Publications Cosetta BARONI

2024

Article

• C. Baroni, G. Lamporesi, M. Zaccanti Quantum mixtures of ultracold gases of neutral atoms, (2024-11-06), (ID: 721287) Toggle Abstract
  After decades of improvements in cooling techniques of several atomic species and in finding methods for the achievement of stable quantum mixtures, the field is now ready for an extensive use of such a versatile experimental platform for the investigation of various physical problems. Among them, relevant examples are the dynamics of impurities in a quantum gas, the miscibility condition of different gases, the study of exotic topological structures, the interplay between magnetism and superfluidity, the formation of artificial molecules or new few-body states. We illustrate the differences among possible quantum mixtures — whether homonuclear spin mixtures or heteronuclear ones — and show how they can be exploited to investigate a plethora of topics from the few-body to the many-body regime. In particular, we discuss quantum mixtures of ultracold gases under three different perspectives: systems made of a few atoms of different kinds, single impurities immersed in a host quantum gas and quantum mixtures of two interacting gases. We restrict the discussion to single harmonic or flat traps, predominantly in a 3D configuration. A selection of results on recent experiments and possible interesting future directions are given.

2023

Articles

• C. Baroni, G. Gori, M. Chiofalo, A. Trombettoni Interacting two-mode model for ultracold quantum interferometers, J. Phys. Conf. Ser. 12030 (2023-12-18), http://dx.doi.org/10.1088/1742-6596/2667/1/012030 doi:10.1088/1742-6596/2667/1/012030 (ID: 721172) Toggle Abstract
  Ultracold gases provide an excellent platform for the realization of quantum interferometers. In the case of implementations based on Bose-Einstein condensates in double well potentials, an effective two-mode model allows to study how the interactions among particles affect the sensitivity of the interferometer. In this work we review the properties of such a model and its application to interferometric protocols, focusing on the achievable sensitivity in the presence of interactions turned on. In particular we study the full interferometric sequence when the initial state is a Twin Fock state, which is perfectly number squeezed. We found that in the presence of interactions and for certain values of the holding time in which a phase difference between the two modes is accumulated, the same sensitivity as in the non interacting case is recovered when using the population imbalance between the two wells as observable. Finally, we characterize the behaviour of the sensitivity by looking at the δ-derivative and the variance of the operator used for the measurement and studying the squeezing parameters.
• C. Baroni, B. Huang, I. Fritsche, E. Dobler, G. Anich, E. Kirilov, R. Grimm, M. A. Bastarrachea-Magnani, P. Massignan, G. Bruun Mediated interactions between Fermi polarons and the role of impurity quantum statistics, Nature Phys. 20 (2023-10-26), http://dx.doi.org/10.1038/s41567-023-02248-4 doi:10.1038/s41567-023-02248-4 (ID: 721077) Toggle Abstract
  The notion of quasi-particles is essential for understanding the behaviour of complex many-body systems. A prototypical example of a quasi-particle, a polaron, is an impurity strongly interacting with a surrounding medium. Fermi polarons, created in a Fermi sea, provide a paradigmatic realization of this concept. As an inherent and important property such quasi-particles interact with each other via modulation of the medium. While quantum simulation experiments with ultracold atoms have significantly improved our understanding of individual polarons, the detection of their interactions has remained elusive in these systems. Here, we report the unambiguous observation of mediated interactions between Fermi polarons consisting of K impurities embedded in a Fermi sea of Li atoms. Our results confirm two landmark predictions of Landau's Fermi-liquid theory: the shift of the polaron energy due to mediated interactions, linear in the concentration of impurities, and its sign inversion with impurity quantum statistics. For weak to moderate interactions between the impurities and the medium, we find excellent agreement with the static (zero-momentum and energy) predictions of Fermi-liquid theory. For stronger impurity-medium interactions, we show that the observed behaviour at negative energies can be explained by a more refined many-body treatment including retardation and molecule formation
• C. Baroni, B. Huang, I. Fritsche, E. Dobler, G. Anich, E. Kirilov, R. Grimm, M. A. Bastarrachea-Magnani, P. Massignan, G. Bruun Mediated interactions between Fermi polarons and the role of impurity quantum statistics, Nature Phys. (2023-10-26), http://dx.doi.org/10.1038/s41567-023-02248-4 doi:10.1038/s41567-023-02248-4 (ID: 721140) Toggle Abstract
  The notion of quasi-particles is essential for understanding the behaviour of complex many-body systems. A prototypical example of a quasi-particle is a polaron, formed by an impurity strongly interacting with a surrounding medium. Fermi polarons, created in a Fermi sea, provide a paradigmatic realization of this concept. Importantly, such quasi-particles interact with each other via the modulation of the medium. However, although quantum simulation experiments with ultracold atoms have substantially improved our understanding of individual polarons, the detection of their interactions has so far remained elusive. Here we report the observation of mediated interactions between Fermi polarons consisting of K impurities embedded in a Fermi sea of Li atoms. Our results confirm two predictions of Landau’s Fermi-liquid theory: the shift in polaron energy due to mediated interactions, which is linear in the concentration of impurities; and its sign inversion with impurity quantum statistics. For weak-to-moderate interactions between the impurities and the medium, our results agree with the static predictions of Fermi-liquid theory. For stronger impurity–medium interactions, we show that the observed behaviour at negative energies can be explained by a more refined many-body treatment including retardation and dressed molecule formation.

Dissertation

• C. Baroni Polarons and their Mediated Interactions in Fermi-Bose and Fermi-Fermi Mixtures of Potassium and Lithium, (2023-06-06), (ID: 721107)

Preprint

• R. Grimm, C. Baroni Fermionic quantum mixtures with tunable interactions, (2023-11-02), http://dx.doi.org/10.48550/arXiv.2311.01220 doi:10.48550/arXiv.2311.01220 (ID: 721139) Toggle Abstract
  The topic of the present lecture notes are two-species quantum mixtures composed of a deeply degenerate Fermi gas and a second component, the latter being fermionic or bosonic. A key ingredient is the possibility to tune the s-wave interaction between the different species by means of magnetically controlled Feshbach resonances, which allow us to investigate regimes of strong interactions. In two case studies, we review our experiments on mixtures of 6Li fermions with fermionic 40K or bosonic 41K atoms and on mixtures of fermionic 161Dy with 40K atoms. We cover various topics of fermionic quantum many-body physics, ranging from impurity physics and quasiparticles over phase separation to the formation of ultracold molecules and progress towards novel superfluids.

2021

Article

• I. Fritsche, C. Baroni, E. Dobler, E. Kirilov, B. Huang, R. Grimm, G. Bruun, P. Massignan Stability and breakdown of Fermi polarons in a strongly interacting Fermi-Bose mixture, Phys. Rev. A 103 53314 (2021-05-17), http://dx.doi.org/10.1103/PhysRevA.103.053314 doi:10.1103/PhysRevA.103.053314 (ID: 720640) Toggle Abstract
  We investigate the properties of a strongly interacting imbalanced mixture of bosonic 41K impurities immersed in a Fermi sea of ultracold 6Li atoms. This enables us to explore the Fermi polaron for large impurity concentrations including the case where they form a Bose-Einstein condensate. The system is characterized by means of radio-frequency injection spectroscopy for tunable interactions using an interspecies Feshbach resonance. We find that the energy of the Fermi polarons formed in the thermal fraction of the impurity cloud remains rather insensitive to the impurity concentration, even as we approach equal densities for both species. The apparent insensitivity to high concentration is consistent with the theoretical prediction, based on Landau's quasiparticle theory, of a weak effective interaction between the polarons. The condensed fraction of the bosonic 41K gas is much denser than its thermal component, which leads to a break-down of the Fermi polaron description. Instead, we observe a new branch in the radio-frequency spectrum with a small energy shift, which is consistent with the presence of Bose polarons formed by 6Li fermions inside the 41K condensate. A closer investigation of the behavior of the condensate by means of Rabi oscillation measurements support this observation, indicating that we have realized Fermi and Bose polarons, two fundamentally different quasiparticles, in one cloud.

2020

Article

• C. Baroni, G. Gori, M. L. Chiofalo, A. Trombettoni Effect of Inter-Well Interactions on Non-Linear Beam Splitters for Matter-Wave Interferometers, Condensed Matter 5 31 (2020-04-21), http://dx.doi.org/10.3390/condmat5020031 doi:10.3390/condmat5020031 (ID: 720521)

2019

Article

• B. Huang, I. Fritsche, R. Lous, C. Baroni, J. T. Walraven, E. Kirilov, R. Grimm Breathing Mode of a BEC Repulsively Interacting with a Fermionic Reservoir, Phys. Rev. A 99 41602 (2019-04-03), http://dx.doi.org/10.1103/PhysRevA.99.041602 doi:10.1103/PhysRevA.99.041602 (ID: 720106) Toggle Abstract
  We investigate the fundamental breathing mode of a small-sized elongated Bose-Einstein condensate coupled to a large Fermi sea of atoms. Our observations show a dramatic shift of the breathing frequency when the mixture undergoes phase separation at strong interspecies repulsion. We find that the maximum frequency shift in the full phase-separation limit depends essentially on the atom number ratio of the components. We interpret the experimental observations within a model that assumes an adiabatic response of the Fermi sea, or within another model that considers single fermion trajectories for a fully phase-separated mixture. These two complementary models capture the observed features over the full range of interest.