PhD Student

Research group: String Theory

Thesis

"Celestial amplitudes and holography"
Advisor : Andrea Puhm

Research interests

Quantum gravity, Holography, Amplitudes, Conformally soft theorems, Celestial amplitudes
 

Publications

[1] Y. Pano, S. Pasterski, and A. Puhm, ''Conformally soft fermions," Journal of High Energy Physics 2021 no. 12, (2021) 1-38.

[2] Y. Pano, A. Puhm, and E. Trevisani, ''Symmetries in Celestial CFT_d,'' arXiv:2302.10222 [hep-th].

Post Doctoral Fellow

Research group: String Theory 

Research interests :

My work concerns infinite-dimensional symmetries (and more generally group actions) that appear in various domains of physics. This includes asymptotic symmetries in gravitation and gauge theory, but also condensed matter physics (e.g. the quantum Hall effect) and fluid mechanics. Indeed, all these systems admit rich underlying group structures, with typically striking implications, whose study is often hampered by the complexity of infinite-dimensional manifolds. My goal is to unveil the physical, possibly observable, effects of these groups, for instance through certain Berry phases that are associated with them.

"It is only slightly overstating the case to say that physics is the study of symmetry."
[Philip W. Anderson, "More is different", Science, 1972]

Thesis:

"BMS Particles in Three Dimensions", Université Libre de Bruxelles, 2016.
arXiv:1610.08526
Published in Springer Theses:  https://www.springer.com/gp/book/9783319618777

Publications:

List of publications can be found on Inspire : https://inspirehep.net/authors/1626583

Selection of publications:

- B. Oblak and G. Kozyreff, “Berry Phases in the Reconstructed KdV Equation,” arXiv:2002.01780.

- B. Oblak, “Berry Phases on Virasoro Orbits,” JHEP 10 (2017) 114, arXiv:1703.06142.

- G. Barnich, H. A. Gonzalez, A. Maloney, and B. Oblak, “One loop partition function of three- dimensional flat gravity,” JHEP 2015 (2015), no. 4, arXiv:1502.06185.

- B. Oblak, “Characters of the BMS Group in Three Dimensions,” Commun. Math. Phys. 340 (2015), no. 1, 413-432, arXiv:1502.03108.

- G. Barnich and B. Oblak, “Notes on the BMS group in three dimensions: I. Induced representations,” JHEP 2014 (2014), no. 6, arXiv:1403.5803.

Hepeng Yao (groupe Matière condensée)

Soutiendra publiquement ses travaux de thèse intitulés

"Strongly-correlated one-dimensional bosons in continuous and quasiperiodic potentials"

dirigés par Laurent Sanchez-Palencia

Soutenance prévue le mardi 20 Octobre 2020 à 14h00

Lieu : viso conférence via zoom (https://us02web.zoom.us/j/83369858239?pwd=MXBybjV1MmJRNFNIR1pvSnpMZ25MZz09)

Abstract

We investigate the properties of one-dimensional bosons in various types of systems, focusing on the phase transitions or crossovers between different quantum degeneracy regimes. Combining quantum Monte Carlo with other standard techniques such as exact diagonalization and thermal Bethe ansatz, we can compute the behavior of 1D bosons in different cases where the results are still lacking. First, in the case of harmonically trapped continuous bosons, we provide a full characterization of a quantity called Tan's contact. We show that the contact exhibits a maximum versus temperature and that it is a signature of the crossover to fermionization in the strongly-interacting regime. Secondly, we study the localization and fractal properties of 1D ideal gases in shallow quasiperiodic potentials. We determine the critical localization properties of the system, the critical potential, mobility edges and critical exponents which are universal. Moreover, we calculate the fractal dimension of the energy spectrum and find it is non-universal but always smaller than unity, which shows the spectrum is nowhere dense. Finally, we move to the study of the interacting case. With the quantum Monte Carlo calculations, we compute the phase diagram of Lieb-Liniger bosons in shallow quasiperiodic potentials. A Bose glass, surrounded by superfluid and Mott phases, is found. At finite temperature, we show that the melting of the Mott lobes is characteristic of a fractal structure and find that the Bose glass is robust against thermal fluctuations up to temperatures accessible in experiments.

Jury

Rapporteurs:
Guillaume Roux (Université Paris-Saclay)
Ulrich Schneider (University of Cambridge)

Examinateurs :
Thierry Giamarchi (University of Geneva)
Anna Minguzzi (Université Grenoble Alpes)
Hanns-Christoph Nägerl (University of Innsbruck)

Directeur de thèse :
Laurent Sanchez-Palencia (CPHT, Ecole Polytechnique)

Fan Yang (groupe Matière condensée)

Soutiendra publiquement ses travaux de thèse intitulés

"Topological Majorana Fermion Models and New Applications"

dirigés par Karyn Le Hur

Soutenance prévue le lundi 21 Septembre 2020 à 14h30

Lieu : viso conférence via zoom (https://zoom.us/j/91425257071)

Abstract

We present a theoretical study of topological models hosting Majorana fermion which is its own anti-particle, with relevant probe of quantum entanglement and experimental protocol for quantum engineering in cQED. For the first part, we focus on Kitaev spin liquids that can be exactly solved in a Majorana fermion representation. We introduce valence bond fluctuations to characterize phase transitions between Abelian and non-Abelian phases, and find a general relation with the entanglement entropy. To simulate these many-body Majorana states, we propose a driven superconducting box circuit with generalizations to coupled box ensembles. In the second part, by proximity effects we address the topological superconducting wire systems, where Majorana fermions emerge as zero-energy modes at edges. By varying strengths of inter-wire couplings and changing fluxes of orbital magnetic fields, we show a physical realization towards topological p-wave superconductivity.

Jury

Président :
Nicolas Regnault (LPA, ENS and Princeton University)

Rapporteurs:
Johannes Knolle (Technical University of Munich)
Yuval Oreg (Weizmann Institute of Science)

Examinateurs :
Pasquale Calabrese (SISSA and INFN, Sezione di Trieste)
Benoit Douçot (LPTHE, Sorbonne Université)
Ion Garate (Université de Sherbrooke)
Pascal Simon (LPS, Université Paris-Saclay)

Directeur de thèse :
Karyn Le Hur (CPHT, Ecole Polytechnique)

Alice Moutenet (groupe Matière condensée)

Soutiendra publiquement ses travaux de thèse intitulés

"Novel algorithms for strongly correlated quantum systems in and out of equilibrium"

dirigés par Monsieur Antoine GEORGES et Monsieur Michel FERRERO

Soutenance prévue le vendredi 03 juillet 2020 à 14h30

Lieu : Collège de France 11, place Marcelin Berthelot 75005 Paris

Salle : viso conférence + Salle 2

Abstract

What do stars in a galaxy, drops in a river, and electrons in a superconducting cuprate levitating above a magnet all have in common? All of these systems cannot be described by the isolated motion of one of their parts. These singular properties emerge from particles and their interactions as a whole: we talk about the many-body problem.
In this Thesis, we focus on properties of strongly-correlated systems, that obey quantum mechanics. Analytical methods being rapidly limited in their understanding of these materials, we develop novel numerical techniques to precisely quantify their properties when interactions between particles become strong.

First, we focus on the equilibrium properties of the layered perovskite Sr2IrO4, a compound isostructural to the superconducting cuprate La2CuO4, where we prove the existence of a pseudogap and describe the electronic structure of this material upon doping.
Then, in order to address the thermodynamic limit of lattice problems, we develop extensions of determinant Monte Carlo algorithms to compute dynamical quantities such as the self-energy. We show how a factorial number of diagrams can be regrouped in a sum of determinants, hence drastically reducing the fermionic sign problem.

In the second part, we turn to the description of nonequilibrium phenomena in correlated systems. We start by revisiting the real-time diagrammatic Monte Carlo recent advances in a new basis where all vacuum diagrams directly vanish. In an importance sampling procedure, such an algorithm can directly address the long-time limit needed in the study of steady states in out-of-equilibrium systems.
Finally, we study the insulator-to-metal transition induced by an electric field in Ca2RuO4, which coexists with a structural transition. An algorithm based on the non-crossing approximation allows us to compute the current as a function of crystal-field splitting in this material.

Jury

M. Antoine GEORGES, Collège de France, Directeur de thèse
M. Michel FERRERO, École polytechnique, Co-directeur de thèse
Mme Laura MESSIO, Sorbonne Université, Examinateur
M. Marco SCHIRO, CEA-Saclay, Examinateur
M. Philipp WERNER, Université de Fribourg, Rapporteur
M. Sylvain CAPPONI, Université Paul Sabatier de Toulouse, Rapporteur

Charles Marteau (String theory)

Boundary structures and holographic fluids in gravity

Abstract

This thesis is devoted to the study of several aspects of dynamical spacetimes with boundaries. An emphasis is put on asymptotic boundaries such as the conformal boundary of AdS or the null infinity in flat space. In AdS the geometry of the conformal boundary is pseudo-Riemannian since the boundary is time-like. In flat space, we will show how the geometry of the null infinity can be described in terms of Carroll structure. The latter emerges as the ultra-relativistic limit, or c → 0 limit, of a pseudo-Riemannian geometry. In particular, the flat limit in the bulk maps to this ultra-relativistic limit on the boundary. We will also see how the symmetries of asymptotically flat gravity translate into global symmetries of this exotic boundary geometry. This analysis is of central importance in fluid/gravity correspondence since the fluid is expected to live on the boundary. In this context we find integrability conditions on the boundary fluid that allow for a resummation of the so-called Derivative Expansion in AdS. The flat limit gives rise to the notion of Carrollian fluid on the boundary whose hydrodynamical expansion maps to a flat version of the Derivative expansion in the bulk, thus providing a notion of fluid/gravity correspondence in flat space. A second type of boundary that we study is the one formed by the horizon of a black hole. There, another type of fluid/gravity correspondence exists: the membrane paradigm. We revisit this concept and propose a novel interpretation of the Damour–Navier–Stokes equation in terms of ultra-relativistic conservation laws.

Jury

Dionysios ANNINOS, Kings College, Londres
Glenn BARNICH, Université Libre de Bruxelles
Daniel GRUMILLER, TU Wien, Vienne
Niels OBERS, Niels Bohr Institute, Copenhague et Nordita, Stockholm
Marios PETROPOULOS, École Polytechnique, IP Paris (directeur de thèse)
Andrea PUHM, École Polytechnique, IP Paris

Organisation

The defense will take place by zoom conference on Friday, June 19, 2020 at 10:30 am. Reception will open from 10 am. Latecomers will not be allowed in the virtual hall before intermission. 

Connection details can be obtained on request from Malika Lang, Charles Marteau or Marios Petropoulos.

Annulé

Frank Redig will give six lectures, grouped into three sessions, at the following dates : 19 May, 26 May, 2 June, from 10:30 to 12:30, in the conference room Louis Michel.

The lectures will be centred around the theory and applications of duality in the context of interacting particle systems and non-equilibrium statistical physics models. Duality is a powerful tool in Markov process theory, allowing to connect two processes (the process under study and its dual) via a so-called duality function. Often the dual process is simpler, e.g., in systems coupled with reservoirs in  the dual the reservoirs are replaced by absorbing boundaries, or in infinite interacting systems, the dual is a system of finitely many particles, or in the context of diffusion processes, the dual is a simple discrete jump process.
We show how to understand dualities from the point of view of an underlying Lie algebra of symmetries (operators commuting with the generator).
This approach gives several new dualities, and new duality functions, such as orthogonal duality functions. It also allows to constructively define processes with dualities, and to find ``correct’’ asymmetric versions of symmetric models with dualities (via so-called q-deformation).
We will explain this theory starting from simple examples (such as independent random walkers, exclusion process, inclusion process), and provide several applications in the description of non-equilibrium steady states as well as in hydrodynamic limits and fluctuation fields.

Lecture 1: Introduction, motivation, some background from Markov process theory

Lecture 2: Duality: definition, duality and symmetries, duality and intertwining, duality and change of representation. Examples.

Lecture 3: Dualities for independent random walkers: self-duality, duality with deterministic system, averaging models. Applications to hydrodynamic limits and fluctuation fields and to ergodic theory.

Lectures 4-5: The symmetric inclusion process (SIP) and its dual processes: Lie algebraic construction via co-product of the Casimir.
Discrete and continuous representations, diffusion processes dual to SIP. Thermalization, models of KMP type, inhomogeneous systems.

Lecture 6: Non-equilibrium steady states: duality with reservoirs. Consistency, orthogonal polynomial duality. Universal properties of correlation functions of non-equilibrium steady states.

Séminaire annulé

Salle de conférence  Louis Michel

Speaker: Grégory Schehr, LPTMS 

Title: Large deviations of the top eigenvalue of random matrices and applications in statistical physics

Abstract: The statistical properties of the largest eigenvalue of a random matrix are of interest in diverse fields such as in the stability of large ecosystems, in disordered systems and related stochastic growth processes, in statistical data analysis and even in string theory. In this talk I will discuss the developments in the theory of extremely rare fluctuations (large deviations) of the largest eigenvalue using a Coulomb gas approach. I will discuss in particular the third-order phase transition which separates the left tail from the right tail, a transition akin to the so-called Gross-Witten-Wadia phase transition found in 2-d lattice quantum chromodynamics.

Salle de Conférence Louis Michel

14:00-15:00 : Bart van Ginkel

Title: Hydrodynamic limit of the Symmetric Exclusion Process on compact Riemannian manifolds.

Abstract: In this talk I will first explain the concept of the hydrodynamic limit of an interacting particle system. The idea is that one wants to show that when both space and time are rescaled (appropriately), the limiting densities of particles satisfy some PDE. This will be illustrated with the Symmetric Exclusion Process. Then we move this basic particle system to a hard context: Riemannian manifolds. I will highlight which challenges arise in the curved setting and how we deal with them. Joint work with Frank Redig.

15:00-16:00 : Rik Versendaal

Title: Large deviations for geodesic random walks.

Abstract: The theory of large deviations is concerned with the limiting behaviour on the exponential scale of a sequence of random variables. A fundamental result is Cramér’s theorem, which states that the empirical mean of a sequence of i.i.d. random variables satisfies a so called large deviations principle. Mogulskii’s theorem is concerned with the corresponding path space large deviations. To study the analogue of these theorems for a Riemannian manifold, we introduce a generalization of random walks to a manifold, called geodesic random walks. We present the analogues of Cramér’s and Mogulskii’s theorem for geodesic random walks and write down the rate function for these large deviations principles.

Une première édition digitale aura lieu le  MARDI 16 JUIN DE 14H A 16H30

Découvrez le programme et inscrivez-vous ici

Organisée par des chercheurs pour les chercheurs et les doctorants, l’objectif de cette journée conviviale est de réunir les dix communautés de l’Institut afin d’offrir un panorama de le recherche réalisée au sein de ses laboratoires. Au programme : des présentations dynamiques de 5 minutes sur des thématiques variées montrant la richesse des recherches de nos laboratoires, des discours scientifiques croisés faisant le lien entre des sujets de plusieurs disciplines, des échanges autours de posters scientifiques, etc. Plus de détails seront bientôt communiqués.

Comité d’organisation : Isabelle Bloch (Telecom Paris), Jean-René Chazottes (École polytechnique), Arnak Dalalyan (Ensae), Maryline Laurent (Telecom SudParis), Jean-François Semblat (Ensta Paris), avec l’aide de Chloé Aubisse et Solange Ricard (direction de la communication de l'École polytechnique).

Une édition en présentiel aura lieu à une date qui reste à définir en fonction de l'évolution des conditions sanitaires.