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Quantum transport in a disordered two-dimensional ultracold Fermi gas

Periodic Reporting for period 1 - VorDIST (Quantum transport in a disordered two-dimensional ultracold Fermi gas)

Okres sprawozdawczy: 2019-09-01 do 2021-08-31

The field of ultracold atoms has undergone rapidly diversifying research subjects during the last decade, flourishing across quantum optics, quantum information, condensed matter physics, quantum chemistry, etc. Meanwhile, thanks to the recent technological advancements such as high-resolution microscope systems and digital micromirror devices, arbitrary manipulation and engineering of optical potentials at short-length scales in atomic systems have become more available. These developments indeed allow for more systematic experimental studies that have been practically challenging. In this project, the main objectives are to resolve compelling questions on quantum transport phenomena in strongly correlated superfluids by maximally utilizing the controllability and tunability of atomic quantum gases. First, we have addressed the problems of measuring condensate fraction in strongly interacting superfluids with dc Josephson supercurrent. Second, We have revealed the details of quantum vortex decay by building a programmable vortex collider in a homogeneous, planar superfluid. Our experimental work constitutes a clear step beyond the state of the art, which could deepen our understanding of quantum transport.
We exploit controlled Josephson currents through a moving tunable tunneling barrier to probe condensation in strongly correlated fermionic superfluids of ultracold atoms. In the absence of any applied chemical potential difference, for different junction parameters and interaction strengths throughout the BEC-BCS crossover, we extract the Josephson critical current. By comparing the experimental results with analytical prediction based on chemical potential and condensate density of the system, we observed that the critical current indeed depends on the condensate density (not the superfluid density) and extracted condensate fraction across the BEC-BCS crossover. This work demonstrates the effectiveness of coherent Josephson transport for pinning down superfluid order parameters, even in strongly correlated superfluids [1]. Furthermore, we have extended our work by measuring the temperature dependence of the dc Josephson supercurrent. We have observed the critical breakdown of coherent Josephson transport with the increase of temperature, by which we have extracted the critical temperature around the unitary-limited interaction. By generalizing our analytic model to finite temperature including the thermal depletion of the condensate density, we have made a comparison between the experimental results and the model prediction. We have found a good agreement between them which evidences a firm link between the critical current and the condensate fraction at any temperature [2].

A deep understanding of the elementary mechanisms behind quantum vortex energy dissipation has been restricted by the scarcity of experimental signatures. We have addressed this outstanding problem by realizing a programmable vortex collider in a thin, homogeneous fermionic superfluid across the BEC-BCS crossover. We create on-demand vortex configurations and engineer collisions within and between vortex-antivortex pairs. These allow us to decouple dissipation of the vortex energy due to sound emission and due to mutual (thermal) friction. By controlling the dipole sizes for vortex dipole-dipole collisions, we directly visualize how the annihilation of two colliding dipoles radiates a sound pulse, converting the whole energy stored in the swirling field and vortex cores into compressible sound energy. Our experiments provide a comprehensive picture of quantum vortex decay arising from mutual friction, vortex-sound interaction, and a possible contribution from fermionic core-bound states inside a vortex core [3].

[1] W. J. Kwon et al., "Strongly correlated superfluid order parameters from dc Josephson supercurrents", Science 369, 84 (2020).
[2] G. Del Pace et al., "Tunneling Transport of Unitary Fermions across the Superfluid Transition", Physical Review Letters 126, 055301 (2021).
[3] W. J. Kwon et al., "Sound emission and annihilations in a programmable quantum vortex collider" arxiv: 2105.15180 (2021).
Ultracold atomic system provides a pristine platform for systematically studying many fundamental problems in condensed matter physics. In this project, by employing high controllability of strongly interacting atomic superfluids combined with dynamical manipulation of optical potentials, we have addressed the elementary questions related to transport properties of quantum fluids. In particular, we have revealed that the Josephson critical currents in strongly interacting Fermi superfluids represent a reliable quantifier of the condensate fraction of superfluids, whose direct determination has typically been hindered by strong interactions thus remained inconclusive. As a second part of the project, we have unveiled the dissipation mechanisms for quantum vortex decay across the BEC-BCS crossover by performing vortex collision experiments, which could throw new light on the understanding of dissipative vortex dynamics. This vortex work could be extended towards investigations of general aspects of vortex-sound interaction and of fundamental differences in quantum vortices originating from the microscopic nature of superfluids.