, especially in three dimensions. Here, we unravel how the density and phase coherence of a Bose–Einstein condensate of ^{6}Li_{2} molecules respond upon quenching on or off an optical speckle potential. We find that, as the disorder is switched on, long-range phase coherence breaks down one order of magnitude faster than the density of the quantum gas responds. After removing it, the system needs two orders of magnitude longer times to reestablish quantum coherence, compared to the density response. We compare our results with numerical simulations of the Gross–Pitaevskii equation on large three-dimensional grids, finding an overall good agreement. Our results shed light on the importance of long-range coherence and possibly long-lived phase excitations for the relaxation of nonequilibrium quantum many-body systems