Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - SCOUTFermi2D (Strongly correlated ultracold fermions in two-dimensional tailored optical potentials: pairing, superfluidity and disorder)

Teaser

The experimental study of strongly correlated materials and their description using simplified models are among the fundamental challenges of modern quantum sciences. Ultracold atomic simulators offer an effective approach to address open problems in quantum many-body physics:...

Summary

The experimental study of strongly correlated materials and their description using simplified models are among the fundamental challenges of modern quantum sciences. Ultracold atomic simulators offer an effective approach to address open problems in quantum many-body physics: clean and controllable quantum systems can be directly realized, with the possibility of externally adjusting the key parameters, such as interparticle interactions and dimensionality. Owing to the impressive advances in atom trapping and manipulation during the last decade, paradigmatic models can be implemented, opening new avenues for the realization of “designer materials” with tailored quantum properties. In this context, it has recently become possible to engineer nearly arbitrary optical potentials for cold atoms at the micrometer scale: the light patterns generated by digital micromirror devices (DMD) can be imprinted onto the atoms through high-resolution optical microscopes. This, in combination with optical lattices or other confining potentials, allows for realizing tunable geometries with unprecedented precision and flexibility, to emulate for instance the transport structures typical of electronic devices within the emerging field of atomtronics.
In this project, we aimed at realizing an atomic Fermi gas with tunable interactions, trapped in tailored optical potentials within different two-dimensional geometries. By implementing advanced probing techniques, such as high-resolution imaging and matter-wave interference, in combination with macroscopic transport measurements, we can access essential observables of the system such as spatial correlations, collective modes and transport coefficients. We have completed a versatile and stable experimental setup, featuring high-resolution imaging of ultracold lithium gases, fast high-precision radiofrequency (RF) spectroscopy and arbitrary high-resolution potential imprinting. The combination of such advanced tools offers novel, unique possibilities for exploring the nature of fermionic superfluidity from three to two spatial dimensions, also in the presence of controllable disorder or multi-flavored fermionic mixtures.

Work performed

During this project, several tasks of both technical and scientific nature have been undertaken in parallel, in order to reach the planned objectives.

- We have developed and characterized various technical advances to the experimental setup, which can produce Fermi gases and paired superfluids trapped in nearly arbitrary sub-micrometer optical structures and imaged in situ with high optical resolution. Absorption imaging with sub-micrometer resolution is afforded by a newly implemented optical system, featuring a bi-chromatic microscope objective that has been characterized before integration into the experimental machine. Arbitrary potentials are imprinted onto the quantum gas by projecting DMD patterns (see Fig. 1). The pattern fed to the DMD array is optimized through a newly developed feedback algorithm, which compensates the imperfections of the illumination field and of the optical setup. The DMD setup performance has been characterized before integration into the main experimental apparatus, as summarized in Ref. [1]. The setup has been already shown to produce binary-disorder potentials with sub-micrometer correlation length (see Fig. 2).
- We have made important and necessary steps towards the reliable production of quasi-two-dimensional atomic Fermi gases. First, we have implemented and characterized a new optical setup for creating a single quasi-two-dimensional optical trap to confine the atomic gas in a single two-dimensional layer. The setup generates a highly anisotropic TEM01-mode Gaussian beam (see Fig. 3 and Ref. [2]), which satisfies the pointing and intensity stability requirements. Second, we have designed, built and characterized a novel large-spacing optical lattice, based on a minimal interferometric scheme, featuring exceptional intrinsic fringe stability < d/20 over several hours (see Fig. 4 and Ref. [3]). Both these potentials are presently undergoing integration into the main apparatus. The combination of the realized setups will also allow in the future to study individual one-dimensional Fermi systems with unprecedented in-situ resolution and probing capabilities.
- We have developed a fast RF spectroscopic technique with resolution < 100Hz, allowing a rapid, accurate manipulation of the Fermi gas spin composition and the coherent control over lithium lowest hyperfine levels. This tool has been already exploited to explore the still debated physics of strongly repulsive Fermi gases. In particular, in a first study we have probed the properties of repulsive spin impurities immersed in a Fermi gas (see Ref. [4]), characterizing the so-called polaron quasiparticles. More recently, we have investigated the dynamics of a balanced spin mixture undergoing a sudden change of interparticle interactions, using a RF pump-probe spectroscopy scheme (see Fig. 5 and Ref. [5]).
- As a fundamental issue in itself, and in preparation to the study of superfluid transport in two dimensions, we have studied the relation between dissipative transport and superfluid excitations in a planar Josephson junction-like geometry in three dimensions. We have found that vortex-induced phase slippage is the dominant microscopic source of dissipation across the BEC-BCS crossover. The results of this study are published in Ref. [6].

[1] G. Del Pace, Master Thesis (University of Florence, 2017)
[2] M. Bertrand, Research internship report (ESPCI, Paris and LENS, Florence, 2016)
[3] E. Lippi, Master Thesis (University of Florence, 2017)
[4] F. Scazza et al., Phys. Rev. Lett. 118, 083602 (2017)
[5] A. Amico et al., Phys. Rev. Lett. 121, 253602 (2018)
[6] A. Burchianti et al., Phys. Rev. Lett. 120, 025302 (2018)

Final results

Strongly correlated fermionic gases are a very active field of research, as they are closely related to the physics of strongly correlated electron materials. The experiments performed within this project explore fundamental concepts in condensed matter physics, serving as a testbed in the global effort to solve long-lasting problems in material sciences. The rapid development of fermionic quantum gas machines is necessary for cold atom platforms to remain competitive in providing insights in condensed matter theory. In this project, several important steps beyond the state of the art have been accomplished, with a considerable potential impact on a broad community. The observation of well-defined repulsive quasiparticles and the emergence of a repulsive Fermi liquid instability, both in the impurity limit of a spin-imbalanced Fermi gas and in the spin-balanced case, represent fundamental findings in the context of itinerant ferromagnetism, a problem of high relevance well beyond the cold atom community. The developed RF spectroscopic technique, large-spacing optical lattice scheme and high-resolution disordered potential generation are highly general, and could be applied in diverse systems, while offering exciting prospects for the further exploration of poorly understood dynamical regimes of fermionic matter. Finally, our observation of vortex-induced dissipative transport in a superfluid fermionic Josephson junction represent an important step for understanding the interplay between coherent transport and dissipation phenomena in quantum many-body systems at the microscopic level.