Onset of intermittency in stochastic Burgers hydrodynamics
le mardi 16 avril 2019 à 11h00

Séminaire LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2

Résumé : A number of theoretical efforts have been devoted to the study of intermittent fluctuations of fluid dynamic observables in the stochastic Burgers model, where the presence of velocity shocks leads to large negative fluctuations of the velocity gradient. I am going to discuss how the response functional approach, where specific velocity field configurations - the instantons - are conjectured to be the dominant strucutures for a statistical account of large negative fluctuations, is meaningful only if the effects of fluctuations around instantons are taken into account.

Acoustic bubbly metamaterials: subwalength focusing, negative refraction and super-absorption
le vendredi 12 avril 2019 à 11h00

Colloque CPTGA

Personne à contacter : Bart van Tiggelen ()

Lieu : Amphithéâtre, maison des Magistères

Résumé : I will show that air bubbles, in water or trapped in a soft solid, are excellent candidates for creating acoustic metamaterials. They indeed exhibit a strong low-frequency monopolar resonance, which can lead to interesting effective acoustic properties at wavelengths that can be hundreds of times larger than the radius of the bubbles. First, I will show the possibility of focusing inside a bubbly metamaterial with a subwavelength resolution. The demonstration will be based on numerical results obtained with a Multiple Scattering Theory (MST) code that fully incorporates multiple-scattering effects. Then, I will explain how to create a 3D disordered double negative metamaterial composed solely of monopolar resonators. Finally, I will demonstrate that acoustic superabsorption can be achieved over a broad frequency range by tuning the parameters of a single layer of bubbles, referred to as a metascreen, which is confirmed by both finite element simulations and experiments.

Density-wave steady-state phase of dissipative ultracold fermions with nearest-neighbor interactions
le mercredi 10 avril 2019 à 13h30

Séminaire LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2

Résumé : I will describe our recent results about the effect of local dissipation on density-wave ordering in the extended Fermi-Hubbard model with both local and nearest-neighbor interactions. For this purpose, we used a recent variant of nonequilibrium dynamical mean-field theory with the auxiliary master equation approach which allows to treat nonperturbatively both local dissipation and local interaction. I will show how density-wave order seems to be robust against dephasing effects up to a critical point, where the system becomes homogeneous with no spatial ordering. I will also show how this model can be realized in ultracold atom experiments by the dressing of fermionic atoms with highly-excited Rydberg states in an optical lattice.

Coherent control of light transport in a dense atomic medium
le mercredi 10 avril 2019 à 11h00

Séminaire LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2

Résumé : Light-matter interfaces play a crucial role in the context of quantum information
networks, enabling for instance the reversible mapping of quantum state of light onto
quantum states of matter. A promising approach for the realization of such interfaces is
based on ensemble of neutral atoms. A critical figure of merit of such interfaces is the
overall storage-and-retrieval efficiency, which is mainly determined by technical losses and
atomic decoherence, and depends on the storage mechanism and matter properties.
Collective and cooperative effects manifistable in an atomic ensemble could provide
essential enhancement of the coupling strength between the light and atomic systems. In this
context, one of the strongest requirements to obtain a high efficiency is a large optical
depth, which can be achieved by increasing the size of the atomic system or atomic density
in the system. In addition, the interaction between light and atoms can be enhanced by
trapping atoms in the vicinity of a nanoscale waveguide due to strong confinement of the
light.
In this talk I will discuss light propagation in a spatially dense atomic ensemble,
where the average distance between atoms is comparable with the resonant wavelength. In
such dense atomic configurations dipole-dipole interaction play an important role and can
lead to manifestation of super and subradiance effects. I will consider the light propagation
in both free space and trapped near nanofiber surface atomic ensembles. The light scattering
in such dense atomic configuration is described in terms of microscopic approach based on
the standard scattering matrix and Resolvent operator formalism. We show theoretically and
experimentally that spatially dense atomic ensembles allow obtaining effective light-matter
interface and reliable light storage with essentially fewer atoms than it can be achieved in
dilute gases. Furthermore, we show that the presence of an optical nanofiber modifies the
character of atomic interaction and results in long-range dipole-dipole coupling between
atoms not only via the free space, but also through the waveguide mode.

Rigorous combination of wave-function methods and density-functional theory for electronic-structure calculations
le vendredi 5 avril 2019 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : I will first give a brief overview of the main goals of quantum chemistry and of the two families of electronic-structure computational methods used to solve the many-electron Schrödinger equation in this domain, namely wave-function methods and density-functional theory. I will then explain the advantages of combining these two approaches and how this can be done in a rigorous way based on a partition of the Coulomb electron-electron interaction into long-range and short-range contributions. The idea is to use a many-body wave-function method for the long-range contribution, coupled with a density-functional approximation for the short-range contribution. I will show two specific realizations of this range-separated wave-function/density-functional theory using for the wave-function method:
1) a random-phase approximation, which allows us to describe van der Waals intermolecular interactions;
2) a selected configuration-interaction approach, which allows us to describe strong electron correlation effects.

Critical properties of the Random field Ising Model
le mercredi 3 avril 2019 à 11h00

Séminaire LPMMC

Personne à contacter : Léonie Canet ()

Lieu : Salle de lecture 2

Résumé : The random field Ising model is a classic of statistical mechanics, which was proposed more than 40 years ago by Imry and Ma. Because of its simplicity, it is relevant for describing many physical situations, both at equilibrium and out-of-equilibrium. After describing some of these experimental realizations, I will present the most striking features that were encountered in the theoretical study of this model (dimensional reduction and its breaking, static avalanches ...). I will explain what are the minimal ingredients needed to describe such situations from an analytic perspective. I will finally present the
results we obtained in the last decade, by making use of the functional renormalization group.

Quantum rifling and some quantum goodies from hybrid structures
le mardi 26 mars 2019 à 13h30

Séminaire nano-électronique quantique

Personne à contacter : Robert Whitney ()

Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Quantum mechanics postulates that a measurement forces the wave-function of a qubit to collapse to one of its two eigenstates. The result of the measurement can then be recorded as a discrete outcome designating the particular eigenstate the qubit collapsed to. I will show that this well-accustomed picture of quantum measurement breaks down when the qubit is strongly driven during measurement. More specifically, when the evolution speed exceeds a threshold defined by the characteristic measurement time, the measurement outcome does not contain any information about the initial state of the qubit and thus the measurement does not generate any back-action. We call this phenomenon quantum rifling, as the fast spinning of the Bloch vector protects it from being deflected into either of its two eigenstates. We study this phenomenon with two superconducting qubits coupled to one to the same detector and demonstrate that the quantum rifling allows us to measure either one of the qubits on demand while protecting the state of another one from the measurement back-action.
If time permits it, I will also cover the results of my PhD from TU Delft, where I have been studying the Josephson effect in semiconducting InSb nanowires. These nanowires bare exotic electronic properties, such as large g-factor and spin-orbit interaction, leading to peculiar behaviour of the Josephson supercurrent. Specifically, the switching current exhibits non-monotonic behaviour with increasing magnetic field, due to the orbital interference of many modes in the wire[1]. For certain magnetic field values, we observe supercurrent flowing at zero phase difference, otherwise known as a Josephson-phi0 junction[2]. Such phi0-junctions can serve as smoking gun signatures of Majorana fermions.
[1] Zuo, K., Mourik, V., Szombati, D.B., Nijholt, B., Van Woerkom, D.J., Geresdi, A., Chen, J., Ostroukh, V.P., Akhmerov, A.R., Plissard, S.R. and Car, D., 2017. Supercurrent interference in few-mode nanowire Josephson junctions. Physical review letters, 119(18), p.187704.
[2] Szombati, D.B., Nadj-Perge, S., Car, D., Plissard, S.R., Bakkers, E.P.A.M. and Kouwenhoven, L.P., 2016. Josephson phi0-junction in nanowire quantum dots. Nature Physics, 12(6), p.568.

Generation of pure superconducting spin current in superconducting heterostructures via non-locally induced magnetism
le mercredi 20 mars 2019 à 13h30

Séminaire LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2

Résumé : Superconducting spintronics aims at carrying spin currents via equal spin Cooper pairs in superconducting/ferromagnetic heterostructures [1]. In this talk, I will present a mechanism for the generation of pure superconducting spin-currents carried by equal-spin triplet Cooper pairs in a superconductor (S) sandwiched between a ferromagnet (F) and a normal metal (Nso) with intrinsic spin-orbit coupling [2]. I will show that in the presence of Fermi-liquid interactions, the superconducting proximity effect can induce non-locally a ferromagnetic exchange field in the normal layer, which disappears above the superconducting transition temperature of the structure. The internal Fermi-liquid exchange field leads to the onset of a spin supercurrent associated with the generation of long-range spin-triplet superconducting correlations in the trilayer. I will show that the magnitude of the spin supercurrent, as well as the induced magnetic order in the Nso layer, depends critically on the superconducting proximity effect between the S layer and the F and Nso layers and the magnitude of the relevant Landau Fermi-liquid interaction parameter. These results provide a mechanism for the generation of equal spin Cooper pairs that is compatible with recent experimental results [3]. I will also give a brief discussion on our ongoing work on non-equilibrium spin currents in superconducting structures.

[1] J.Linder and J.W.A. Robinson, Nat. Phys. 11 307 (2015)

[2] X. Montiel and M. Eschrig, Phys. Rev. B 98, 104513 (2018)

[3] K.-R. Jeon, C. Ciccarelli, A. J. Ferguson, H. Kurebayashi, L. F. Cohen, X. Montiel, M. Eschrig, J. W. A. Robinson, M. G. Blamire Nat. Mat. 17, 499 (2018)

Benjamin Lenz (Centre de physique théorique, École polytechnique)

Effects of non-local correlations on spectral properties of doped Sr2IrO4
le mercredi 20 mars 2019 à 11h00

Séminaire LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2

Résumé : The spin-orbit Mott insulator Sr_{2}IrO_{4} has been in the spotlight in recent years due to its striking similarities to isostructural high-T_{c} superconducting copper oxides. In particular, upon doping the system recent photoemission experiments found pseudogap behavior at low temperatures, which raises the question of its relation to the pseudogap found in cuprate superconductors.
In this talk, I will present new insights into the spectral properties of this 5d transition metal system as a function of electron- and hole-doping by means of a combined ab-initio electronic structure and oriented cluster dynamical mean-field approach. Within this treatment, important ingredients like spin-orbit coupling and distortions of the oxygen octahedra as well as Hubbard interactions and non-local charge fluctuations are taken into account. The calculated spectral function of pure Sr_{2}IrO_{4} compares well with angular-resolved photoemission measurements, both in the low-temperature antiferromagnetic and high-temperature paramagnetic phase, and allows to study emerging changes under electron- and hole-doping. Special emphasis of my talk will be placed on pseudogap features of the momentum-resolved spectral function of electron-doped Sr_{2}IrO_{4}, which are found to be in good agreement with experiment.

A hierarchy of strongly correlated modes in quantum wires
le vendredi 15 mars 2019 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : The natural excitations of an interacting one-dimensional system at low energy are hydrodynamic modes of Luttinger liquid, protected by the Lorentz invariance which originates from the linearised dispersion. In this talk, I will show that beyond low energy, where quadratic dispersion reduces the symmetry to Galilean, the organisational principle of the many-body excitations changes into a hierarchical structure: calculations of dynamic correlation functions for fermions show that the spectral weights of the excitations are proportional to integral powers of R^2/L^2, where R is the interaction radius and L is the system length. Thus, only small numbers of excitations carry the principal spectral power in representative regions on the energy-momentum planes. For example, in the spectral function the first-level (strongest) excitations form a mode with parabolic dispersion, like that of a renormalised single particle. The second-level excitations produce a singular power-law line shape to the first-level mode and multiple power-laws at the spectral edge. Crossover from this hierarchy in the nonlinear regime to Luttinger liquid at low energy will be illustrated by a calculation of the local density of state at all energy scales using Bethe ansatz. I will also give a brief discussion of experiments on quantum wires realised in GaAs double-well heterostructures. The momentum-resolved tunnelling in this setup directly probes the spectral function of electrons at all energy scales giving access to the spin-charge separation of spinful Luttinger liquid in the linear and to the hierarchy of strongly correlated modes in the nonlinear regime.

Modes collectifs de "Higgs" dans les condensats fermioniques
le mercredi 13 mars 2019 à 11h00

Séminaire LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2

Résumé : Je discuterai l'existence d'une branche collective de "Higgs" dans les
condensats fermioniques, gaz ultra-froids ou supraconducteurs. Cette
question, qui bénéficie désormais de plusieurs résultats expérimentaux,
demeure controversée d'un point de vue théorique du fait de la présence d'un
continuum (lié à la brisure des paires de Cooper) aux énergies où la branche
collective devrait se trouver. En particulier, à vecteur d'onde nul, il
n'est pas certain qu'il existe un mode collectif clairement distinct du bord
du continuum à 2Δ (deux fois la bande interdite). À vecteur d'onde
petit mais non nul, plusieurs prédictions contradictoires de la relation de
dispersion coexistent dans la littérature.
Nous montrons que la branche existe bel et bien à potentiel chimique μ
positif et à vecteur d'onde strictement positif et inférieur à
sqrt(2m*μ)/ℏ. Pour cela, nous traitons le couplage au continuum de
façon non perturbative en faisant un prolongement analytique à travers la
ligne de coupure qui lui est associée dans la fonction de Green du champ de
paire. Nous obtenons ainsi la relation de dispersion du mode collectif quel
que soit le vecteur d'onde et la force des interactions.
Enfin, nous montrons que la résonance associée au mode collectif est bien
visible dans la fonction de réponse du champ de paires.

Microscopic charged fluctuators as a limit to the coherence of disordered superconductor devices
le mardi 12 mars 2019 à 14h00

Séminaire nano-électronique quantique

Personne à contacter : Robert Whitney ()

Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : By performing experiments with thin-film resonators of NbSi, we elucidate a decoherence mechanism at work in disordered superconductors. This decoherence is caused by charged Two Level Systems (TLS) which couple to the conduction electrons in the BCS ground state; it does not involve any out-of-equilibrium quasiparticles, vortices, etc. Standard theories of mesoscopic disordered conductors enable making predictions regarding this mechanism, notably that decoherence should increase as the superconductor cross section decreases. Given the omnipresence of charged TLS in solid-state systems, this decoherence mechanism affects, to some degree, all experiments involving disordered superconductors. In particular, we show it easily explains the poor coherence observed in quantum phase slip experiments and may contribute to lowering the quality factors in some disordered superconductor resonators.

Shaping the properties of condensed-matter systems with light
le mercredi 6 mars 2019 à 11h00

Séminaire LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2

Résumé : Using strong light-matter interactions to control the quantum properties of condensedmatter
systems is an ongoing broad research effort. In particular, it is known that transport
[1,2] and superconductivity [3,4] can be modified using an external radiation. In the
absence of the latter, it is an interesting question whether these properties may be also
affected by coupling the relevant matter excitations to the vacuum field of a cavity-type
structure. In this talk, I will present different systems and regimes where this idea can be
exploited. Particularly interesting is the so-called “ultrastrong” light-matter coupling regime
[5], which occurs when the coupling strength is comparable to the bare transition
frequency. I will first show that this regime can be achieved in quantum Hall systems
coupled to a terahertz resonator [6,7], which can affect the magneto-transport properties of
the electron gas [8,9]. I will also explain how quantum Hall systems embedded in a
photonic band gap material can be used to reach an interesting regime featuring very large
cooperativities [10,11]. A simple model to study the interplay between charge transport and
light-matter interactions in a chain of two-level systems will then be presented. Using nonequilibrium
Green’s functions and quantum master equations, I will show that in the
dissipative regime where the cavity photon decay rate is the largest parameter, the lightmatter
coupling is responsible for a steady-state current enhancement scaling with the
cooperativity [12,13]. Possible applications of these results will be also discussed. Finally, I
will present some recent theoretical results showing that ultrastrong coupling without
external resonators can be achieved in certain metallic crystals where plasmons coexist
with phonons exhibiting large oscillator strengths. These strong light-matter interactions
give rise to intrinsic surface plasmon-phonon polaritons, which offer the unique possibility
to control the phonon properties by tuning the electron density and the crystal thickness
[14]. In particular, these surface polaritons can lead to large enhancements of the electronphonon
scattering, which could have a profound inuence, e.g. on the superconducting
properties of certain crystals. I will conclude by giving some perspectives of this work.

References

[1] M. A. Zudov et al. Phys. Rev B64, 201311(R) (2001)
[2] R. Mani et al. Nature420, 646 (2002)
[3] A. F. G. Wyatt et al., Phys. Rev. Lett.16, 1166 (1966)
[4] D. Fausti et al., Science331, 189 (2011)
[5] C. Ciuti, G. Bastard, and I. Carusotto, Phys. Rev. B72, 115303 (2005)
[6] D. Hagenmüller, S. De Liberato, and C. Ciuti, Phys. Rev. B81, 235303 (2010)
[7] G. Scalari et al., Science335, 1323 (2012)
[8] N. Bartolo and C. Ciuti, Phys. Rev. B98, 205301 (2018)
[9] G. L. Paravicini-Bagliani et al., Nat. Phys.15, 186 (2019)
[10] D. Hagenmüller, Phys. Rev. B93, 235309 (2016)
[11] Q. Zhang et al., Nat. Phys.12, 1005 (2016)
[12] D. Hagenmüller et al., Phys. Rev. Lett.119, 223601 (2017)
[13] D. Hagenmüller et al., Phys. Rev. B97, 205303 (2018)
[14] D. Hagenmüller et al., arXiv:1810.10190 (2018)

Imaging thermoelectric transport through quantum nanostructures
le mardi 5 mars 2019 à 14h00

Séminaire nano-électronique quantique

Personne à contacter : Robert Whitney ()

Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : We developed a new scanning probe technique to image thermoelectric transport in two- dimensional devices: Thermoelectric Scanning Gate Microscopy (TSGM). This technique is derived from Scanning Gate Microscopy (SGM), that consists in mapping changes in a device's electrical conductance induced by a moving electrostatic perturbation, generated with a biased AFM tip [1]. TSGM consists in recording the devices’ Seebeck coefficient instead of its electrical conductance. To perform this measurement, we heat one side of the device and record the thermoelectric voltage arising across the device in response to this temperature difference. We then scan the electrically biased tip above the surface while recording this signal.
We apply this technique to investigate the low density regime of quantum point contacts (QPCs), where strong electron-electron interactions give rise to conductance [2,3] and thermoeletcric [4] anomalies. By scanning the polarized tip in front of the QPC, we create a Fabry-Pérot cavity between the QPC channel and the tip-depleted region [5], which induces interference fringes in both the conductance and the thermopower. Surprisingly, the interference in the thermoelectric signal exhibit an abrupt phase shift by π at very low QPC transmission, which is invisible in the conductance. We propose a model to explain these differences, based on the spontaneous localization of electrons in the QPC channel [6,7]. Our work illustrates that the combination of scanning gate microscopy and thermoelectric measurements can unveil elusive phenomena that escape transport measurements [8].
[1] M.A Topinka et al., Nature, 416, 183-186 (2001).
[2] K.J Thomas, Phys. Rev. Lett. 77, 135 (1996).
[3] S. M. Cronenwett, Phys. Rev. Lett. 88, 226805 (2002). [4] N. J. Appleyard, Phys. Rev. B 62, 8 R16275 (2000) [5] B. Brun et al., Phys. Rev. Lett. 116, 136801 (2016).
[6] M. J. Iqbal et al. Nature, 501, 79 (2013)
[7] B. Brun et al., Nat. Com., 5, 4290 (2014)
[8] B. Brun et al., arXiv:1804,00075

Confronting Theory and Data in Cosmology
le vendredi 1er mars 2019 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : The mysteries of the cosmic beginning, gravitational clustering, and cosmic acceleration persist. How can we distill relevant cosmological information from the next generation of data sets? Taking examples from the cosmic microwave background, large scale structure, and supernova cosmology, I will discuss inference strategies, artificial intelligence, machine learning, and computational approaches that promise to extract more information from current and upcoming data sets. The philosophy is to allow maximum freedom to design realistic forward models, to be robust to systematic nuisances, accurately combine multiple probes, move beyond simplistic likelihood assumptions, naturally allow quantitative model comparison, characterize tensions in the data, and maintain (near-)optimality whenever possible.

Bogoliubov theory in the Gross-Pitaevskii regime
le vendredi 15 février 2019 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : Since 1947 Bogoliubov theory has represented the guide model to thinking
about weakly interacting Bose gases. Remarkably, such a theory predicts a
linear excitation spectrum and provides expressions for the thermodynamic
functions which are believed to hold in the dilute limit. However, so far,
there are only a few cases where the predictions of Bogoliubov theory can
be obtained by rigorous mathematical analysis. In particular, one of the
main mathematical issues is to recover the physical intuition that the
correct parameter to appear in the expressions of the physical quantities
is the scattering length of the interaction.
In this talk I will discuss how the validity of Bogolibov theory can be
proved in the case of systems of N interacting bosons trapped in a box with
volume one and interacting through a repulsive potential with scattering
length 1/N (Gross-Pitaevskii regime). This is a joint work with C. Boccato,
C. Brennecke and B. Schlein.

Turbulent flow and soliton interaction in resonantly-driven polaritons superfluids
le mercredi 13 février 2019 à 13h30

Séminaire LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2

Résumé : Exciton-polaritons, microcavity half-matter half-light quasi-particles, when resonantly driven exhibit a superfluid regime. Accordingly, topological excitations similar to those predicted in equilibrium superfluids may spontaneously appear [1,2]. However, the non-equilibrium nature of polaritons requires the system to be continuously pumped to compensate for losses. This driving plays a crucial role in the formation and dynamics of such topological excitations tending to inhibit their formation [1].
I will present a recent breakthrough allowing to simultaneously extended the fluid propagation distance and to release the constraints imposed by the resonant driving [3]. This fully optical method, exploiting optical bistability present in these systems, allows for accurate hydrodynamics study of polariton superfluid and for a deterministic control of excitation taking place is this unconventional fluid of light. Experimental validation of the proposal will be reported. I will also discuss prospects open thanks to this method towards non-linear statistical physics and quantum correlation.

Quantum correlations close to quantum critical points : entanglement and beyond
le mercredi 13 février 2019 à 11h00

Séminaire LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2

Résumé : Second-order quantum phase transitions host coherent superpositions and entanglement at all length scales. Theoretically, it is expected that a quantum critical point (QCP) induces a specific scaling of thermodynamic observables in an extended region of the finite-temperature phase diagram (the so-called quantum critical fan). So far, characterizing the extent of the quantum critical fan has however proved challenging, given the interplay of quantum and thermal fluctuations around the QCP. After introducing a simple procedure to isolate the coherent contribution to the fluctuations of an arbitrary observable, we will propose such a characterization for paradigmatic spin models of quantum phase transitions. In a second part of the talk, we will explain why the entanglement generated close to a QCP is a potential resource for quantum interferometry, and illustrate this general property by describing a counter-intuitive, genuinely quantum, mechanism, for the suppression of certain fluctuations at the QCP of the quantum Ising model (leading to spin-squeezing).

Black holes in fluid flows
le mercredi 6 février 2019 à 13h30

Séminaire LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2

Résumé : I will discuss the possibility to reproduce black hole physics in fluid flows.
The starting point is an analogy discovered by Unruh between the propa-
gation of sound in a flowing fluid and waves around a black hole. I will
discuss the analogue of the Hawking effect through which a black hole loses
its mass, and its recent experimental verifications. I will also present a recent
water wave experiment, where we have observed the analogue of black hole
superradiance, that is, the amplification of waves by extraction of angular
momentum from a rotating flow.

Quantum many-body physics with nonlinear propagating light
le mercredi 6 février 2019 à 11h00

Séminaire LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2

Résumé : The propagation of a paraxial and quasimonochromatic quantum light field in a dispersive and nonlinear dielectric is considered. In this all-optical setup, the space propagation of the field's envelope can be mapped onto the time evolution of a quantum fluid of interacting photons. The resulting quantum many-body system constitutes a particular class of quantum fluids of light and presently attracts growing interest as a platform for quantum simulation. I will review recent theoretical and experimental progresses in this rapidly emerging research field, including investigations on superfluidity, elementary excitations, quantum quenches (possibly in the presence of disorder), thermalization, Bose-Einstein condensation, and topology.
Selected references:

P.-É. Larré and I. Carusotto, Phys. Rev. A 92, 043802 (2015)

A. Chiocchetta, P.-É. Larré, and I. Carusotto, EPL 115, 24002 (2016)

P.-É. Larré, S. Biasi, F. Ramiro-Manzano, L. Pavesi, and I. Carusotto, Eur. Phys. J. D 71, 146 (2017)

P.-É. Larré, D. Delande, and N. Cherroret, Phys. Rev. A 97, 043805 (2018)

C. Michel, O. Boughdad, M. Albert, P.-É. Larré, and M. Bellec, Nat. Commun. 9, 2108 (2018)

Precision couplings and tailored couplings for high-fidelity quantum computing
le vendredi 1er février 2019 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : This is a two-part talk, one is about consideration of longitudinal rather than transverse coupling for qubit-resonator systems, and the other part is about a critical examination of the rotating wave approximation.

A semiclassical perspective for quantum many-body systems
le mercredi 30 janvier 2019 à 13h30

Séminaire LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2

Résumé : One way to characterize complex quantum systems is to consider their corresponding classical counterpart. Such a program, dubbed as quantum chaos, has been highly successful for low-dimensional (mainly one-body) systems. There is a growing interest in transferring these techniques towards many-body systems. First I will explain how to describe the energy spectrum and the associated eigenstates for the seminal Bose-Hubbard system. Second I will consider the disordered Anderson problem on a random graph. I used extensive numerical techniques in order to characterise the localised/delocalised transition and whether the delocalised phase is ergodic.

Many-body localization and thermalization in one-dimensional quantum systems
le mercredi 30 janvier 2019 à 11h00

Séminaire LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2

Résumé : At high energy, isolated quantum systems generically thermalize, their macroscopic observables obeying the classical laws of thermodynamics. In many-body localized systems however, transport is prevented, effectively breaking thermalization. The phenomenon of many-body localization (MBL) is interesting (1) from a applied point of view, to engineer states robust to decoherence effects, and (2) from a fundamental point of view, as a genuinely quantum phenomenon whose understanding is instrumental in crafting a quantum theory of thermodynamics. In this talk, I will review the progresses made in that direction, notably discussing the minimal ingredients needed for MBL to arise, and presenting a picture of the MBL phase as a fractal delocalized phase on a complex graph. If time permits, I will extend the discussion to out-of-equilibrium open systems.

Selected references

Multifractal scalings across the many-body localization transition
N. Macé, F. Alet and N. Laflorencie
arXiv:1812.10283

Many-body localization in a quasiperiodic Fibonacci chain
N. Macé, N. Laflorencie and F. Alet
arXiv:1811.01912

The Sachdev-Ye-Kitaev model, its holographic dual and quantum conformal fluctuations
le vendredi 25 janvier 2019 à 11h00

Séminaire théorie

Personne à contacter : Serge Florens ()

Lieu : Salle Rémy Lemaire, K223, Bâtiment K, Insitut Néel

Résumé : The fascinating Sachdev-Ye-Kitaev (SYK) model describing a large number of randomly interacting Majorana fermions represents an ultimate example of the AdS/CFT correspondence in 1+1 space-time dimensions. As pointed out by Kitaev, both the SYK model and its gravity dual possess an emergent conformal symmetry which is spontaneously broken in the infra-red. As such, the soft Goldstone mode in the spectrum of the model emerges which is known to be described by the so-called 'Schwarzian' action. In my talk, after a general exposition to the SYK model, I will concentrate on its quantum deeply infra-red limit where conformal Goldstone fluctuations start to play a paramount role. I will demonstrate how the 'Schwarzian' action can be mapped onto a 'Liouvillian' quantum mechanics and study a long-time limit of 2- and 4-point correlation functions of Majoranas. The range of new results predicted by such mapping encompasses universal power-law decays of correlators as well as an emergent Coulomb blockade physics in the 'complex' version of the SYK model reminiscent to that of conventional mesoscopic quantum dots.

Static and dynamic properties of spin-orbit-coupled Bose gases
le mercredi 23 janvier 2019 à 11h00

Séminaire LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2

Résumé : The realization of synthetic spin-orbit coupling represents one of the most important achievements in the physics of ultracold atomic gases. In this talk I shall illustrate some of my theoretical predictions about the properties of two-component Bose-Einstein condensates with equal-weighted Rashba and Dresselhaus spin-orbit couplings. Their phase diagram includes different structures, such as a spin-polarized plane-wave phase, and a stripe phase featuring density modulations. Because of the simultaneous presence of superfluidity and of a crystalline structure, the stripe phase exhibits the long-sought phenomenon of supersolidity. Several relevant features of this configurations, recently observed in an experiment by Ketterle’s group at MIT, will be discussed.

Quantum Information Beyond the Circuit Model
le mercredi 16 janvier 2019 à 11h00

Séminaire LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2

Résumé :
The standard circuit model to quantum information has proven a powerful tool not just for quantum computation, but understanding quantum communication tasks, quantum networks and diverse problems including, e.g., quantum metrology. Recently, the potential of going beyond the circuit model, for example by applying operations in superpositions of different orders, has garnered interest, showing new advantages in some computational and communication tasks. I will discuss some of the advantages that can be obtained in such "indefinite causal structures” and the challenges involved in using this as a resource for quantum information. I will finish by briefly mentioning another related approach that may allow some similar advantages while remaining within a causal framework.