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Quantum 2021 : Dynamics and local control of impurities in complex quantum environments



In recent years there has been a tremendous progress in experimental techniques addressing individual particles (impurities) embedded in quantum many-body environments of varying nature. Examples include static and mobile impurities in ultra cold atomic and molecular gases, impurity atoms and molecules in superfluids, individual atoms in cavity QED, Kondo-like and central spin states in solid-state devices. With exquisite resolution approaching the quantum limit it is now possible to exert local control on the quantum state of a single impurity particle, switching between the impurity’s states in ultra-fast regime. Local structure of the entangled state formed between the impurity and the surrounding medium can be mapped with outstanding spatial resolution. Time-resolved measurements enable using impurities for probing out-of-equilibrium quantum many-body dynamics, quantum diffusion and drag, quenches, and Floquet evolution. One can identify four loosely overlapping communities where work related to the quantum impurity problem is currently underway. Those are in the areas of quantum solid state devices, quantum optics, ultra cold atomic systems, and physical chemistry.

The aim of the workshop is to bring together specialists representing these different streams of research in order to facilitate exchange of results and ideas based on the unifying topic of quantum impurity problems. The ultimate goal is to aggregate a unique body of usually non-overlapping expertise around one central topic in order to create favourable conditions for breakthroughs in physics and engineering of quantum devices at the nano-scale.

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  • Alberto Biella
  • Alberto Cappellaro
  • Anna Minguzzi
  • Areg Ghazaryan
  • Artem Volosniev
  • Atac Imamoglu
  • Christophe Salomon
  • Dimitri Gangardt
  • Emanuel Gull
  • Fabian Essler
  • Frédéric Chevy
  • Fumika Suzuki
  • Félix Werner
  • Georg Bruun
  • Giulia De Rosi
  • Grigory Astrakharchik
  • Guido Masella
  • Guillaume Roux
  • Hanns-Christoph Nägerl
  • Igor Cherepanov
  • Kris Van Houcke
  • Kushal Seetharam
  • Lars Fritz
  • Leonardo Mazza
  • Lorenzo Gotta
  • Lorenzo Rosso
  • Luis Ardila
  • Marco Schiro
  • Martin Zwierlein
  • Maurizio Fagotti
  • Meera Parish
  • Michael Knap
  • Michał Suchorowski
  • Mikhail Lemeshko
  • Mikhail Maslov
  • Mikhail Zvonarev
  • Milena Horvath
  • Nikolay Prokofiev
  • Oleg Lychkovskiy
  • Oleksandr Gamayun
  • Petrov Dmitry
  • Richard Schmidt
  • Savvas Malikis
  • Sebastian Hofferberth
  • Sudipta Dhar
  • Thierry Giamarchi
  • Tobias Donner
  • Vadim Cheianov
  • Volker KARLE
  • Xavier Waintal
  • Yevheniia Cheipesh
  • Yossi Paltiel
    • 11:00 12:00
      Strongly correlated electrons and excitons in moire superlattices 1h

      Twisted bilayers of transition metal dichalcogenides offer a wealth of new phenomena, ranging from dipolar excitons to correlated insulator states. An example of qualitatively new phenomena in this system is our recent observation of an electrically tunable two-dimensional Feshbach resonance in exciton-hole scattering [1], which allows us to control the strength of interactions between excitons and holes located in different layers. Our findings enable hitherto unexplored possibilities for optical investigation of many-body physics, as well as realization of degenerate Bose-Fermi mixtures with tunable interactions.

      [1] I. Schwartz, Y. Shimazaki, C. Kuhlenkamp. K. Watanabe, T. Taniguchi, M. Kroner, A. Imamoglu, arXiv:2105.03997 (2021).

      Speaker: Prof. Atac Imamoglu (ETH Zurich)
    • 12:00 13:30
      Lunch 1h 30m
    • 10:00 11:00
      Polaron interactions and bipolarons in one-dimensional Bose gases in the strong coupling regime 1h

      Bose polarons, quasi-particles composed of mobile impurities surrounded by cold Bose gas, can experience strong interactions mediated by the many-body environment and form bipolaron bound states. Here we present a detailed study of heavy polarons in a one-dimensional Bose gas by formulating a non-perturbative theory and complementing it with exact numerical simulations. We develop an analytic approach for weak boson-boson interactions and arbitrarily strong impurity-boson couplings. Our approach is based on a mean-field theory that accounts for deformations of the superfluid by the impurities and in this way minimizes quantum fluctuations. The mean-field equations are solved exactly in Born-Oppenheimer (BO) approximation leading to an analytic expression for the interaction potential of heavy polarons which is found to be in excellent agreement with quantum Monte-Carlo (QMC) results. In the strong-coupling limit the potential substantially deviates from the exponential form valid for weak coupling and has a linear shape at short distances. Taking into account lowest-order BO corrections we calculate bipolaron binding energies and find excellent agreement with QMC results for impurity-boson mass ratios as low as 3.

      Speaker: Prof. Grigory Astrakharchik (UPC, Barcelona, Spain)
    • 12:00 13:30
      Lunch 1h 30m
    • 11:30 13:00
      Lunch 1h 30m
    • 11:30 13:00
      Lunch 1h 30m
    • 11:30 13:00
      Lunch 1h 30m
    • 10:00 11:00
      Anderson localization of composite particles 1h

      We investigate the effect of coupling between translational and internal degrees of freedom of composite quantum particles on their localization in a random potential. We show that entanglement between the two degrees of freedom weakens localization due to the upper bound imposed on the inverse participation ratio by purity of a quantum state. We perform numerical calculations for a two-particle system bound by a harmonic force in a 1D disordered lattice and a rigid rotor in a 2D disordered lattice. We illustrate that the coupling has a dramatic effect on localization properties, even with a small number of internal states participating in quantum dynamics.

      Speaker: Dr Fumika Suzuki (IST Austria)
    • 11:00 12:00
      Coexistence and phase separation of pairs and fermions in a one-dimensional model with pair-hopping 1h

      We consider a simple model of spinless fermions in which the kinetic energy competes with a pair-hopping term. We show by means of numerical calculations that there exists a phase in which part of the fermions are paired while the others remain unpaired. These elementary components makes two mixed Luttinger liquids, one for pairs and one for fermions. A simple two-fluid model accounts remarkably well for the observed numerical data [1]. Adding nearest-neighbour interaction leads to a rich phase diagram [2] in which we observe a regime in which the two previous Luttinger liquids get phase separated. In the context of impurity physics, this model on a finite size chain allows for the creation of a single pair interacting with a fermionic bath or a single fermion interacting with a paired fermions bath.

      Speaker: Prof. Guillaume Roux (Université Paris Saclay)
    • 12:00 13:30
      Lunch 1h 30m
    • 11:00 12:00
      High-order diagrammatic expansion around BCS: polarized superfluid phase of the attractive Hubbard model 1h

      In contrast to conventional QMC methods, expansions of intensive quantities in series of connected Feynman diagrams can be formulated directly in the thermodynamic limit. Over the last decade, diagrammatic Monte Carlo algorithms made it possible to reach large expansion orders and to obtain state-of-the-art results for various models of interacting fermions in 2 and 3 dimensions, mostly in the normal phase.

      We obtained first results inside a superconducting phase, for the 3D attractive Hubbard model [1]. Spontaneous symmetry breaking is implemented by expanding around a BCS Hamiltonian. All diagrams up to 12 loops are summed thanks to the connected determinant algorithm [2] with anomalous propagators. Working on the BCS side of the strongly correlated regime, we observe convergence of the expansion, and benchmark the results against determinant diagrammatic Monte Carlo [3]. In presence of a polarizing Zeeman field (where unbiased benchmarks are unavailable due to the fermion sign problem) we observe a first-order superconducting-to-normal phase transition, and a thermally activated polarization of the superconducting phase well described by a dilute gas of quasiparticles. We also discuss the large-order behavior of the expansion and its relation to Goldstone and instanton singularities.

      [1] G. Spada, R. Rossi, F. Simkovic, R. Garioud, M. Ferrero, K. Van Houcke, F. Werner, arXiv:2103.12038
      [2] R. Rossi, PRL 119, 045701 (2017)
      [3] E. Burovski, N. Prokof'ev, B. Svistunov, M. Troyer, PRL 96, 160402 (2006)

      Speaker: Dr Félix Werner (LKB, ENS)
    • 12:00 13:30
      Lunch 1h 30m
    • 11:00 12:00
      Quantum Impurities coupled to Markovian and Non-Markovian Environments 1h


      Speaker: Dr Marco Schiro (CNRS and College de France)
    • 12:00 13:30
      Lunch 1h 30m
    • 10:00 11:00
      Local dynamic control of spin impurities using chiral molecules 1h

      Using the chiral induced spin selectivity (CISS) effect we were able to induce local spin impurities on magnetic and superconducting material. Dynamic control of spin impurities was also achieved. The CISS is an electronic phenomenon in which electron transmission through chiral molecules depends on the direction of the electron spin. Thus charge displacement and transmission in chiral molecules generates a spin-polarized electron distribution. This effect is metastable and may generate local magnetic defect that can be enhanced or removed by electric dipole. Also selective process may organize the molecules adsorption.In my talk I will present the CISS effect and its importance, both for applications and basic science. I will show that when chiral molecules are adsorbed on the surface of thin ferromagnetic film,they induce magnetization perpendicular to the surface, without the application of current or external magnetic field. On superconductors chiral molecules generate Shiba like states, as well as change the order parameter of the superconductor. Lastly, I will also point to open questions regarding dynamic control of skyrmions, and the CISS effect.

      Speaker: Prof. Yossi Paltiel (The Hebrew University of Jerusalem)
    • 12:00 13:30
      Lunch 1h 30m
    • 10:00 11:00
      Electric and heat transport in a charge two-channel Kondo device 1h

      Motivated by the experimental realization of a multi-channel charge Kondo device [Iftikhar et al., Nature 526, 233 (2015)], we study generic charge and heat transport properties of the charge two-channel Kondo model. We present a comprehensive discussion of the out-of-equilibrium and time-dependent charge transport, as well as thermal transport within linear response theory. The transport properties are calculated at, and also in the vicinity of, the exactly solvable Emery-Kivelson point, which has the form of a Majorana fermion resonant level model. We focus on regimes where our solution gives exact results for the physical quantum dot device, and highlight new predictions relevant to future experiments.

      Speaker: Prof. Lars Fritz (Utrecht University)
    • 12:00 13:30
      Lunch 1h 30m
    • 11:00 12:00
      The spatial structure of magnetic polarons in strongly interacting antiferromagnets 1h

      The properties of mobile impurities in quantum magnets are fundamental for our understanding of strongly correlated materials and may play a key role in the physics of high-temperature superconductivity. Hereby, the motion of hole-like defects through an antiferromagnet has been of particular importance. It creates magnetic frustrations that lead to the formation of a quasiparticle, whose complex structure continues to pose substantial challenges to theory and numerical simulations. In this article, we develop a non-perturbative theoretical approach to describe the microscopic properties of such magnetic polarons. Based on the self-consistent Born approximation, which is provenly accurate in the strong-coupling regime, we obtain a complete description of the polaron wave function by solving a set of Dyson-like equations that permit to compute relevant spin-hole correlation functions. We apply this new method to analyze the spatial structure of magnetic polarons in the strongly interacting regime and find qualitative differences from predictions of previously applied truncation schemes. Our calculations reveal a remarkably high spatial symmetry of the polaronic magnetization cloud and a surprising misalignment between its orientation and the polaron crystal momentum. The developed framework opens up a new approach to the microscopic properties of doped quantum magnets and will enable detailed analyses of ongoing experiments based on cold-atom quantum simulations of the Fermi-Hubbard model.

      Speaker: Prof. Georg Bruun (Aarhus University)
    • 12:00 13:30
      Lunch 1h 30m
    • 09:30 10:30
      Thermal instability, evaporation, and thermodynamics of one-dimensional liquids in weakly interacting Bose-Bose mixtures. Important perspectives for impurities. 1h

      We study the low-temperature thermodynamics of weakly interacting uniform liquids in one-dimensional attractive Bose-Bose mixtures. The Bogoliubov approach is used to simultaneously describe quantum and thermal fluctuations. First, we investigate in detail two different thermal mechanisms driving the liquid-to-gas transition, the dynamical instability, and the evaporation, and we draw the phase diagram. Then, we compute the main thermodynamic quantities of the liquid, such as the chemical potential, the Tan's contact, the adiabatic sound velocity, and the specific heat at constant volume. The strong dependence of the thermodynamic quantities on the temperature may be used as a precise temperature probe for experiments on quantum liquids.

      The liquid-to-gas transition should also occur by reducing the concentration of one component in the mixture. By reaching the highly unbalanced mixture limit of our theory, one can explore the Bose polaron problem with attractive impurity-bath interaction, at finite temperature and in one spatial dimension where quantum and thermal fluctuations, which are both consistently taken into account in our theory, are strongly enhanced.

      Speaker: Giulia De Rosi (UPC – Universitat Politècnica de Catalunya)
    • 12:00 13:30
      Lunch 1h 30m