Name: Quantum 2021 : Dynamics and local control of impurities in complex quantum environments
Start: 2021-08-23T08:00:00+02:00
End: 2021-09-17T19:00:00+02:00
Location: No location set

lundi 23 août 2021

12:00 Lunch
Lunch
12:00 - 13:00
13:30 Ipa Director's Welcome - Yves Balkanski (Institut Pascal)
Ipa Director's Welcome
- Yves Balkanski (Institut Pascal)
13:30 - 13:55
14:00 The 2N+1 many-body body problem; an impurity immersed in a spin 1/2 ferminonic superfluid (ONSITE presentation) - Frédéric Chevy (Ecole Normale Supérieure)
The 2N+1 many-body body problem; an impurity immersed in a spin 1/2 ferminonic superfluid (ONSITE presentation)
- Frédéric Chevy (Ecole Normale Supérieure)
14:00 - 15:00
The polaron is one of the paradigmatic concepts of many-body physics that describes the properties of the quasi-particle arising from the interaction between an impurity and a many-body environment. First introduced by Landau and Pekard to describe the behaviour of an electron shrouded by a cloud of phonons in a crystal, it has recently been applied to ultracold atoms where the properties of atomic impurities immersed in weakly interacting Fermi and Bose gases have been studied both theoretically and experimentally. In my talk I will describe how recent experiments on mixtures of ultracold Bose and Fermi gases have further extended the scope of polaronic physics by addressing the properties of impurities immersed in a strongly interacting medium. In particular I will discuss the crucial role played by three body-physics in this problem.
mardi 24 août 2021

12:00 Lunch
Lunch
12:00 - 13:00
14:00 Polaron interactions and bipolarons in one-dimensional Bose gases in the strong coupling regime (ONLINE presentation) - Grigory Astrakharchik (UPC, Barcelona, Spain)
Polaron interactions and bipolarons in one-dimensional Bose gases in the strong coupling regime (ONLINE presentation)
- Grigory Astrakharchik (UPC, Barcelona, Spain)
14:00 - 15:00
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.
15:00 Smooth Polaron-Molecule Transition in a Degenerate Fermi Gas with Multiple Impurities (ONLINE presentation) - Yoav Sagi (Technion - Israel Institute of Technology)
Smooth Polaron-Molecule Transition in a Degenerate Fermi Gas with Multiple Impurities (ONLINE presentation)
- Yoav Sagi (Technion - Israel Institute of Technology)
15:00 - 16:00
Understanding the behavior of a spin impurity strongly-interacting with a Fermi sea is a long-standing challenge in many-body physics. For short-range interactions and zero temperature, most theories predict a first-order phase transition between a polaronic ground state and a molecular one. We study this question with an ultracold Fermi gas [1]. Experimentally, the impurity problem poses a challenge: the signals from the minority atoms are inherently very weak. To overcome this difficulty, we have developed novel sensitive rf and Raman spectroscopic techniques, which are based on fluorescence detection. Raman spectroscopy allows us to isolate the quasiparticle contribution and extract the polaron energy [2-3]. As the interaction strength is increased, we observe a continuous variation of all observables, in particular a smooth reduction of the quasiparticle weight as it goes to zero beyond the transition point. Our observations are explained by a theoretical model where polaron and molecule quasiparticle states are thermally occupied according to their quantum statistics. At the experimental conditions, polaron states are hence populated even at interactions where the molecule is the ground state and vice versa. The emerging physical picture is thus that of a smooth transition between polarons and molecules and a coexistence of both in the region around the expected transition. A smooth transition happens even at zero temperature, while a true first-order transition occurs only in the single impurity limit. [1] G. Ness et. al., Phys. Rev. X 10, 041019 (2020) [2] C. Shkedrov, Y. Florshaim, G. Ness, A. Gandman, and Y. Sagi, PRL 121, 093402 (2018). [3] C. Shkedrov, G. Ness, Y. Florshaim, and Y. Sagi, PRA 101, 013609 (2020).
mercredi 25 août 2021

12:00 Lunch
Lunch
12:00 - 13:00
14:00 Quantum impurities in 1D chains and ladders (ONSITE presentation) - Thierry Giamarchi (University of Geneva)
Quantum impurities in 1D chains and ladders (ONSITE presentation)
- Thierry Giamarchi (University of Geneva)
14:00 - 15:00
How an environment can modify the physical properties of a quantum particle is a long standing question. I will discuss this physics both for quantum bosonic chains for which the presence of the bath can lead to subdiffusion, but also for bosonic ladders for which the transverse degrees of freedom play a crucial role. I will discuss these theories in connection with experiments in cold atomic gases.
15:00 Dynamical quantum Cherenkov transition of fast impurities in quantum liquids (ONLINE presentation) - Kushal Seetharam (MIT/Harvard)
Dynamical quantum Cherenkov transition of fast impurities in quantum liquids (ONLINE presentation)
- Kushal Seetharam (MIT/Harvard)
15:00 - 16:00
The challenge of understanding the dynamics of a mobile impurity in an interacting quantum many-body medium comes from the necessity of including entanglement between the impurity and excited states of the environment in a wide range of energy scales. In this talk, I will discuss the motion of a finite mass impurity injected into a three-dimensional quantum Bose fluid as it starts shedding Bogoliubov excitations. We uncover a transition in the dynamics as the impurity’s velocity crosses a critical value which depends on the strength of the interaction between the impurity and bosons as well as the impurity’s recoil energy. We find that in injection experiments, the two regimes differ not only in the character of the impurity velocity abatement, but also exhibit qualitative differences in the Loschmidt echo, density ripples excited in the BEC, and momentum distribution of scattered bosonic particles. The transition is a manifestation of a dynamical quantum Cherenkov effect, and should be experimentally observable with ultracold atoms using Ramsey interferometry, RF spectroscopy, absorption imaging, and time-of-flight imaging.
jeudi 26 août 2021

12:00 Lunch
Lunch
12:00 - 13:00
14:00 Static and dynamic properties of a one-dimensional mobile impurity (ONSITE presentation) - Oleksandr Gamayun (University of Amsterdam)
Static and dynamic properties of a one-dimensional mobile impurity (ONSITE presentation)
- Oleksandr Gamayun (University of Amsterdam)
14:00 - 15:00
A single impurity particle in an ultracold atomic gas is a prospective model for probing relaxation dynamics of an interacting out-of-equilibrium quantum system. I will focus on McGuire's model that describes an impurity that propagates in a one-dimensional gas of free fermions. It is the simplest and yet fundamental model capturing the peculiar physics and mathematics of the non-equilibrium processes. The integrability of the model allows one to obtain a complete nonperturbative solution and express physical quantities in terms of the Fredholm determinants. After a detailed analysis of these determinants, I will discuss several striking physical phenomena such as incomplete relaxation and momentum-dependent impurity statistics. The universal properties of such one-dimensional systems can be described by the nonlinear Luttinger liquid (nLL). I will explain the microscopic derivation of nLL from the form-factors and discuss the generalization to finite temperatures.
vendredi 27 août 2021

12:00 Lunch
Lunch
12:00 - 13:00
samedi 28 août 2021
dimanche 29 août 2021

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