Entanglement Complexity and the Emergence of Irreversibility in Quantum Mechanics
le vendredi 29 juin 2018 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : The onset of irreversibility in physics is one of the great questions at the heart of statistical mechanics.
The second principle of thermodynamics in essence states that spontaneous processes happen in one direction. In classical physics irreversibility can only happen with some seed of randomness or coarse graining and topological mixing. Since coarse graining and the counting of micro states is arbitrary in classical physics, firmer grounds for statistical mechanics must be found in the quantum domain.
At a first glance the quantum case looks even harder. In a closed system evolution is unitary, and therefore the entropy of a quantum state cannot increase. Moreover, unitary evolution is always reversible, so irreversibility is strictly speaking impossible. In classical mechanics irreversibility is due to chaos, that is, high sensitivity to initial conditions. But in quantum mechanics, unitarity implies that slightly different initial conditions do not evolve into highly different states.
In this talk we take seriously the idea that the defining feature of quantum mechanics is entanglement. As such, irreversibility must be a consequence of entanglement. As we shall see, it is not the amount of entanglement per se that is important, but its complexity. We show that complexity of entanglement classifies the dynamical behavior of a isolated quantum many-body system and determines its irreversibility and the approach to thermalization.

Controlled dissipation and dynamics in quantum simulators
le vendredi 22 juin 2018 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : Over the past few years, several experimental platforms from Atomic, Molecular and Optical (AMO) physics have begun to be used as quantum simulators. These are devices where we have excellent microscopic understanding and control – allowing us to write down microscopic models under well-controlled approximations, and to adjust the parameters of these models to explore a wide range of phenomena arising from many-body physics. An important aspect of these systems that is often not widely discussed is that we also have excellent microscopic understanding of the dissipative dynamics of these systems, and means to engineer this dissipation (e.g., via controlled light scattering, or introduction of a reservoir gas). This provides new possibilities to observe the effects of dissipation on many-body dynamics, and also new tools to produce interesting many-body quantum states.
I will discuss this, with examples from our recent theoretical work on dissipative engineering of spin-entangled states, and exploration of light scattering and its effects on the dynamics and decoherence of many-body states in optical lattices.

Fractionalization between the vacua: from QCD to quantum magnetism
le vendredi 15 juin 2018 à 11h00

Séminaire théorie

Personne à contacter : Serge Florens ()

Lieu : Amphithéâtre, maison des Magistères

Résumé : Quantum Chromodynamics (QCD) -- the theory of strong nuclear forces -- has baffled the physics community and remains one of the poorly understood parts of the standard model. Its quintessential property: the confinement of quarks into protons, neutrons and mesons, while verified both experimentally and numerically, remains an elusive theoretical problem. The various cousins of QCD are however possible to understand to varying degrees and precision. In some of these theories the vacuum state is degenerate, and hence allows for domain walls -- a surface excitation which interpolates between two vacua of the theory. These domain walls have a remarkable property that quarks become liberated on them, and the domain wall excitation spectrum is very different from that of the bulk. Such QCD cousins are, unfortunately, not the physical theory, and they do not occur in nature. QCD however has another unlikely cousin: the Valence Bond Solid (VBS) state of the quantum anti-ferromagnet, where spin 1/2 excitations (or spinons) are bound into spin 1 excitations by a mechanism very similar to confinement of quarks. Perhaps surprisingly the low energy theory describing the behavior of the VBS phase is virtually identical to its QCD cousins under certain conditions. Further the VBS phase may have multiple vacua, and thus support domain walls, which in turn support liberated spinon excitations absent in the bulk. This has been verified numerically in the so-called J-Q model. These domain wall modes can in fact be seen as edge modes akin to those of the symmetry protected topological state. A multidisciplinary effort is slowly emerging to understand such phenomena, from the theoretical aspects of fundamental and condensed matter physics, to the numerical efforts in trying to understand QCD and quantum magnets.

Dynamical properties of impenetrable bosons in optical lattices
le mercredi 13 juin 2018 à 11h00

Séminaire interne LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé : The study of strongly correlated quantum systems is one of the most interesting and
intriguing research field in physics. The framework of ultracold gases in optical
lattices allows to explore equilibrium and non-equilibrium properties of such systems.
One example is the Tonks-Girardeau (TG) gas, in which the infinite repulsive delta-
like interaction mimics the Pauli exclusion principle and is reflected into the well
known mapping to non-interacting fermions. While local quantities are identical to the
fermionic case, all non local ones, like correlations or momentum distribution, are
significally different.
We develop a powerful method to study the spectral function of the TG gas by using
only single particle orbitals, and apply it to inspect the behaviour of ultracold gases in a periodic lattice with hard-wall confining. Moreover, the efficient
implementation of the one body green’s functions provide an instrument to
investigate energy and mass transport in periodic media and quasicrystals via the
scheme of non equilibrium green’s functions.

Interaction without back-action in the context of quantum manipulation
le vendredi 8 juin 2018 à 11h00

Séminaire théorie

Personne à contacter : Markus Holzmann ()

Lieu : Amphithéâtre, maison des Magistères

Résumé : We study the interaction between two quantum systems (A and B) that is mediated by their common linear environment. If the environment is out of equilibrium the resulting interaction violates Onsager relations and cannot be described by a Hamiltonian. In simple terms the action of system A on system B does not necessarily produce a back action. We derive general quantum equations describing the situation and analyze in details their classical correspondence. Changing the properties of the environment one can easily change and engineer the resulting interaction. It is tempting to use this for quantum manipulation of the systems. However the resulting quantum gate is not always unitary and may induce a loss of quantum coherence. For a relevant example we consider systems A and B to be spins of arbitrary values and arrange the interaction to realize an analogue of the two-qubit CNOT gate. The direction of spin A controls the rotation of spin B while spin A is not rotated experiencing no back-action from spin B. We solve the quantum dynamics equations and analyze the purity of the resulting density matrix. The resulting purity essentially depends on the initial states of the systems. We attempt to find a universal characteristics of the purity optimizing it for the worst choice of initial states. For both spins s_{A}=s_{B}=1/2, the optimized purity is bounded by 1/2 irrespective of the details of the gate. We also study in detail the semiclassical limit of large spins. In this case the optimized purity is bounded by (1+π/2)^{-1 } ≈ 0.39. This is much better than the typical purity of a large spin state ∼ s^{-1}. We conclude that although the quantum manipulation without back-action inevitably causes decoherence of the quantum states the actual purity of the resulting state can be optimized and made relatively high.

Quantum Critical Behavior at the Mott Point: the Status Quo
le vendredi 1er juin 2018 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : According to early ideas of Mott and Anderson, the interaction-driven metal-insulator transition – the Mott transition – remains a sharp T=0 phase transition even in absence of any spin or charge ordering. Should this phase transition be regarded as a quantum critical point? This basic question has long remained controversial, although it bears direct relevance to many puzzling phenomena in strongly correlated electronic systems. This talk will provide a brief overview of several new theoretical ideas and methods, developed in the last 25 years, that shed new light on this important problem. Especially useful information has very recently been obtained from the study of a class of “maximally frustrated” Hubbard models, which can be solved using non-perturbative methods of Dynamical Mean-Field Theory (DMFT). This theory identified the relevant quantum critical region associated with the Mott metal-insulator transition and found remarkable scaling behavior of transport properties, characteristic of quantum criticality. Precisely this kind of behavior was in very recent experiments on organic Mott systems [1,2]. The detailed mapping of this family of experimental systems to the theoretical phase diagram, identifying the Quantum Widom Line as the relevant organizing principle, has been provided by very recent optical conductivity studies [3]. Further relevance of these ideas for other systems will also be discussed, including the long-standing chicken-and-egg problem of the Mott-Pierls transition in VO2 [4].
[1] Quantum criticality of Mott transition in organic materials, Tetsuya Furukawa, K. Miyagawa, H. Taniguchi, R. Kato & K. Kanoda, Nature Physics 11, 221–224 (2015); See also: http://condensedconcepts.blogspot.ae/2015/03/quantum-criticality-near-mott.html
[3] Quantum spin liquids unveil the genuine Mott state, A. Pustogow, M. Bories, A. Löhle, R. Rösslhuber, E. Zhukova, B. Gorshunov, S. Tomić, J.A. Schlueter, R. Hübner, T. Hiramatsu, Y. Yoshida, G. Saito, R. Kato, T.-H. Lee, V. Dobrosavljević, S. Fratini, M. Dressel, arXiv:1710.07241.
[4] Resolving the VO2 controversy: Mott mechanism dominates the insulator-to-metal transition, O. Nájera, M. Civelli, V. Dobrosavljević, and M. J. Rozenberg, Phys. Rev. B 95, 035113 (2017); Phys. Rev. B 97, 045108 (2018).

Keith Gilmore (European Synchrotron Radiation Facility, Grenoble)

Reproducing dynamical excitations observed in resonant inelastic X-ray scattering through Bethe-Salpeter calculations
le vendredi 25 mai 2018 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : Resonant inelastic X-ray scattering (RIXS) is a relatively new technique for probing low energy excitations in materials. In addition to traditional techniques, such as angle resolved photoemission, it has become an important, high precision characterization tool of strongly correlated electron materials. To calculate RIXS, and related core and valence level spectra, we solve the Bethe-Salpeter equation (BSE) based on a self-energy corrected density functional theory electronic structure. I outline our implementation of the BSE and use SrVO3 for demonstration. Non fluorescence features in RIXS arise from a dynamic response of the system to the intermediate state perturbation. Since the Bethe-Salpeter equation is typically reduced to the static limit in practice, these dynamic excitations are generally not reproduced. To include interactions beyond the static BSE I introduce the cumulant expansion. Spectral functions derived from a GW self-energy are typically inadequate when the dressed Green’s function is built via the Dyson equation. With the same GW self-energy, a superior Green’s function and spectral function, implicitly including vertex corrections, is obtained through the cumulant expansion. I consider application of cumulant spectral functions to photoemission, photoabsorption, and X-ray scattering. Lastly, vibronic coupling has important impacts on these spectra. I show how to calculation the phonon contribution to photoemission, absorption and scattering with a vibronic cumulant.

Conservation laws and nonequilibrium dynamics
le vendredi 4 mai 2018 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : One of the simplest examples of nonequilibrium dynamics varying in the presence of a conservation law is the coarsening process of the kinetic Ising model, whose exponent changes from 1/2 to 1/3 if the magnetization is conserved. We start showing that this difference may be strongly enhanced by increasing the interaction range of the ferromagnetic coupling, which determines a speed-up in nonconserved coarsening while it almost freezes conserved coarsening [1].
Conserved quantities are also relevant to obtain a condensation-like transition in non interacting systems: we will discuss the relaxation process of a simple model, which may be considered as a rough approximation of the Discrete Nonlinear Schroedinger Equation [2], whose dynamics is actually much, much slower.
[1] Federico Corberi, Eugenio Lippiello and Paolo Politi
Effective mobility and diffusivity in coarsening processes
EPL 119, 26005 (2017)
[2] Stefano Iubini, Antonio Politi and Paolo Politi
Relaxation and coarsening of weakly-interacting breathers in a simplified DNLS chain
J. Stat. Mech.: Theory and Experiments, 073201 (2017)

Quenching the Kitaev honeycomb model
le vendredi 20 avril 2018 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : The Kitaev honeycomb model is one of the few exactly solvable models with a spin liquid ground state. Here I will discuss the dynamics of an initial antiferromagnetic state time evolved with the Kitaev model. I find a dynamic crossover to a valence bond solid. When the spin interactions are anisotropic, an exponentially long prethermalized regime appears.
Reference: arXiv:1710.09761

Emergence of non-Abelian magnetic monopoles and Anyonic statistics in quantum impurity problems
le vendredi 23 mars 2018 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : By virtue of emergent gauge fields in quantum impurity problems, we demonstrate that in experimentally realized regime the angulon, a quantum rotor dressed by bosonic excitations, can be seen as a point charge on a two-sphere interacting with a gauge field of non-Abelian monopole. We find a topological transition associated with making the monopole Abelian, which takes place in the vicinity of the previously reported angulon instabilities. Furthermore, we show that identical impurities interacting with a two dimensional many-particle environment obey anyonic statistics. In particular, we find that due to the many-body interactions between impurities and the bath, each of the impurities can be viewed as a flux-tube-charged-particle composite described by fractional statistics. This amounts to a novel configuration with emerging anyons, which is fundamentally different from the previously studied fractional quantum Hall and Kitaev model settings.

Damping of Josephson oscillations in strongly correlated one-dimensional atomic gases
le mercredi 14 mars 2018 à 11h00

Séminaire interne LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé : We study the Josephson oscillations of two strongly correlated one-dimensional bosonic clouds separated by a localized barrier. Using a quantum-Langevin approach and the exact Tonks-Girardeau solution in the impenetrable-boson limit, we determine the dynamical evolution of the particle-number imbalance, displaying an effective damping of the Josephson oscillations which depends on barrier height, interaction strength and temperature. We show that the damping originates from the quantum and thermal fluctuations intrinsically present in the strongly correlated gas. Thanks to the density-phase duality of the model, the same results apply to particle-current oscillations in a one-dimensional ring where a weak barrier couples different angular momentum states.

Laws of thermodynamics and fluctuation theorems for quantum machines
le vendredi 9 mars 2018 à 11h00

Séminaire théorie

Personne à contacter : Serge Florens ()

Lieu : Amphithéâtre, maison des Magistères

Résumé :
Consider quantum dots (or other nanostructures) which can convert a heat flow into electrical power, or can use power to move heat from a cold reservoir to a hot one.
Do such systems obey the same laws of thermodynamics as macroscopic heat engines?
How does one get the second law of thermodynamics when the Schrodinger equation is invariant under time-reversal?
What approximations can one make on the dynamics without unphysical violations of the laws of thermodynamics?
I consider these questions in the context of a real-time Keldysh theory of transport for a nanostructure coupled to reservoirs of electrons and phonons,
with particular interest in the difficult case of strong system-reservoir coupling.
I explicitly formulate how our lack of information leads to entropy production with what I call a "no Maxwell demons" assumption.
I then show that a simple microscopic symmetry in the Feynman diagrams enables one to show that
the first and second law of thermodynamics are obeyed on average, but that fluctuations violate them.
I show that these fluctuations obey a variety of fluctuation theorems (Jarzynski equality, Crooks equation, etc).
Finally I mention the well-known approximations that satisfy this symmetry, and thus will not lead to unphysical thermodynamics.

Quantum dynamics in the presence of interactions
le mercredi 14 février 2018 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : Interactions between atoms often introduce large amounts of complexity into many-particle systems, but they can also lead to new and interesting physical regimes. In this presentation I will discuss several examples of interacting ultra cold atomic systems, where tuneable interactions allow to access new dynamical situations or create new control techniques.
The first example will be taken from our efforts towards a full set of techniques to coherently control the external state of small samples of ultracold atoms using spatial adiabatic passage, and in the second an increased interaction in a multicomponent BEC system is shown to lead to emergence of classical behaviour in a fully quantum mechanical system.

Fractionalized particles on defects in topological insulators and superconductors
le vendredi 9 février 2018 à 11h00

Séminaire théorie

Personne à contacter : Serge Florens ()

Lieu : Amphithéâtre, maison des Magistères

Résumé : Recent experiments on one-dimensional and two-dimensional materials have been very successful in realizing topological states of electrons, with one of ultimate goals being the creation of emergent Majorana particles. Such emergent particles can have fractional charge, spin, and/or quantum statistics, so they are interesting both fundamentally and for quantum computing applications, but remain hard to realize.
We will discuss the use of topological defects as a way to realize fractional particles. By combining analytic and numerical approaches we predict Majorana particles, having favorable energetic properties, in certain vortices of two-dimensional superconductors. Our predictions fully explain puzzling features of recent experiments at INSP Jussieu. We will also focus on lattice dislocations in topological insulators as realizations of more complex fractional particles. General properties of fractionalized particles and open questions will also be discussed.

Static and dynamical properties of ultracold gas in a periodic and incommensurate potential
le mercredi 7 février 2018 à 11h00

Séminaire interne LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé : We explore static and dynamical properties of fermionic and hardcore bosonic cold
atomic gas trapped by the combination of two potentials (bichromatic lattice) with
incommensurate periods.
Firstly we study the effect of the metal to insulator transition, and the presence of a
mobility edge, into many-body measurable quantities, such as the momentum
distribution.
Then we study the long time dynamics of the gas, subject to a quantum quench, by
looking at both single particle and global quantities, namely the single particle
Green's function and the Loschmidt echo. In the case of a periodic lattice we show
that the asymptotic dynamics manifests the Anderson Orthogonality Catastrophe
(AOC) and we find a general analytic expression for the power law exponent which is
then compared with our numerical results. On the other hand in the case of an
incommensurate potential, which shows a mtal-to-insulator transition, we observe
the suppression of the AOC and an anomalous spreading of correlations as the
transition point is approached. We discuss these results from the point of view of the
nature of the single particle energy spectrum and show that these anomalous
features come from the particular geometry of the system.

Jerome Dubail (Institut Jean Lamour, Université de Lorraine)

Hydrodynamics of 1d bosons with delta repulsion
le vendredi 2 février 2018 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Salle de lecture 2, maison des Magistères

Résumé : Describing and understanding the motion of quantum gases out of equilibrium is a tremendous challenge for theorists. In 2006, the groundbreaking Quantum Newton Cradle experiment [1], where it was observed that two 1d clouds of cold atoms bounce against each other indefinitely without relaxation, provided impetus for many developments on the effects of low dimensionality in out-of-equilibrium quantum physics. But it is only thanks to the breakthrough of « Generalized HydroDynamics (GHD) » in 2016 [2] that one now possesses the adequate tools for an effective large-scale description of that experiment [3].
The purpose of this talk will be to give an introduction to those recent theoretical advances.
Refs:
[1] Kinoshita, Wenger and Weiss, Nature 440, 900, 2006
[2] Castro-Alvaredo, Doyon and Yoshimura, PRX 6, 041065, 2016 and Bertini, Collura, de Nardis and Fagotti, PRL 117, 207201, 2016
[3] Caux, Doyon, Dubail, Konik, Yoshimura, arXiv:1711.00873