The key player problem in complex oscillator networks and electric power grids
le vendredi 20 décembre 2019 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : In network theory, two issues of central importance are (i) how to assess the global robustness of a network-coupled system and (ii) how to identify its local vulnerabilities.
The second issue is related to the historical and fundamental problem of identifying the key player. That may be for instance the player who, once removed, leads to the biggest changes in the other player’s activity in game theory, or to the biggest structural change in a social network. That problem has been tackled with the introduction of graph-theoretic descriptors, in particular centrality indices. Additionally, centralities averaged over the whole system provide a global indicator of how tightly bound a network is, which helps in dealing with the first issue.
The purely graph theoretic, centrality-based approach cannot be straightforwardly applied to deterministic network-coupled dynamical systems. Assessing such a network’s global robustness and identifying its most critical components must go beyond computing graph centralities and needs to incorporate the coupling dynamics into account. In my talk I will discuss methods recently developed to deal with these two issues in physical network-coupled dynamical systems. The talk will survey a number of already obtained results and present a tentative discussion of future challenges.
Reference: M. Tyloo, L. Pagnier, P. Jacquod, "The key player problem in complex oscillator networks and electric power grids: Resistance centralities identify local vulnerabilities",
Science Advances 5, eaaw8359 (2019)

Universality and stability of the edge states of chiral-symmetric topological semimetals and surface states of the Luttinger semimetal
le vendredi 13 décembre 2019 à 11h00

Séminaire théorie

Personne à contacter : Serge Florens ()

Lieu : Amphithéâtre, maison des Magistères

Résumé : We theoretically demonstrate that the chiral structure of the nodes of nodal semimetals is responsible for the existence and universal local properties of the edge states in the vicinity of the nodes. We perform a general analysis of the edge states for an isolated node of a 2D semimetal, protected by chiral symmetry and characterized by the topological winding number N. We derive the asymptotic chiral-symmetric boundary conditions and find that there are N+1 universal discrete classes of them. The class determines the numbers of flat-band edge states on either side off the node in the 1D spectrum and the winding number N gives the total number of edge states. We then show that the edge states of chiral nodal semimetals are robust: they persist in a finite-size stability region of parameters of chiral-asymmetric terms. This significantly extends the notion of 2D and 3D topological nodal semimetals. We demonstrate that the Luttinger model with a quadratic-node for j=3/2 electrons (Luttinger semimetal) is a 3D topological semimetal in this new sense and predict that alpha-Sn, HgTe, possibly Pr2Ir2O7, and many other semimetals described by it are topological and exhibit surface state.
Reference: M. Kharitonov, J.-B. Mayer, and E. M. Hankiewicz, Phys. Rev. Lett. 119, 266402 (2017).

Daniel Hernangómez Pérez (University of Regensburg)

Aspects of topology in organic quantum wires
le vendredi 6 décembre 2019 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : In the past forty years, polyacetylene molecular wires have attracted a long-standing interest: these wires support propagation of topological domain-wall states, so-called solitons, which provide a paradigm for spin-charge separation. Recent experimental developments have shown that individual polyacetylene chains can be synthesized on metallic substrates. Motivated by this breakthrough, we propose a way for chemically
supported “soliton-design” in these systems. We demonstrate how to control the soliton position and how to read-it out by electrical means. This provides a step toward functional electronic devices based on soliton manipulation, i.e. solitonics.

A non-equilibrium system as a demon
le vendredi 29 novembre 2019 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : A Maxwell demon is a creature (or machine) that reduces the entropy of a system without performing any work on it.
It performs this apparent violation the second-law of thermodynamics through the intricate action of measuring individual particles and subsequently performing feedback. Bennett (building on work of Landauer) argued that the second-law is restored
once one takes into account the fact that information recorded by the demon is a physical resource like heat or work.
Here we show that much simpler setups can also act as demons: we demonstrate that it is sufficient to exploit a non-equilibrium distribution to seemingly break the second law of thermodynamics. No particle-by-particle measurement or feedback is necessary.
We call this an N-demon (with the "N" for non-equilibrium), and show that it can reduce the entropy of a system without doing work or exchanging heat with that system. We then show that the second-law is restored by treating ``non-equilibrium'' as a physical resource like heat, work or information.
We propose both an electronic and an optical implementation of this phenomenon, realizable with current technology.
These examples make it clear that the non-equilibrium distribution can be classical or quantum in nature.
Ref: Rafael Sánchez, Janine Splettstoesser, Robert S. Whitney, to appear in Phys. Rev. Lett. Eprint - arXiv:1811.02453

Probing and verifying quantum simulators and quantum computers with randomized measurements
le mardi 26 novembre 2019 à 14h00

Séminaire nano-électronique quantique

Personne à contacter : Robert Whitney ()

Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Randomized measurements have emerged as a new tool to probe quantum simulators and quantum computers beyond standard observables. In this talk, I will describe the framework of our randomized measurement protocols that can measure entanglement, out-of-time-ordered correlations, and many-body topological invariants. I will also show some experimental results (Collaboration with the group of Rainer Blatt, IQOQI Innsbruck).

Confined phases of fermions coupled to Z2 gauge fields
le vendredi 22 novembre 2019 à 11h00

Séminaire théorie

Personne à contacter : Serge Florens ()

Lieu : Amphithéâtre, maison des Magistères

Résumé : After briefly summarizing my long-term interest in quantum physics of low-dimensional spinless fermions that attract each other, I will present our recent study of a quantum many-body lattice system of one-dimensional fermions interacting with a dynamical Z2 gauge field. The gauge field mediates long-range attraction between fermions resulting in their confinement into bosonic dimers. At strong coupling we developed an exactly solvable effective theory of such dimers with emergent constraints. I will show that even at a generic coupling and fermion density, the model can be rewritten as a local spin 1/2 chain and forms a Luttinger liquid. In a finite chain we observed the doubling of the period of Friedel oscillations which paves the way towards experimental detection of confinement in this system. Finally, I will also discuss the possibility of a Mott phase at the commensurate filling 2/3, connection to quantum scars and our plans to extend this study to two spatial dimensions in pursuit of exotic p-wave superfluidity.

Average field approximation for almost bosonic anyons in a magnetic field
le mercredi 20 novembre 2019 à 11h00

Séminaire interne LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé : We study the ground state of a large number N of 2D anyons in an external magnetic field. We consider a scaling limit where the statistics parameter α is proportional to N^{-1} when N → ∞. The model is that of bosons in a magnetic field and interacting through long-range magnetic potential generated by magnetic charges carried by each particle, smeared over discs of radius R. Our method allows to take R → 0 not too fast at the same time as N → ∞. We use the information theoretic version of the de Finetti theorem of Brandão and Harrow to justify the so-called "average field approximation": the particles behave like independent, identically distributed bosons interacting via a self-consistent magnetic field.

Superradiant Quantum Phase transition in Rashba Cavity QED
le mardi 19 novembre 2019 à 14h00

Séminaire nano-électronique quantique

Personne à contacter : Robert Whitney ()

Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé :

In cavity quantum electrodynamics (QED), the interaction between an atomic transition and the cavity field is measured by the vacuum Rabi frequency omega_0. The regime with omega_0 comparable to the two-level transition frequency is called the ultrastrong coupling regime. In such a regime, and for a large number of atoms coupled to the same cavity mode, a superradiant quantum phase transitions (SQPT) has been predicted, e.g. within the Dicke model.

In this theoretical seminar, I will briefly describe the SQPT at equilibrium, discuss the experimental context, and present our recent proposal where a 2DEG with Rashba spin-orbit coupling placed inside an optical cavity can exhibit the SQPT.

Ref: P. Nataf, T. Champel, G. Blatter, and D. M. Basko, Rashba cavity qed: a route towards the superradiant quantum phase transition, arXiv:1907.02938 (2019)

Leszek Sirko (Institute of Physics of the Polish Academy of Sciences)

Properties of open quantum graphs and microwave networks
le vendredi 15 novembre 2019 à 11h00

Colloque CPTGA

Personne à contacter : Julia Meyer ()

Lieu : Amphithéâtre, maison des Magistères

Résumé : We will discuss interesting properties of open quantum graphs and microwave
networks [1]. We will demonstrate that there exist graphs which do not obey the Weyl’s law
N(R) = LR/π + O(1), where O(1) is a function which for R going to infinity is bounded by a
constant. The Weyl’s law directly links the counting function N(R) of the number of
resonances with the square root of energy k, 0(k), and the total length L of the internal
edges of a graph. Graphs which do not obey the Weyl’s law are called non-Weyl graphs. We
show that for standard coupling conditions the transition from a Weyl graph to a non-Weyl
graph occurs if we introduce a balanced vertex. A vertex of a graph is called balanced if the
numbers of infinite leads and internal edges meeting at a vertex are the same. The
experimental results demonstrating the existence of non-Weyl networks are in excellent
agreement with the theoretical predictions [2].

Acknowledgements

This work was supported in part by the National Science Centre,
Poland, Grant No. 2016/23/B/ST2/03979 and the Operational Programme Innovative
Economy Grant No. POIG.01.01.02-00-008/08. J. L. was supported by the internal grant
project “Introduction to quantum mechanics on graphs” of the University of Hradec Kralove.

References

[1]. O. Hul, S. Bauch, P. Pakoński, N. Savytskyy, K. Życzkowski, and L. Sirko, Phys. Rev. E
69, 056205 (2004).

[2]. M. Ławniczak, J. Lipovsky, and L. Sirko, Phys. Rev. Lett. 122, 140503 (2019).

Olivier Coquand (German Aerospace Center, Cologne)

Dynamics of granular fluids
le mercredi 6 novembre 2019 à 11h00

Séminaire LPMMC

Personne à contacter : Léonie Canet ()

Lieu : Salle de lecture 2

Résumé : Granular fluids are omnipresent in our everyday life, from
the physics of geological phenomena to food processing. However, very
few theoretical models are able to provide reliable predictions for
these systems in density ranges close to those encountered on Earth.
In this seminar, I will discuss how the combination of the
mode-coupling theory, and the integration through transients formalism
can be used to describe granular flows at moderate densities (close to
the transition to the amorphous solid state). On the particular
example of a planar shear flow, the results of this model will be
compared to the phenomenological laws of the so-called mu(I) rheology
known from experiments

Developing InSb quantum wells as a platform for topological superconductivity
le mardi 5 novembre 2019 à 14h00

Séminaire nano-électronique quantique

Personne à contacter : Robert Whitney ()

Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé :

Two-dimensional systems coupled to superconductors offer the opportunity to explore a variety of quantum phenomena. These include the study of novel Josephson effects , superconducting correlations in quantum (spin) Hall systems, hybrid superconducting qubits and emergent topological states in semiconductors with spin-orbit interaction. In this talk I will focus on our efforts to develop InSb two-dimensional electron gases (2DEGs) to engineer topological superconductivity.

We show that Josephson junctions in InSb 2DEGs support supercurrent transport over several microns and display clear signatures of ballistic superconductivity. Furthermore, we exploit the large Landé g-factor and gate tunability of the junctions to control the current-phase relation, and drive transitions between the 0 and π-states. This control over the free energy landscape allows us to construct a phase diagram identifying these 0 and π-regions, in agreement with theory. In addition to induced superconductivity I will discuss the operation of stable quantum confined structures (quantum dots - QDs) in these 2DEGs, which are essential elements for the manipulation and readout of topological qubits. We show that the specific material properties of InSb are strongly reflected in the transport through the QD. The small effective mass of InSb (~ 0.02 me) results in a large single-particle confinement energy, giving rise to an even-odd variation in the size of the Coulomb diamonds, related to the spin-dependent filling of orbital levels in the QD. Studying the evolution of Coulomb peaks in a magnetic field allows us to determine the ground state spin configuration and directly extract the g-factor (~30). An important consequence of this large g-factor is that the ground state of the QD changes from a spin-singlet (spin zero) to spin-triplet (spin one) at magnetic fields as low as 0.3 T. Together, these results establish InSb 2DEGs as a promising new platform to study topological superconductivity.

1. Ballistic superconductivity and tunable π-junctions in InSb quantum wells
C. T. Ke, C. M. Moehle et al., Nat. Commun. 10, 3764 (2019) arXiv:1902.10742
2. Stable quantum dots in an InSb two-dimensional electron gas
I. Kulesh et al., arXiv:1910.07309

Multi-component Gauge Dependent Quantum Gases
le vendredi 18 octobre 2019 à 10h00

Soutenance de thèse

Personne à contacter : Anna Minguzzi ()

Lieu : Amphithéâtre, maison des Magistères

Résumé : Dear Colleagues,
I am glad to invite you all to my PhD defense,

[TITRE]

The discussion will take place on Friday (18/10/2019)
at 10:00 in the amphitheater of Maison des Magistères.

Abstract

A ladder is the simplest geometry where one can get some insight on two-dimensional quantum systems subjected to a synthetic gauge field.
We consider a system of bosons trapped in two one-dimensional coupled lattice rings subjected to different fluxes in each leg. This specific bosonic ladder
corresponds to different boundary conditions with respect to the case of a linear ladder. We explore the mesoscopic effect in this system with generic flux scheme
threading the system. Effect of commensuration between flux and quantized momentum will be shown as well as fragmentation effects in the infinite
interaction between boson.
We will also investigate the physics of interacting exciton-polaritons in honeycomb lattice structure.
The discussion will be followed by a buffet !
Chères et chers collègues,
je vous invite à ma soutenance de thèse,

Gaz quantiques à plusieurs composantes sous champ de jauge

L'oral aura lieu vendredi (18/10/2019)
à 10h00 dans l’amphithéâtre de la Maison des Magistères.

Résumé

Une échelle est la géométrie la plus simple où l'on peut avoir un aperçu de la physique des systèmes quantiques à deux dimensions soumis à un champ de jauge synthétique.
Nous considérons un système de bosons piégés dans un réseau formé de deux anneaux unidimensionnels couplés soumis à des flux différents. Ce système bosonique spécifique
correspond à différentes conditions aux limites par rapport au cas d'une échelle linéaire. Nous explorerons les effets mésoscopiques dans ce système avec un schéma de flux générique.
L'effet de la commensuration entre le flux et la quantité de mouvement quantifiée sera exposé ainsi que la fragmentation du système à interaction faible et infinie entre les bosons.
Nous étudierons également la physique des exciton-polaritons dans un réseau en nid d'abeilles.
La discussion sera suivie d'un buffet !

Riccardo Rossi (Flatiron Insitute, Smons Foundation, New York)

Two-terminal conductance measurements in Majoranas SAG nanowires
le mardi 15 octobre 2019 à 14h00

Séminaire nano-électronique quantique

Personne à contacter : Robert Whitney ()

Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé :

Majorana quasiparticles have generated years of intense research following the first observation of zero-bias anomalies in semiconductor-superconductor heterostructures [1] due to the promises they hold in the field of quantum computing. However, despite these efforts, definitve proofs of the topological nature of these excitations are still being sought after. In particular, one of the most significative prediction for Majorana fermions is that the zero-bias anomalies are to be found at both sides of a wire. Performing a simultaneous conductance measurements at both sides of a wire would be a significative step forward allowing us to verify this theoretical prediction. Unfortunately, usual InAs nanowires are grown on a substrate before being transferred onto another chip before being processed, which prevents from defining a well-defined electrical ground without defects in the center of the wire. An alternative to these standing wires are the so-called SAG [2] (selective area growth) wires that grow directly on a chip that can be directly processed and can be connected electronically through the epitaxial Al thin film deposited in MBE. Using this technique, we realized three-terminals nanowire structures allowing us to probe both sides of the same wire simultaneously [3,4]. In this presentation, I will discuss the advantage of this SAG wires and present results we obtained in these systems in relation to topological signatures as well as future possible developments using this technique.

[1] V. Mourik et al., Science 336, 1003 (2012)
[2] S. Vaitiekenas et al., Phys. Rev. Lett. 121, 147701 (2018)
[3] G. Anselmetti et al., arXiv:1908.05549 (2019)
[[4] G. Ménard et al., arXiv:1905.05505 (2019)

Exploring Kardar-Parisi-Zhang universality class: from the dynamics of exciton-polariton condensates to stochastic interface growth with temporally correlated noise
le mercredi 9 octobre 2019 à 14h00

Soutenance de thèse

Personne à contacter : Léonie Canet ()

Lieu : Amphithéâtre, maison des Magistères

Résumé : Dear Colleagues,
I am glad to invite you all to my PhD defense,
Exploring Kardar-Parisi-Zhang universality class: from the dynamics of exciton-polariton condensates to stochastic interface growth with temporally correlated noise.
The discussion will take place on Wednesday (09/10/2019) at 14:00 in the amphitheater of Maison des Magistères.
Abstract:
Kardar-Parisi-Zhang (KPZ) equation stands as a paradigmatic model for the investigation of Out-of-Equilibrium critical phenomena. In this thesis, we study realizations of the KPZ equation in two different physical systems. The first is a non-equilibrium condensate of Exciton-Polaritons, quasi-particle excitations resulting from the interaction between confined photons and excitons. A link between the dynamics of the condensate phase and the KPZ dynamics was established in the literature. Using a model and parameters close to real experimental configurations, we show that universal KPZ properties are observable in current one-dimensional experimental systems, and we extend this analysis by examining the geometry-dependent nature of the probability distributions of the phase of the condensate. We also show that these universal effects are robust with respect to finite-size-effects and disorder. The second physical systems we study are classical growth surfaces whose microscopic dynamics involve temporal correlations over time. These phenomena are described by a KPZ equation in which the noise is correlated temporally. This correlation breaks one of the main symmetries of the KPZ equation and possibly leads to a new fixed point. Using the non-perturbative renormalization group (NPRG) technique, we study short- and long-term temporally correlated systems in one and two dimensions. In the one-dimensional case, we show that the pure fixed point KPZ persists in the short-term and in the long-term case to a critical value of the correlation exponent. This clarifies a long-standing debate about the effects of an infinitesimal temporal correlation in the KPZ equation. In two dimensions, we find a similar image. No other result, except one-loop perturbative computation, existed in the literature for two dimensions.
The defense will be followed by an italian buffet.
_______________________________________________________________________________________________________________
Chères et Chers Collègues,
Je suis heureux de vous inviter tous à ma soutenance de thèse,
Exploration des propriétés universelles Kardar-Parisi-Zhang: de la dynamique des condensats d'exciton-polariton à la croissance stochastique d'interfaces avec un bruit temporellement corrélé.
La discussion aura lieu mercredi (09/10/2019) à 14h00 dans l'amphithéâtre de la Maison des Magistères.
Résumé:
Dans cette thèse, nous étudions des réalisations de l'équation de Kardar-Parisi-Zhang (KPZ) dans deux systèmes physiques différents. Le premier est un condensat hors équilibre d’Exciton-Polaritons, des excitations quasi-particulaires issues de l'interaction entre photons confinés et excitons. Un lien entre la dynamique de la phase du condensât et la dynamique KPZ avait été établi dans la littérature. En utilisant un modèle et des paramètres proches des configurations expérimentales réelles, nous montrons que des propriétés universelles KPZ sont observables dans des systèmes expérimentaux actuels à une dimension, et nous étendant cette analyse en examinant les propriétés dépendantes de la géométrie. On montre aussi que ces propriétés ils sont robustes par rapport a des effets des taille finie et au désordre. Les deuxièmes systèmes physiques que nous étudions sont des surfaces de croissance classiques dont la dynamique microscopique implique des corrélations temporelles dans le temps. Ces phénomènes sont décrits par une équation de KPZ dans laquelle le bruit est corrélé temporellement. Cette corrélation brise l'une des symétries principales de l'équation de KPZ et conduit possiblement à un nouveau point fixe. En utilisant la technique du groupe de renormalisation non perturbartive (NPRG), nous étudions des systèmes corrélés temporellement à court et à long terme dans une et deux dimensions. Dans le cas unidimensionnel, nous montrons que le point fixe pur KPZ persiste dans le cas à court terme et dans celui à long terme jusqu’à une valeur critique de l’exposant de corrélation. Ceci clarifie un débat de longue date sur les effets d'une corrélation temporelle infinitésimale dans l’équation de KPZ. En deux dimensions, nous trouvons une image similaire. Aucun autre résultat, à l'exception d'un calcul perturbatif à une boucle, n'existait dans la littérature pour deux dimensions.
La discussion serait suivie par un pot de thèse italien.

Quantum phase slips in one-dimensional superconductors
le mercredi 2 octobre 2019 à 14h00

Soutenance de thèse

Personne à contacter : Denis Basko ()

Lieu : Amphithéâtre, maison des Magistères

Résumé :

Chers collègues,

J'ai le plaisir de vous inviter à ma soutenance de thèse

Quantum phase slips in one-dimensional superconductors

mercredi (02/10/2019).
La présentation commence à 14h00
dans l'amphithéâtre de la Maison des Magistères.

La présentation sera suivie d'un pot avec des spécialités russes.

Dear colleagues,

I am glad to invite you to my Ph.D. defense

Quantum phase slips in one-dimensional superconductors

on Wednesday (02/10/2019).
The presentation starts at 14:00
in the amphitheater of Maison des Magistères.

The presentation will be followed by buffet with some Russian specialities.
In this thesis quantum phase slips in one-dimensional superconductors are studied. One-dimensional superconductors can be represented by two physical systems: a superconducting wire and a Josephson junction chain. A superconducting wire can be considered one-dimensional, if its transverse dimensions are smaller than the superconducting coherence length. In one-dimensional systems fluctuations strongly influence the system properties. The quantum phase slips correspond to quantum tunneling between different phase configurations along the superconductor. They can be of two types. Coherent quantum phase slips do not involve dissipation and only shift energy levels of the system. Incoherent quantum phase slips lead to a dissipative relaxation in the system.
We start with studying an incoherent phase-slip process in a single underdamped current-biased Josephson junction. This process corresponds to dissipative tunneling between weakly broadened levels in neighboring minima of the tilted washboard potential. We derive an expression for the voltage peaks near the resonant values of the external current, which correspond to matching energies of the lowest level in one minimum and an excited level in the lower neighboring minimum. This process is analogous to resonant Zener breakdown known for electrons in a superlattice subject to a strong electric field.
We continue with studying coherent quantum phase slips in a Josephson junction chain. First, we determine the amplitude of a coherent quantum phase slip in a homogeneous chain. It has already been shown that the amplitude is determined by the imaginary-time instanton action, which can be divided into the local (corresponding to phase winding by 2π on one junction) and environmental (corresponding to phase readjustment in the rest of the chain, which is determined by gapless Mooij-Schön modes) parts. We derive a numerical correction to the environmental part of the action, going beyond logarithmic precision. Second, we study the effect of spatial periodic modulations of the chain parameters on the coherent quantum phase slip process. We calculate the corrections both to the local and environmental part of the coherent quantum phase slip action and show that both of them can be significant, depending on the chain and modulations parameters. Then, we study the effect of two types of quenched disorder: random spatial modulation of the junction areas and random induced background charges. The main result is that the dominant contribution to the coherent quantum phase slip action is local. We also study the statistics of the mesoscopic fluctuations of the quantum phase slips amplitude and show that it can be non-Gaussian for chains which are not sufficiently long.
Finally, we consider one-dimensional superconducting wires. There is no microscopic theory available for the fast phase winding in the phase-slip core, where the order parameter is suppressed. However, the slow phase readjustment process, determined by the Mooij-Schön modes with frequencies lower than 2Δ, is analogous to that in Josephson junction chains, so the resulting environmental part of the coherent quantum phase slip action takes the same form. Therefore, we discuss how our results, obtained for Josephson junction chains, can be applied to inhomogeneous superconducting wires.

Thermodynamics in presence of quantum measurements
le mercredi 25 septembre 2019 à 11h00

Séminaire LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé : Much progress has been done recently in identifying the consequences of quantum mechanics on thermodynamics. One of the key differences with classical systems is the disturbance induced by quantum measurement on the measured object. This effect is accompanied with energy and entropy changes of the measured system. Consequently, quantum measurement can be considered a thermodynamic resource, and can be used e.g. to fuel quantum engines or refrigerators with no classical analogues. Such engines have different constraints than thermal engines, and can e.g. reach unit efficiency at non-zero power in certain limits.
Due to the intimate link between decoherence and measurement, this approach gives new tracks to understand the thermodynamic constraints on quantum protocols, such that quantum computation algorithms.

Quantum signatures in the heat flow: the case of the fluorescence of a driven two-level atom
le mardi 24 septembre 2019 à 14h00

Séminaire nano-électronique quantique

Personne à contacter : Robert Whitney ()

Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : A lot of attention was devoted recently to understand the differences between the thermodynamic behavior of classical and quantum systems, and design engines exploiting these differences. One fundamental difficulty to characterize energy transfers in the quantum world is the existence of coherent superpositions of state of different energies. The impact of this property can be investigated by looking at the minimal example of a two-level atom driven quasi-resonantly and in contact with a thermal bath. The driving continuously induces coherences in the energy eigenbasis of the free qubit, which is also the eigenbasis of the state realizing thermal equilibrium with the bath. Consequently, the atom reaches a steady-state which is out-of-equilibrium in a genuinely quantum way. The continuous decoherence induced by the bath is associated with a quantum contribution to the total heat flow provided by the bath which can be interpreted as the energy cost to erase coherences. Identifying allows to derive a quantum version of the first and second law taking into account the presence of coherences, and allowing to study important unsolved problems like the characterization of the energy transfers occuring during a quantum measurement, or the evaluation of the cost required to operate a quantum computer.

Chiral quantum optics with atomic arrays and superconducting circuits
le jeudi 19 septembre 2019 à 11h00

Séminaire LPMMC

Personne à contacter : Benoît Vermersch ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé :

As the size and complexity of quantum processors increase, the ability to route quantum information between distant components in a reliable and controllable way becomes a necessity. In most experiments with superconducting circuits, this task is taken care of by ferrite junction circulators; however, as these devices are bulky and use large magnetic fields, they are not suitable for on-chip integration and new, scalable alternative must be developed in the near future.

In this talk I will present the design of a passive integrated architecture for realizing on-chip photon routing. In contrast to other recent proposals, our scheme does not rely on breaking time-reversal symmetry; rather, the collective emission of pairs of superconducting artificial atoms in a microwave transmission line is engineered such that orthogonal atomic transitions spontaneously emit and absorb photons propagating in opposite directions. I will show how the resulting cascaded interactions between distant atoms can be exploited to passively probe and measure programmable many-body operators, which will be illustrated with the generation and manipulation of the toric code.
Finally, I will discuss how some of these results can be translated to the optical domain for cold atom experiments, and, in particular, show that photon routing can be realized in free-space with defect-free atomic arrays.

Un niveau quantique discret fortement couplé à un continuum avec une structure de bandes
le mercredi 18 septembre 2019 à 14h30

Soutenance de thèse

Personne à contacter : Robert Whitney ()

Lieu : Amphithéâtre, maison des Magistères

Résumé :

English version below

Chers collègues,
J'ai le plaisir de vous inviter à ma soutenance de thèse qui aura lieu
le mercredi 18 septembre à 14h30
dans l'amphithéâtre de la Maison des Magistères.

Un niveau quantique discret fortement couplé à un continuum avec une structure de bandes

Thèse dirigée par Robert Whitney

La présentation (en anglais) sera suivie d'un pot où vous pourrez déguster des spécialités bourguignonnes.

Résumé

Suivant les progrès technologiques de la révolution industrielle, la thermodynamique classique a été développée au XIXème siècle dans le but de comprendre la conversion de la chaleur en travail intervenant dans les machines thermiques nouvellement élaborées. Les travaux de Boltzmann apportèrent une autre révolution conceptuelle avec la physique statistique. Il démontra l'origine microscopique des lois de la thermodynamique, celles-ci ne décrivant en fait que le comportement macroscopique de systèmes pour lesquels la thermalisation locale est plus rapide que toutes les autres échelles de temps. Cependant, conséquemment à l'intérêt grandissant pour les nanotechnologies, il est aujourd'hui possible de manipuler des systèmes microscopiques pour lesquels la thermalisation est plus lente que les échelles de temps associés aux flux d'électrons. Une avancée technologique majeure dans ce domaine provient de l'utilisation de boîtes quantiques, des dispositifs nanométriques permettant de confiner les électrons sur des distances si petites qu'ils se répartissent sur des niveaux d'énergie discrets. Il est alors évidemment indispensable de prendre en compte les effets quantiques pour l'étude de ce type de systèmes, c'est-à-dire de concevoir des outils théoriques alliant thermodynamique et mécanique quantique.

Les problèmes de thermodynamique quantique sont souvent abordés dans le cadre de la théorie des systèmes quantiques ouverts. L'idée générale de ce formalisme est d'étudier la dynamique d'un « petit » système quantique lorsqu'il est couplé à un autre système supposé bien plus « gros » et représentant l'environnement. On démontre alors que l'évolution temporelle du petit système peut être décrite par une équation maîtresse dans la limite où il est faiblement couplé à l'environnement. Cependant, il semble intuitivement qu'une machine pourra délivrer une puissance plus importante dans un contexte de fort couplage.

Pour les problèmes de transport électronique, le formalisme de Landauer-Büttiker permet de décrire le régime de fort couplage. Dans ce cadre, les électrons sont supposés ne subir que des processus de diffusion élastique dans le système central. Toutes les propriétés thermoélectriques de la machine peuvent alors être caractérisées grâce aux propriétés de transmission du diffuseur. Cependant, ce formalisme souffre aussi d'une importante limitation, la structure de bandes des réservoirs étant ignorée.

Ici nous avons choisi d'adopter un point de vue différent pour aborder le régime de fort couplage en étudiant un modèle exactement soluble. Nous analysons donc le modèle de Fano-Anderson décrivant un niveau discret couplé à un continuum. Nous nous intéressons particulièrement à l'influence de la densité d'états des réservoirs. On démontre en effet que, sous certaines conditions, des états liés discrets apparaissent dans les bandes interdites des réservoirs. Ces états jouent un rôle prépondérant sur la dynamique du niveau discret à temps longs : leur contribution dépend de la préparation initiale du système et peut donner lieu à des oscillations permanentes de l'occupation du niveau discret.

Nous commençons par expliciter la solution exacte du modèle en nous concentrant particulièrement son comportement à temps longs. Nous analysons ensuite deux cas particuliers. En premier lieu, nous nous intéressons aux propriétés de transport d'une boîte quantique à un niveau couplée à un semi-conducteur présentant une unique bande interdite. Un état lié apparaît dans cette bande lorsque le couplage au réservoir dépasse une valeur critique ce qui affecte fortement les propriétés de transport du système. Nous étudions ensuite le cas de réservoirs décrit par un modèle de liaisons fortes dont la densité d'états ne comporte qu'une bande finie d'énergie. Nous montrons qu'un niveau discret couplé à un tel réservoir se comporte comme un système à plusieurs niveaux, sa densité d'états locale et sa fonction de transmission présentant de multiples résonances.

Bien cordialement,
Étienne Jussiau

Dear colleagues,

It is my pleasure to invite to my PhD defense which will take place on
Wednesday, 18th September
in the lecture hall of the Maison des Magistères.

The presentation (in English) will be followed by a buffet where you will be able to taste specialities from Burgundy.

A discrete quantum level strongly coupled to a continuum with a band structure

PhD thesis supervised by Robert S. Whitney

Abstract

Following the technological advances of the Industrial Revolution, classical thermodynamics was developed in the 19th century in order to understand the conversion of heat into work in newly designed machines. The works of Boltzmann brought another conceptual revolution with statistical mechanics. He demonstrated the microscopical origin of the laws of thermodynamics which actually only describe the macroscopic behaviour of systems in which local thermalization is faster than all other timescales. However, following the growing interest for nanotechnologies, it is now possible to manipulate microscopic systems in which thermalization is slower than the timescales for electron flow. A major technological advance in this field stems from the use of quantum dots, nanoscale devices which confine electrons on such small scales that they spread on discrete energy levels. It is then essential to take into account quantum effects for the study of this type of systems, that is to say to design theoretical tools combining thermodynamics and quantum mechanics.

Problems of quantum thermodynamics are often tackled in the framework of the theory of open quantum systems. The general idea of this formalism is to study the dynamics of a “small” quantum system when it is coupled to another much bigger representing the environment. One can then show that the time evolution of the small system can be described by a master equation in the limit where it is weakly coupled to the environment. However, it intuitively seems that the power output of machine would be higher in the context of strong coupling.

For problems of electronic transport, the Landauer-Büttiker formalism allows to describe the strong-coupling regime. In this framework, electrons are assumed to solely undergo elastic scattering processes in the central system. All the thermoelectric properties of the machine can then be characterized thanks to the transmission properties of the scatterer. However, this formalism has an important limitation; it ignores the band structure of the reservoirs.

Here we have chosen to adopt a different viewpoint to tackle the strong-coupling regime by studying an exactly soluble model. We therefore analyze the Fano-Anderson model describing a discrete level coupled to a continuum. We are particularly interested by the influence of the reservoirs' band structure. One can indeed show that, under certain conditions, discrete bound states appear in the band gaps of the reservoirs. This state play an important rôle on the dynamics of the discrete at long times: their contribution depends on the initial preparation of the system and gives rise to persistent oscillations of the occupation of the discrete level.

We start by deriving the exact solution of the model especially focusing on its long-time behaviour. We then analyze two special cases. First, we study the transport properties of a single-level quantum dot coupled to a semiconductor with single a band gap. A bound state appears in this gap when the coupling to the reservoir exceeds a critical value. We show that this greatly affects the transport properties of the device. We then study the case of reservoirs described by a tight-binding model which density of states consists of a single finite-range energy band. We show that a discrete level coupled to such reservoir behaves like a many-level system as its local density of states and transmission function exhibits multiple resonances.

A strong-coupling approach to electronic quantum transport
le mardi 17 septembre 2019 à 14h00

Séminaire nano-électronique quantique

Personne à contacter : Robert Whitney ()

Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé :

The reaction-coordinate mapping is a way of redefining the boundary
between system and reservoir that allows to treat limits inaccessible
with standard approaches. It is implemented by identifying collective
reservoir degrees of freedom and including them -- at the level of the
Hamiltonian -- into a redefined system.

In particular regimes, this enlarged system can then be treated with
standard methods. Within the context of electronic quantum transport,
it is straightforward to apply a fermionic version of the mapping
individually to every reservoir. This allows to revisit nonequilibrium
phenomena from the perspective of strong-system reservoir couplings
and non-Markovian effects.

In particular, I will demonstrate the benefits of the method by showing
that performance of a continuously operating quantum heat engine may
increase in the strong-coupling regime. Furthermore, for explicit
feedback loops, the method can also be used to identify the
thermodynamic cost of measurement and control operations, which for
example allows for a revisiting of electronic Maxwell demons.
Even models that are anyways exactly solvable may profit from
conceptual insight gained from such transformations,
which e.g. allows to identify non-Markovian limits by broken
thermodynamic uncertainty relations.

Papers:
+ A. Nazir and G. Schaller, in F. Binder et al. (eds.),
Thermodynamics in the quantum regime - Recent Progress and Outlook
(Springer International Publishing), (2019).
+ G. Schaller et al., PRB 97, 195104 (2018)
+ P. Strasberg et al., ibid., 205405 (2018).
+ N. Martensen and G. Schaller, EPJB 92, 30 (2019).

Tomas Ramos (Characterizing photon-photon interactions and correlated noise in nanophotonic systems)

Characterizing photon-photon interactions and correlated noise in nanophotonic systems
le vendredi 13 septembre 2019 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : salle lecture 2, maison des Magistères

Résumé : We present new spectroscopic methods for characterizing correlated dephasing noise and multiphoton scattering processes in experiments with photons propagating in microwave transmission lines or optical waveguides.
First, we show how correlated dephasing is manifested in the line shapes of quantum emitters, and how the same noise information contained in standard time-resolved Ramsey experiments can be extracted spectroscopically from single-photon scattering [1,2].
Second, we present a spectroscopic method based on coherent states and homodyne detection to measure the multiphoton scattering matrix of any photonic quantum device [3]. We exemplify the protocol by reconstructing the two-photon scattering matrix of a single two-level quantum emitter, and provide preliminary experimental results using a quantum dot inside a photonic crystal waveguide [4].
[1] T. Ramos, J.J. García-Ripoll, NJP 20, 105007 (2018).
[2] P. Eder, T. Ramos, …, J.J. García-Ripoll, F. Deppe, R. Gross, Supercond. Sci. Technol. 31 115002 (2018).
[3] T. Ramos, J.J. García-Ripoll, Phys. Rev. Lett. 119, 153601 (2017).
[4] H. Le Jeannic, T. Ramos, …, J.J. García-Ripoll, P. Lodahl, in preparation.

Is the Solar System stable?
le jeudi 12 septembre 2019 à 11h00

Séminaire théorie

Personne à contacter : Serge Florens ()

Lieu : Salle Rémy Lemaire K223, Bât K, Institut Néel

Résumé : The search for the Solar System's stability is a fascinating adventure that started with Newton's discovery of the law of gravitation. While this question has been the source of major discoveries both in mathematics and in physics for about three centuries, it is still not fully answered. In the past 30 years, a breakthrough occurred with the numerical discovery that the Solar System is chaotic with a Lyapunov time of about 10 million years. In particular, it has been shown that planetary collisions are possible between the four smallest terrestrial planets. Chaotic motion thus prevent any long-term accurate prediction of planetary positions, and requires us to invent new techniques to predict the state of the Solar System on a timescale comparable to its lifetime. In the present talk, I will show how the methods issued from statistical physics can be used to study the long-term stability of the Solar System. I will explain how the probability of fast destabilizations can be predicted using the theory of rare events.

Kardar-Parisi-Zhang Equation with temporally correlated noise: a non-perturbative renormalization group approach
le mercredi 11 septembre 2019 à 11h00

Séminaire interne LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé : We investigate the universal behavior of the Kardar-Parisi-Zhang equation with temporally correlated noise. The presence of time correlations in the microscopic noise breaks the
statistical tilt symmetry, or Galilean invariance, of the original KPZ equation with delta-correlated noise (denoted SR-KPZ). Thus it is not clear whether the KPZ universality class is preserved in this case. Conflicting results exist in the literature, some advocating that it is destroyed even in the limit of infinitesimal temporal correlations, while others find that it persists up to a critical range of such correlations. Using non-perturbative and functional renormalization group techniques, we study the influence of two types of temporal correlators of the noise: a short range one with a typical time-scale τ , and a power-law one with a varying exponent θ. We show that for the short-range noise with any finite τ , the symmetries (the Galilean symmetry, and the time-reversal one in D = 1 + 1) are dynamically restored at large scales, and the long-distance properties are governed by the SR-KPZ fixed point. In the presence of a power-law noise, we find that the SR-KPZ fixed point is still stable for θ below a critical value θc, in accordance with previous RG results, while a long-range (LR) fixed-point controls the critical scaling for θ > θc, and we evaluate the θ-dependent critical exponents at this LR fixed point, in both D = 1 + 1 and D = 2 + 1 dimensions. While the results in D = 1 + 1 can be compared to previous estimates, no other prediction was available in D = 2 + 1.

Detection and manipulation of dopants and atoms in a high-Tc superconductor using MHz current noise
le mardi 10 septembre 2019 à 14h00

Séminaire nano-électronique quantique

Personne à contacter : Robert Whitney ()

Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé :

Dopants and impurities are crucial in shaping the ground-state of host materials: semiconducting technology is based on their ability to donate or trap electrons, and they can even be used to transform insulators into high temperature superconductors. Due to limited time resolution, most atomic scale studies of the latter materials focussed on the effect of dopants on the electronic properties averaged over time. To lift this limitation, I will first present how we implemented cryogenic circuitry operating in the MHz regime into our home-built scanning tunnelling microscope in order to gain access to time-dependent information, including shot-noise, at the atomic scale [1]. After discussing the details of the circuitry, I will show how it enabled us to detect remarkable charge dynamics at select atomic sites in the high temperature superconductor Bi2Sr2CaCu2O8+x [2]. Lastly, I will demonstrate how we can use these sites, as well as other individual atoms, to manipulate superconductivity.

[1] F. Massee et al., Rev. Sci. Instrum. 89, 093708 (2018)
[2] F. Massee et al., Nature Communications 10, 544 (2019)

Superconductivity in a disordered vortex lattice
le lundi 9 septembre 2019 à 11h00

Séminaire théorie

Personne à contacter : Serge Florens ()

Lieu : Salle Nevill Mott D420, Bât D, Institut Néel

Résumé : Orbital magnetic field and strong disorder weaken superconducting correlations acting individually on a type-II s-wave superconductor. The Abrikosov vortex lattice, resulting from the applied magnetic field, melts with an increase of the strength of the field, turning the system into a metal. Similarly, the presence of disorder causes a superconductor to insulator transition beyond a critical strength of disorder. Here we show that the interplay of these two perturbations, when present simultaneously in a two-dimensional superconductor, causes its intriguing evolution. In particular, we show that the local superconductivity can actually strengthen due to interesting spatial reorganization or order parameters in the presence of strong disorder. While at weak disorder strengths the critical magnetic field for the suppression of superconducting energy gap matches with the critical field at which superfluid density vanishes, the two critical fields diverge from each other with the increase of the disorder strengths. Our results have important consequences for the strong magnetoresistance peak observed in disordered superconducting thin films. We illustrate this by calculating the dynamical conductivity and analyzing its low-frequency behavior. Our results, which emphasize the role of spatial fluctuations in the pairing amplitude, capture the non-monotonic evolution of the magnetoresistance, consistent with experiments. We will also demonstrate that the presence of even weak disorder causes the Caroli-deGennes-Matricon zero-bias peak in vortex-core density of states to disappear. The origin and consequences of such dramatic behaviors will be discussed along with their experimental relevance.
* Work done in collaboration with Anushree Datta, Anurag Banerjee, and Nandini Trivedi

Harnessing Noise in Superconducting Quantum Circuits
le mardi 3 septembre 2019 à 14h00

Séminaire nano-électronique quantique

Personne à contacter : Robert Whitney ()

Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Superconducting quantum circuits show great potential as a practical quantum information technology. However, noise causes decoherence and loss of fidelity in quantum processes, preventing full-scale quantum processors from being built. I will discuss our ongoing experiments to harness noise to improve coherence and fidelity. These include adding engineering the quantum bath to turn bad qubits into a good refrigerators; adding "generalized Markovian" noise to suppress the effects of environmental Markovian noise; and using noise correlations between different qubits to design better quantum error correction algorithms.

Nucleation and Aging Transient Dynamics in the Two-Dimensional Complex Ginzburg-Landau Equation
le mercredi 28 août 2019 à 11h00

Séminaire LPMMC

Personne à contacter : Léonie Canet ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé : The complex Ginzburg-Landau equation (CGL) is a (stochastic)
partial differential equation that describes a remarkably wide
range of physical systems. We numerically investigate nucleation
processes in the transient dynamics of the two-dimensional CGL
towards its "frozen" state with stationary spiral structures,
starting either from the defect turbulence regime or random
initial configurations. Nucleation events of spiral structures
are monitored using the characteristic length between the
emerging shock fronts. We employ an extrapolation method and a
phenomenological formula to account for finite-size effects. The
non-zero barrier for the nucleation of single spiral droplets in
the extrapolated infinite system size limit suggests that the
transition to the frozen state is discontinuous. We also study
the nucleation of spirals for systems that are quenched close to
but beyond the crossover limit, and of target waves which emerge
if a specific spatial inhomogeneity is introduced. In either of
these cases, we observe long, "fat" tails in the distribution of
nucleation times, which also supports a discontinuous transition
scenario. Upon quenching the CGL into the "defocusing spiral
quadrant", we observe slow coarsening dynamics as oppositely
charged topological defects annihilate. We find the physical
aging features in this system to be governed by non-universal
aging scaling exponents. We also investigate systems with control
parameters residing in the "focusing quadrant", and identify slow
aging kinetics in that regime as well. We provide heuristic
criteria for the existence of slow coarsening dynamics and
physical aging behavior in the CGL.
Reference: W. Liu and U.C. Täuber, arXiv:1905.07317;

Hall viscosity induced transverse voltage in two-dimensional Fermi liquids
le vendredi 19 juillet 2019 à 11h00

Séminaire théorie

Personne à contacter : Serge Florens ()

Lieu : Amphithéâtre, maison des Magistères

Résumé : The absence of parity and time-reversal symmetry in two-dimensional
Fermi liquids gives rise to nondissipative transport features
characterized by the Hall viscosity. For non-vanishing magnetic
fields, the Hall viscous force directly competes with the Lorentz
force, since both mechanisms contribute to the Hall voltage. In this
work, we present a channel geometry that allows us to uniquely
distinguish these two contributions and derive, for the first time,
their functional dependence on all external parameters. In particular,
the ratio of Hall viscous to Lorentz force contributions decreases
with the width and slip-length of our channel, while it increases with
its carrier density and electron-electron mean free path. Therefore,
for typical materials such as GaAs, the Hall viscous contribution can
dominate the Lorentz signal by orders of magnitudes up to a few tens
of millitesla. This paves the way to uniquely measure and identify
Hall viscous signals in simple experimental setups.

Quantum complexity, irreversibility, learnability and fluctuations
le vendredi 12 juillet 2019 à 11h00

Séminaire théorie

Personne à contacter : Serge Florens ()

Lieu : Amphithéâtre, maison des Magistères

Résumé : Quantum complexity is a notion characterizing the universality of the entanglement arising from a quantum evolution. A universal evolution will result in a complex entanglement. At the same time, this also corresponds to small fluctuations and to unlearnability from the point of view of machine learning. All these aspects are connected to the different features of k-designs, which are under-samplings of the Hilbert space.
We study the transition in complexity due to the doping of a quantum circuit by universal gates and show that the transition to complex entanglement can be obtained by just a single gate.
These results are relevant for the notions of scrambling, quantum chaos, OTOCs and operator spreading.
We conjecture that the transition to 4−design, W-D and unlearnability are one and the same.

Paolo Zanardi (University of Southern California)
Annulé

Majorana zero modes around skyrmionic textures'
le vendredi 5 juillet 2019 à 11h00

Séminaire théorie

Personne à contacter : Serge Florens ()

Lieu : Amphithéâtre, maison des Magistères

Résumé : Recent scanning tunneling spectroscopy measurements on a
superconducting monolayer of lead(Pb) with nanoscale cobalt islands,
have revealed puzzling quasiparticle in-gap states [1] which demand a
better understanding of two-dimensional superconductivity in presence
of spin-orbit coupling and magnetism. Thus motivated, we
theoretically study a model of two-dimensional s-wave superconductor
with a fixed configuration of exchange field and spin-orbit coupling
terms allowed by symmetry. Using analytics and exact diagonalization
of tight-binding models, we find that a vortex-like defect in the
Rashba spin-orbit coupling binds a single Majorana zero-energy
(mid-gap) state. In contrast to the case of a superconducting vortex
[2], our spin-orbit defect does not create a tower of in-gap
excitation states and our findings match the puzzling features
observed in the experiment. Additionally, these properties indicate
that the system realizes a pair of well-protected Majorana zero mode
(MZM) localized at the core and the rim of the defect [3].
We also discuss how the quasiparticle states of the defect relate to
the states of superconductors on top of magnetic textures, such as skyrmions.
Magnetic skyrmions are nanoscale particle-like spin configurations
that are efficiently created and manipulated. They hold great promises
for next-generation spintronics applications. I will focus on the
theoretical analysis of magnetic skyrmions proximitized by
conventional superconductors. I will show that a topological
superconducting phase can emerge in these systems and uncover a whole
almost flat band of these modes on the edge of the magnetic texture,
in contrast to a previously reported MZM in the core of the skyrmion
[4]. I will discuss in details the origin of these MZMs by relating
this problem to the the extensively-studied Rashba nanowire model. We
have found that these modes are remarkably stable to electronic and
geometric perturbations which we investigate by a combination of
analytical arguments and numerical tight-binding calculations.
Additionally, this analysis reveals that the number of MZMs on the
edge scales linearly with its perimeter [5].
[1] G.C. Ménard et al., Nature Comm. 11, 1013 (2017).
[2] C. Caroli, P.G. de Gennes, and J. Matricon, Physics Letters 9, 307(1964).
[3] G. C. Ménard, et al., arXiv:1810.09541, Nature Comm. 10, 2587 (2019).
[4] G. Yang, P. Stano, J. Klinovaja & D. Loss, PRB 93, 224505 (2016).
[5] M. Garnier, A. Mesaros, P. Simon, arXiv:1904.03005

Multi-particle observables from pure Yang Mills
le jeudi 4 juillet 2019 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : The strong force is governed by a mathematical framework called quantum chromodynamics (QCD). The building blocks of QCD are quarks and gluons, and the interactions of these constituents leads to a rich variety of observed phenomena. A particularly intriguing aspect of QCD physics is the nature and behavior of resonances, short-lived states that decay via the strong force. In this talk I will discuss progress in studying these states, by calculating multi-particle scattering observables in a the simplified framework of pure Yang Mills. This can be achieved by combining field-theoretic ideas with large scale numerical calculations. In particular, I will focus on the idea of using the finite volume required for numerical calculations as a tool, rather than an unwanted artifact, to extract dynamical observables such as two- and (eventually) three-particles scattering amplitudes.

(titre non communiqué)
le mercredi 3 juillet 2019 à 11h00

Séminaire interne LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2

Résumé : Anastasia Gorbunova : Numerical study of turbulence
Bastien Maguet : Symmetries in the stochastic dynamics of interfaces and their supersymmetric formulation

Stefano Roddaro (Scuola Normale Superiore & Università di Pisa)

Field-effect control of the properties of InAs/InP nanowire single-electron trnasistors
le mardi 2 juillet 2019 à 14h00

Séminaire nano-électronique quantique

Personne à contacter : Robert Whitney ()

Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Single-electron transistors based on heterostructured nanowires represent a promising and robust building block for a range of applications in fundamental science as well as in sensing. Many of these require a good degree of control on a set of key device parameters such as the tunnel coupling, level spacing and filling, which is not obvious to combine with the adoption of a heterostructure-defined nanodevice. In my talk I will review our recent results on the field-effect control of InAs/InP quantum dots and demonstrate in particular how tunnel rates can be sharply and controllably increased depending on the kind of orbitals involved in the transport process. Experimental results will be compared with simulations of the nanostructure to identify the mechanisms responsible for the tuning.

Lax Integrability and Cheap Macroscopic Quantum Coherence with Matter-Wave Breathers
le lundi 1er juillet 2019 à 10h00

Séminaire interne LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé : Matter-wave breathers is an example of an area where the macroscopic quantum coherence resource may be already available today but yet overlooked. There, even for a relatively hot soliton, a four-fold quench of the coupling constant will generate a bi-solton state whose relative soliton-soliton motion is in a minimal Heisenberg uncertainty state. The latter will be observable through an eventual separation between the solitons, itself a deep consequence of the Lax integrability and the classical field level and Bethe integrability at the quantum one. The estimates for the separation time range between a few to a dozen of seconds, i.e. within the experimental reach.

Some Empirical Implementations of the Multi-Dimensional Reflection Groups
le vendredi 28 juin 2019 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : In this presentation, I will review some of our recent successes in finding a three-dimensional empirical room for the abstract multidimensional kaleidoscopes. The latter ensure solvability of the former. The areas of implementation include (a) quantum one-dimensional hard-core particles with mon-trivial mass-spectra, on a line, in a box, or in a harmonic potential; (b) a quantum one-dimensional bosonic dimer interacting with a barrier; (c) a field of a static electric charge in a conducting cavity surrounded by four spherical segments. Concrete experimental suggestions include (a) an “entanglement amplifier”, (a’) integrability induced peaks in a relaxation time vs. mass ratio curve for a binary mass mixture, (b) a novel observable selection rule for some one-dimensional chemical processes and the usage thereof for miniaturization of chip-based atom interferometers, and (c) nineteen three-parametric families of solvable electrostatic problems in piece-wise-spherical cavities with conducting grounded walls.

Transition-metal pnictides : electrons and phonons
le mercredi 26 juin 2019 à 11h00

Séminaire LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé : The family of iron-based superconductors display signatures of moderate electronic correlations due to strong Hund's coupling, leading to quasiparticle renormalizations and relatively low coherence temperatures. In this seminar, I will discuss the electronic structure of a series of 122 transition metal pnictides, from chromium to copper, based on photoelectron spectroscopy experiments and ab initio calculations. In a second part, I will also discuss the interplay of electronic and lattice effects in a few iron-based compounds.

Zhihui Peng (Hunan Normal University, Changsha, China )

Coupling of a Cavity and a Transmission Line with a Superconducting Artiﬁcial Atom
le mardi 25 juin 2019 à 14h00

Séminaire nano-électronique quantique

Personne à contacter : Robert Whitney ()

Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé :

We report our experimental results about strong coupling of a cavity and a transmission line with an superconducting artificial atom. With the special architecture, we observed the vacuum-induced Aulter-Townes splitting[1] which has potential application in microwave quantum network. We also observed anomalous resonance fluorescence of an atom-cavity coupled system[2] which is qualitatively different from the driven-atom in free space. Our results show the superconducting artificial atom is an ideal testbed for quantum optics.

References:
1. Z.H. Peng et al., PRA 97, 063809 (2018).
2. Z.H. Peng et al., In preparation.

Assessing many-body quantumness via correlation functions
le vendredi 21 juin 2019 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : Decades of research into the foundational aspects of quantum mechanics — started with the Einstein-Podolsky-Rosen (EPR) paradox on entanglement and non-locality — have brought us a radically new way of thinking about physical systems: the latter can be now viewed as hosts of quantum information encoded in quantum superpositions, and as potential resources for novel quantum technologies. A central question is how to assess the non-classical nature of quantum states, namely their characterization as coherent superpositions featuring non-local quantum correlations. This question becomes particularly intriguing and intricate when moving to many-body systems: the exponential growth of quantum information with the system size makes many-body tomography simply inaccessible, and strategies for a scalable assessment of quantumness need to be devised. This endeavor has obviously a foundational aspect, ultimately aiming at an exploration of the mysterious quantum-classical or micro-macro boundary; but it has also immediate bonuses, since assessing quantumness of many-body states can translate into probing their potential use as resources for quantum information tasks.
In this talk I will address the question: assuming the one has theoretical or even experimental access to correlation functions related to a generic quantum many-body state (pure or mixed, at equilibrium or far from it), can one make conclusive statements about the quantum nature of the state in question? By "quantum nature” I mean here the various forms of increasing non-classicality, namely entanglement; EPR correlations; and Bell correlations. I will show that bipartite entanglement and EPR correlations can be effectively assessed via the knowledge of correlations between two subsystems; and that they are in fact generic features of systems with continuous symmetries in the “canonical” ensemble (namely at fixed magnetisation for quantum spin systems; fixed particle number for lattice quantum gases). Moreover I will illustrate how one can make an exhaustive Bell test on the measured correlations -- unveiling constructively their definite incompatibility with classical physics — without making use of Bell’s inequalities.

Scanning SQUID measurements of domain walls in SrTiO3
le mardi 18 juin 2019 à 14h00

Séminaire nano-électronique quantique

Personne à contacter : Robert Whitney ()

Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé :

The interface between the oxide insulators Lanthanum Aluminate and Strontium titanate hosts a gate tunable 2D electron gas that also becomes SC at low temperatures. It has been demonstrated that the 2DEG can be confined to create devices such as gate defined SQUIDs or a single electron transistor.

In effect the Physics of the SrTiO3 substrate play a major role in the behavior of the interface. SrTiO3 undergoes a structural phase transition at 105K resulting in a dense network of domains separated by nanometer thick twin walls.

I will discuss our recent findings, where we used scanning SQUID microscopy to map the spatial distribution of conduction at the interface. Images of the interface showed quasi-1D channels of modulated current flow, superconductivity and magnetic signal. The domain walls change their location with thermal cycles and with the application of back gate voltage. These findings open exciting possibilities for normal and superconducting devices based on domain walls.

Brijesh Kumar (Jawaharlal Nehru University New Delhi)

Inversion and Quantum Oscillations in Kondo insulators
le vendredi 14 juin 2019 à 11h00

Séminaire théorie

Personne à contacter : Serge Florens ()

Lieu : Amphithéâtre, maison des Magistères

Résumé : Conventionally, the quantum oscillations of magnetisation [the de Haas-van Alphen (dHvA) effect] have come to be exclusively associated with metals. But recent observations of magnetic quantum oscillations in Kondo insulators (SmB6 and YbB12) challenge this conventional view, and call for a reexamination. We study this problem by investigating the basic models of Kondo insulators for their orbital response to uniform magnetic field. By doing a self-consistent theory of the charge dynamics of Kondo insulators in a novel representation for electrons [1], we discover the gapped charge quasiparticles to undergo inversion upon decreasing the Kondo coupling, and establish the inversion to be the key determinant for quantum oscillations to occur as a bulk phenomenon in Kondo insulators [2,3]. The frequency of dHvA oscillations we obtain corresponds to the half of the bulk Brillouin zone, as observed experimentally [4].
References:
[1] Brijesh Kumar, Phys. Rev. B 77, 205115 (2008)
[2] Panch Ram and Brijesh Kumar, Phys. Rev. B 96, 075115 (2017)
[3] Panch Ram and Brijesh Kumar, arXiv:1809.04654; Phys. Rev. B (2019). In production.
[4] B. S. Tan et al, Science 349, 287 (2015)

Anjan K. Gupta (Indian Institute of Technology Kanpur)

Optimization of constriction based niobium µ-SQUIDs for probing nano-magnetism
le mardi 11 juin 2019 à 14h00

Séminaire nano-électronique quantique

Personne à contacter :

Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé :

Magnetometry using micron-size superconducting quantum interference devices (µ-SQUIDs) has been remarkably successful in probing classical as well as quantum regimes of magnetism in single nano-particles. This technique can be further improved for higher speed and sensitivity with hysteresis-free µ-SQUIDs. This is difficult, particularly, at low temperatures, which is essential for probing quantum-magnetism. The hysteresis in these devices arises from thermal instabilities in superconducting weak-links and neighboring region. The heat generated in resistive normal region gives rise to a self sustained hot-spot. This leads to two possible states, hot (normal) and cold (superconducting), and hence bistability. Such hot-spot and hysteresis has been modeled in the past by using steady state thermal heat balance equations.

However, as we approach the hysteresis-free regime by optimizing the relative heat evacuation, another regime of hysteresis is found in which the bistability results due to a phase dynamic steady state. We understand this dynamic regime using a thermal model that helps us quantitatively capture the behavior in both hysteretic and non-hysteretic regimes. Slow relaxation of quasi-particles, which are generated due to phase dynamics, is found to be a bottleneck, which is the case for several superconducting devices including SIN-coolers and superconducting qubits.

We solve the thermal model for different shunting conditions to find that an optimal shunt having resistance and inductance both can eliminate hysteresis at low temperatures and with a good sensitivity. A pure resistive shunt, which works well for hysteresis elimination in usual tunnel junction based SQUIDs, leads to a marked reduction in sensitivity of µ-SQUIDs. This new model also reveals an interesting non-linear dynamical system with various regimes. We successfully test this idea of inductive shunt eliminating hysteresis with good sensitivity. Finally, we present preliminary results on magnetization reversal in permalloy nano-needles by using these optimized non-hysteretic µ-SQUIDs.

Peter Makk (University of Basel & Budapest University of Technology and Economics,)

Engineering exotic states in graphene heterostructures
le mardi 4 juin 2019 à 14h00

Séminaire nano-électronique quantique

Personne à contacter :

Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé :

Graphene is an ideal platform to realize novel, topological states of matter by combining it with other 2D materials using van der Waals stacking. These states include the topological insulator state, the valley Hall state and the appearance Majorana excitations have also been predicted by combining special quantum Hall states with superconducting correlations. Here we show our work towards this direction.

First I will show supercurrent measurements in graphene/hBN heterostructures with a Moiré superlattice formed by alinging graphene lattice to the hBN lattice. Using the supercurrent measurement we extract the DOS of the superlattice and investigate the appearance of edge states using interferometry measurements. We also show tunneling spectroscopy measurements in graphene, where we extract the non-equilibrium distribution function and investigate the electron cooling mechanisms in graphene. Finally we comment on the appearance of SOI from TMDC substrates.

- D. Indolese, et al., Phys. Rev. Lett., 121, 137701 (2018)
- S. Zihlmann, et al. Phys. Rev. B. 7, 075419 (2019)
- L. Wang, et al. Nano Lett., 19, 2371 (2019)
- S. Zihlmann, et al. Phys. Rev. B., 7, 075434 (2019)

From billiards for light to mesoscopic optics
le mardi 28 mai 2019 à 14h00

Séminaire nano-électronique quantique

Personne à contacter : Robert Whitney ()

Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : The investigation of the propagation of light in mesoscopic, i.e. often micrometer-scale, systems is a rich subject providing insights ranging from quantum chaos in open systems to new schemes for realizing microlasers. The concept of quantum-classical, here wave-ray, correspondence, proves to be as useful as for electronic mesoscopic systems such as quantum dots. Whereas there the electrons are confined by means of gate voltages, the confinement of light in optical microresonators is due to total internal reflection, leading to billiards for light. There are, however, semiclassical deviations from the naive ray-picture expectation in the reflection and refraction of light at dielectric interfaces yielding for example to deviations from Snell's law. We illustrate these effects and discuss their impact on the far-field emission characteristics of optical microcavities.
The propagation of electromagnetic waves in three-dimensional optical microcavities requires to pay attention to the evolution of the light's polarization as a new degree of freedom. In systems like dielectric Möbius-strips or cone-shaped microtube cavities, the polarization state of resonant whispering gallery-type modes may differ strongly from the reference case of homogeneous cylinders. Whereas we find that the polarization of the electromagnetic field follows the wall orientation in thin Möbius strips, thereby reflecting the accumulated geometric phase, we observe that the electromagnetic field ignores the Möbius topology when the strip thickness is increased. Breaking of symmetries further influences the morphology of resonances and can induce a transition from linear to elliptical polarization that is both of theoretical interest from the point of view of spin-orbit interaction of light and their interpretation in terms of Berry phases, and relevant for potential applications.

Superfluidity in the inner crust of neutron stars
le vendredi 24 mai 2019 à 11h00

Colloque CPTGA

Personne à contacter : Julia Meyer ()

Lieu : Amphithéâtre, maison des Magistères

Résumé : After a general introduction to neutron stars, I will focus on the
special role of their inner crust. This region is characterized by the
coexistence of a Coulomb lattice of neutron-rich nuclei ("clusters")
in a uniform background of ultrarelativistic degenerate electrons and a
gas of unbound neutrons. The unbound neutrons are supposed to be
superfluid, which gives rise to remarkable phenomena, such as the
famous "glitches" (sudden increases of the neutron star's rotation
frequency) and changes in the star's cooling behaviour. However,
making reliable predictions for the superfluid critical temperature
remains a challenging problem for nuclear many-body theory, mostly
because of medium-polarisation effects. Another important unsolved
problem is the "entrainment" between the neutron gas and the clusters
in the crust, since it determines the density of superfluid neutrons.

In-flight manipulation of single electrons
le mardi 21 mai 2019 à 14h00

Séminaire nano-électronique quantique

Personne à contacter : Robert Whitney ()

Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé :

Over the past decade, an important effort has been made in the field of low-dimensional electronic conductors towards single-electron electronics with the goal of gaining coherent control over single flying electrons in solid-state devices [1]. In this talk I will present our recent advances towards the realization of electronic flying-qubit architectures using ultrashort charge pulses (Levitons) as well as surface acoustic wave (SAW) driven single electrons.

In the first part of the talk I will present time-resolved measurements of ultrashort single-electron charge pulses injected into a quasi-one-dimensional quantum conductor. We show that the velocity of such a single-electron pulse is found to be much faster than the Fermi velocity due to the presence of strong electron-electron interactions and can be tuned over more than an order of magnitude by electrostatic confinement. In addition, our set-up allows us to tune our system continuously from a clean one-channel Tomonaga-Luttinger liquid to a multi-channel Fermi liquid [2]. Our results are in quantitative agreement with a parameter-free theory and demonstrate a powerful new probe for directly investigating real-time dynamics of fractionalisation phenomena in low-dimensional conductors.

In the second part of the talk, I will concentrate on SAW-assisted single-electron transport. I will present our recent results on highly-efficient electron routing in a beam-splitter configuration. For this we connect four quantum dots via two 22 μm long quantum rails that are coupled by a tunnel barrier along a 2 µm long interaction region. Changing the energy detuning in the interaction region we can partition the electrons on-demand into two paths with electron transport efficiencies of 99.7 % [3].
Our results demonstrate the potential of these two approaches for the implementation of an electronic solid-state flying qubit having high relevance in fundamental research and quantum information technology.

[1] C. Bäuerle et al., Rep. Prog. Phys. 81, 056503 (2018)
[2] G. Roussely, E. Arrighi et al., Nature Comm. 9, 2811 (2018)
[3] S. Takada, H. Edlbauer et al., arXiv:1903.00684v1

Raphaël Chétrite (Laboratoire Dieudonné, Université de Nice - Sophia Antipolis)

Analytical Large Deviation and Uncertainly Relation
le vendredi 17 mai 2019 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : In this talk, I will talk about the theory of large deviations. After a general introduction, I will present some recent developments on the large deviations associated with a Markov process and on applications for thermodynamic uncertainty relations.

Bogoliubov Theory at Positive Temperatures
le vendredi 10 mai 2019 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : I shall discuss the homogeneous Bose gas at positive temperatures within
Bogoliubov theory. The theory arises by restricting the Hilbert space to
quasi-free states. I will introduce the free energy functional and discuss
the existence of equilibrium states, phase diagram and critical
temperature. This is joint work with Robin Reuvers and Jan Philip Solovej.

Probing quantum Hall conductors with low and high frequency noise
le mardi 30 avril 2019 à 14h00

Séminaire nano-électronique quantique

Personne à contacter : Robert Whitney ()

Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé :

In my seminar, I will discuss measurements of low and high frequency noise in quantum Hall conductors and how they can probe the elementary excitations propagating along the edge channels in the integer and fractional cases.

I will first present how low frequency noise measurements can be used to extract the electronic states propagating along the edge channels of the integer quantum Hall regime. Combining two-electron Hong-Ou-Mandel interferometry [1] with signal processing techniques, we have implemented a quantum tomography protocol [2,3] able of extracting from any electrical current the generated electron and hole wavefunctions as well as their emission probabilities

In the second part of my presentation, I will discuss the measurement of high frequency noise [4] generated by the random transfer of fractional excitations through a potential barrier biased with a dc voltage Vdc. At high frequencies (few GHz), the emitted noise can be interpreted as the generation of microwave photons in a coaxial measurement line weakly coupled to the sample. We observe that photons are only emitted when their frequency is smaller than the frequency threshold fJ=qVdc/h called the Josephson frequency [5,6] in analogy with the Josephson relation in superconductors. This threshold provides a direct determination of the fractional charge q.

[1] E. Bocquillon et al. Science 339, 1054 (2013)
[2] C. Grenier et al. NJP 13, 093007 (2011)
[3] T. Jullien et al. Nature 514, 603 (2014)
[4] R. Bisognin et al., Nature Communications 10, 1708 (2019).
[5] C. de C. Chamon, D. E. Freed, and X. G. Wen, Phys. Rev. B, 51, 2363 (1995).
[6] M. Kapfer et al., Science 363, 846 (2019).

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.