Quantum Encounters Seminar

Quantum Information, Condensed Matter, Quantum Field Theory


A seminar organized by Vasily Pestun and Slava Rychkov

Pour être informé des prochains séminaires vous pouvez vous abonner à la liste de diffusion.
Ecrivez un mail à sympa@listes.math.cnrs.fr avec comme sujet: “subscribe quantum_encounters_seminar PRENOM NOM”.
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7th June, 2022 - 16:00 Luca Delacretaz (University of Chicago)

Nonlinear Effective Field Theory of Fermi Liquids

Landau's theory of Fermi liquids is a cornerstone of theoretical physics. I will show how to formulate Fermi liquid theory as an effective field theory of bosonic degrees of freedom, using the mathematical formalism of coadjoint orbits. While at the linear level, this theory reduces to existing multidimensional bosonization approaches, it necessarily features nonlinear corrections that are fixed by the geometry of the Fermi surface. These are crucial to reproduce nonlinear response, such as higher-point functions of currents. The effective field theory framework furthermore systematically parametrizes corrections to Fermi liquid behavior, and provides a computationally advantageous approach for non-Fermi liquids -- strongly interacting fixed points obtained by deforming Fermi liquids with relevant interactions.

Participer à la réunion Zoom
https://us02web.zoom.us/j/87394591389?pwd=bFVaZHltckVaOXpZY1YrdWVVZUtHdz09

ID de réunion : 873 9459 1389
Code secret : 573006

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Pour être informé des prochains séminaires vous pouvez vous abonner à la liste de diffusion en écrivant un mail à sympa@listes.math.cnrs.fr avec comme sujet: "subscribe quantum_encounters_seminar PRENOM NOM"
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17th May, 2022 - 16:00 Julio Parra-Martinez (Caltech)

Causality Constraints on Gravitational Effective Theories

In this talk I will describe constraints from causality and unitarity on 2→2 graviton scattering in weakly-coupled gravitational effective field theories. Together, causality and unitarity imply dispersion relations that connect low-energy observables to high-energy data. Using such dispersion relations, I will explain how to derive two-sided bounds on gravitational Wilson coefficients in terms of the mass M of new higher-spin states. Such bounds imply that gravitational interactions must shut off uniformly in the limit G→0, and prove the scaling with M expected from dimensional analysis. In addition they demonstrate the gravity must be weakly coupled at all scales below Planck. Time permitting, I will comment on the experimental implications of the bounds.

Sujet : Quantum Encounter Seminar
Heure : 17 mai 2022 04:00 PM Paris

Participer à la réunion Zoom
https://us02web.zoom.us/j/85890403645?pwd=_WMK9e4w_JBbmNZFpTsnd4zUMmjnSa.1

ID de réunion : 858 9040 3645
Code secret : 301826

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Pour être informé des prochains séminaires vous pouvez vous abonner à la liste de diffusion en écrivant un mail à sympa@listes.math.cnrs.fr avec comme sujet: "subscribe quantum_encounters_seminar PRENOM NOM"
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5th April, 2022 - 10:30 Jean-Noël Fuchs (LPTMC, Sorbonne Université, Paris)

Wegner-Wilson Loops in String-Nets

Lattice gauge theories, when restricted to the pure gauge sector (i.e. no matter, only gauge fields), typically show a phase transition between a topologically-ordered deconfined phase at weak coupling and a trivial confined phase at strong coupling (cf. Wegner 1971, Wilson 1974, Fradkin and Shenker 1979). The diagnosis for such a topological phase transition is a non-local gauge-invariant order parameter known as a Wegner-Wilson loop (WWL) defined along a chosen contour. The WWL features a perimeter law exp(-#P) in the topological phase and an area law exp(-#A) in the confined phase, where P is the perimeter and A the area of the contour. The trivial phase is described as having confined charges (“quark confinement”) and condensed fluxes. Whereas, the topological phase has free (deconfined) charges and fluxes. Two-dimensional quantum lattice gauge theories are special in that the excitations in the deconfined phase are anyons (cf. toric code model, Kitaev 2003). In the toric code, WWL were studied in detail by Halasz and Hamma 2012.

Here we extend such a study of WWL from lattice gauge theories built on gauge groups to string-net models (Levin-Wen 2005) built on more general objects known as unitary modular tensor categories. We use these WWL to study the different kind of anyonic excitations that are believed to be described at low-energy by a topological quantum field theory of the doubled achiral type.

Ref: A. Ritz-Zwilling, J.-N. Fuchs and J. Vidal, arxiv:2011.12609,  Phys. Rev. B 103, 075128 (2021).

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IHES Covid-19 regulations:

- all the participants who will attend the event in person will have to keep their mask on in indoor spaces
and where the social distancing is not possible;
- speakers will be free to wear their mask or not at the moment of their talk if they feel more comfortable
without it;
- Up to 25 persons in the conference room, every participant will be asked to be able to provide a health pass
- Over 25 persons in the conference room, every participant will be asked to provide a health pass which will
be checked at the entrance of the conference room.

==================================================================

Pour être informé des prochains séminaires vous pouvez vous abonner à la liste de diffusion en écrivant un mail à sympa@listes.math.cnrs.fr avec comme sujet: "subscribe quantum_encounters_seminar PRENOM NOM"
(indiquez vos propres prénom et nom) et laissez le corps du message vide.

15th March, 2022 - 14:30 Piotr Tourkine (LAPTh Annecy)

On the Unitarity and Low Energy Expansion of the Coon Amplitude

The Coon amplitude is a deformation of the Veneziano amplitude with logarithmic Regge trajectories and an accumulation point in the spectrum, which interpolates between string theory and field theory. With string theory, it is the only other solution to duality constraints explicitly known and it constitutes an important data point in the modern S-matrix bootstrap. Yet, its basics properties are essentially unknown. In this paper we fill this gap and derive the conditions of positivity and the low energy expansion of the amplitude. On the positivity side, we discover that the amplitude switches from a regime where it is positive in all dimensions to a regime with critical dimensions, that connects to the known d = 26, 10 when the deformation is removed. En passant, we find that the Veneziano amplitudes can be extended to massive scalars of masses up to m^2 = 1/3, where it has critical dimension 6.3. On the low-energy side, we compute the first few couplings of the theory in terms of q-deformed analogues of the standard Riemann zeta values of the string expansion. We locate their location in the EFT-hedron, and find agreement with a recent conjecture that theories with accumulation points populate this space. We also discuss their relation to low spin dominance.

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IHES Covid-19 regulations:

- all the participants who will attend the event in person will have to keep their mask on in indoor spaces
and where the social distancing is not possible;
- speakers will be free to wear their mask or not at the moment of their talk if they feel more comfortable
without it;
- Up to 25 persons in the conference room, every participant will be asked to be able to provide a health pass
- Over 25 persons in the conference room, every participant will be asked to provide a health pass which will
be checked at the entrance of the conference room.

==================================================================

Pour être informé des prochains séminaires vous pouvez vous abonner à la liste de diffusion en écrivant un mail à sympa@listes.math.cnrs.fr avec comme sujet: "subscribe quantum_encounters_seminar PRENOM NOM"
(indiquez vos propres prénom et nom) et laissez le corps du message vide.

1st March, 2022 - 16:00 Yin-Chen He (Perimeter Institute, Waterloo)

Stiefel Liquids: Dirac Spin Liquid and Possible Non-Lagrangian CFTs in Quantum Magnets

I will talk about a new type of critical quantum liquids, dubbed Stiefel liquids, that can emerge in quantum magnets. Our theory is based on 2+1 dimensional nonlinear sigma models on target space SO(N)/SO(4), supplemented with Wess-Zumino-Witten terms. We argue that the Stiefel liquids form a class of 3d CFTs with extraordinary properties, such as large emergent symmetries, a cascade structure, and nontrivial quantum anomalies. We show that the well known deconfined quantum critical point and U(1) Dirac spin liquid (i.e. Nf=4 QED3) are unified as two special examples of Stiefel liquids, with N=5 and N=6, respectively. Furthermore, we conjecture that Stiefel liquids with N>6 are non-Lagrangian, in the sense that under renormalization group they flow to infrared (conformally invariant) fixed points that cannot be described by any renormalizable continuum Lagrangian. I will also discuss a physical way to realize Stiefel liquids (both the Dirac spin liquid and N=7 non-Lagrangian Stiefel liquid) in quantum spin systems, for example, on triangular or kagome lattice, through the intertwinement of symmetry breaking orders.

Participer à la réunion Zoom
https://us02web.zoom.us/j/82757702598?pwd=RnBJZWRZWFIvd3lGU0ZRMG9OUEVhQT09

ID de réunion : 827 5770 2598
Code secret : 498440

==================================================================

IHES Covid-19 regulations:

- all the participants who will attend the event in person will have to keep their mask on in indoor spaces
and where the social distancing is not possible;
- speakers will be free to wear their mask or not at the moment of their talk if they feel more comfortable
without it;
- Up to 70 persons in the conference room, every participant will be asked to be able to provide a health pass
- Over 70 persons in the conference room, every participant will be asked to provide a health pass which will
be checked at the entrance of the conference room.

==================================================================

Pour être informé des prochains séminaires vous pouvez vous abonner à la liste de diffusion en écrivant un mail à sympa@listes.math.cnrs.fr avec comme sujet: "subscribe quantum_encounters_seminar PRENOM NOM"
(indiquez vos propres prénom et nom) et laissez le corps du message vide.

30th November, 2021 - 16:00 Sebastian Mizera (IAS Princeton)

Landau Discriminants

Scattering amplitudes in quantum field theories have intricate analytic properties as functions of the energies and momenta of the scattered particles. In perturbation theory, their singularities are governed by a set of nonlinear polynomial equations, known as Landau equations, for each individual Feynman diagram. The singularity locus of the associated Feynman integral is made precise with the notion of the Landau discriminant, which characterizes when the Landau equations admit a solution. In order to compute this discriminant, we present approaches from classical elimination theory, as well as a numerical algorithm based on homotopy continuation. These methods allow us to compute Landau discriminants of various Feynman diagrams up to 3 loops, which were previously out of reach. For instance, the Landau discriminant of the envelope diagram is a reducible surface of degree 45 in the three-dimensional space of kinematic invariants. We investigate geometric properties of the Landau discriminant, such as irreducibility, dimension and degree.

Participer à la réunion Zoom
https://us02web.zoom.us/j/89109430156?pwd=NnRZNFNHMkFVOUJ5cC92bkJOeDNHQT09

ID de réunion : 891 0943 0156
Code secret : 583871

 

23rd November, 2021 - 16:00 Dominic Else (Harvard)

Emergent Symmetries and Anomalies in Metals: Luttinger's Theorem and Beyond

Metals are an interesting class of gapless quantum many-body systems. Many metals are described by the famous "Fermi liquid theory" at low temperatures, but there are also many metallic materials for which Fermi liquid theory is an inadequate description. In this talk, I will argue that a productive way to think about certain properties of metals, beyond Fermi liquid theory, is in the language of emergent symmetries and anomalies, thus importing ideas originally developed in the context of gapped topological phases of matter and their boundaries. From this point of view, I will show how to derive a vast generalization of Luttinger's theorem, the result that relates the volume enclosed by the Fermi surface of a Fermi liquid to the microscopic charge density. From this one can derive a number of consequences, including strong constraints on the emergent symmetry group of compressible metals. I also discuss implications for electrical resistivity.
 

Participer à la réunion Zoom
https://us02web.zoom.us/j/85119690238?pwd=MC96eVpLZTN6aXhCdHZpTVJYSzFTZz09

ID de réunion : 851 1969 0238
Code secret : 529628

 

16th November, 2021 - 16:00 Dalimil Mazáč (IAS Princeton)

Bootstrapping Automorphic Spectra

I will explain how the conformal bootstrap can be adapted to place rigorous bounds on the spectra of automorphic forms on locally symmetric spaces. A locally symmetric space is of the form H\G/K, where G is a non-compact semisimple Lie group, K the maximal compact subgroup of G, and H a discrete subgroup of G. If we take G = SL(2,R), then spaces of this form are precisely hyperbolic surfaces and hyperbolic 2-orbifolds. Automorphic forms then come in two types: modular forms, and eigenfunctions of the hyperbolic Laplacian, also known as Maass forms. The bootstrap constraints arise from the associativity of function multiplication on the space H\G, and are very similar to the usual correlator bootstrap equations, with G playing the role of the conformal group. For G=SL(2,R), I will use this method to prove upper bounds on the lowest positive eigenvalue of the Laplacian on all closed hyperbolic surfaces of a fixed genus. The bounds at genus 2 and genus 3 are very nearly saturated by the Bolza surface and the Klein quartic. This is based on upcoming work with P. Kravchuk and S. Pal.

Participer à la réunion Zoom
https://us02web.zoom.us/j/82068794423?pwd=TjZ3V0hhTFl5MEtCdEM4Lys1UHlEQT09

ID de réunion : 820 6879 4423
Code secret : 946750

Pour être informé des prochains séminaires vous pouvez vous abonner à la liste de diffusion en écrivant un mail à sympa@listes.math.cnrs.fr avec comme sujet: "subscribe quantum_encounters_seminar PRENOM NOM"
(indiquez vos propres prénom et nom) et laissez le corps du message vide.

9th November, 2021 - 16:00 Andrei B. Bernevig (Princeton University)

Exact Eigenstates in Non-Integrable Systems: A violation of the ETH

We find that several non-integrable systems exhibit some exact eigenstates that span the energy spectrum from lowest to the highest state. In the AKLT Hamiltonian and in several others “special” non-integrable models, we are able to obtain the analytic expression of states exactly and to compute their entanglement spectrum and entropy to show that they violate the eigenstate thermalization hypothesis. This represented the first example of ETH violation in a non-integrable system; these types  of states have gained notoriety since then as quantum Scars in the context of Rydberg atoms experiments. We furthermore show that the structure of these states, in most models where they are found is that of an almost spectrum generating algebra which we call Restricted Spectrum Generating Algebra. This includes the (extended) Hubbard model, as well as some thin-torus limits of Fractional Quantum Hall states. Yet in other examples, such as the recently found chiral non-linear luttinger liquid, their structure is more complicated and not understood.

Participer à la réunion Zoom
https://us02web.zoom.us/j/82078858054?pwd=KzhaZldTbFhJclUrK3YrYkI5VTJVdz09

ID de réunion : 820 7885 8054
Code secret : 560193

Pour être informé des prochains séminaires vous pouvez vous abonner à la liste de diffusion en écrivant un mail à sympa@listes.math.cnrs.fr avec comme sujet: "subscribe quantum_encounters_seminar PRENOM NOM"
(indiquez vos propres prénom et nom) et laissez le corps du message vide.

2nd November, 2021 - 17:00 Jeongwan Haah (Microsoft Research)

Topological Phases of Discrete Time Evolution

In the Heisenberg picture of quantum mechanics, time evolution is a one-parameter family of automorphisms of operator algebra. Restricted to short times the equivalence between time evolution and quantum circuits, especially the property that it maps a local operator to another, has been implanted in theoretical studies of topological phases of matter. In this talk, I will explain recent findings that not all locality-preserving automorphisms, also called quantum cellular automata, can be written as quantum circuits --- there exists a "discrete time dynamics" that cannot have a "Hamiltonian." These are tightly related to static, topological many-body states. I will give results on the classification of these automorphisms, and connect them to locally generated subalgebras in one lower dimension.

Participer à la réunion Zoom
https://us02web.zoom.us/j/83786264059?pwd=Nk81UDhlT3JLSDFkd29KY0NUMFYvZz09

ID de réunion : 837 8626 4059
Code secret : 051224

Pour être informé des prochains séminaires vous pouvez vous abonner à la liste de diffusion en écrivant un mail à sympa@listes.math.cnrs.fr avec comme sujet: "subscribe quantum_encounters_seminar PRENOM NOM"
(indiquez vos propres prénom et nom) et laissez le corps du message vide.

26th October, 2021 - 16:00 Ryan Thorngren (Harvard Univ.)

Higher Berry Phase and Diabolical Points in Phase Diagrams of Many-body Systems

The Berry phase is a well-known phenomenon in quantum mechanics with many profound implications. It describes the response of the phase of the wavefunction to the adiabatic evolution of system parameters, defining a U(1) connection on the parameter space. In many-body systems described by quantum field theory, we may also allow the parameters to vary in space, and we find a higher group connection generalizing the Berry phase. This connection also describes phenomena such as the Thouless pump and its generalizations. It allows us to constrain the global structure of phase diagrams by probing non-contractible cycles in the space of quantum field theories. In a typical phase diagram drawn in R^n, these cycles surround topologically-protected critical loci called diabolical points, in analogy to the quantum mechanical singularities which act as "monopoles" for the Berry connection. I will discuss these concepts in more detail, as well as a bulk-boundary correspondence and some recent applications to phase diagrams of topologically ordered systems. This talk is based on https://arxiv.org/abs/2004.10758 w/ Po-Shen Hsin and Anton Kapustin https://arxiv.org/abs/2110.07599 and its sequel, 2110.xxxx w/ Nathanan Tantivasadakarn, Ashvin Vishwanath, and Ruben Verresen.

Participer à la réunion Zoom
https://us02web.zoom.us/j/87445066109?pwd=U1Bjc211enZlOGFYd0l5REltRWVqQT09

ID de réunion : 874 4506 6109
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Pour être informé des prochains séminaires vous pouvez vous abonner à la liste de diffusion en écrivant un mail à sympa@listes.math.cnrs.fr avec comme sujet: "subscribe quantum_encounters_seminar PRENOM NOM"
(indiquez vos propres prénom et nom) et laissez le corps du message vide.

 

18th October, 2021 - 10:30 Adam Nahum (ENS Paris)

Measurement Phase Transitions and the Statistical Mechanics of Tree Tensor Networks

A many-body quantum system that is continually monitored by an external observer can be in two distinct dynamical phases, distinguished by whether or not repeated local measurements (throughout the bulk of the system) prevent the build-up of long-range quantum entanglement. I will describe the key features of such “measurement phase transitions” and sketch theoretical approaches to their critical properties that make connections with topics in classical statistical mechanics, such as percolation and disordered magnetism. Finally I will discuss random tensor networks with a tree geometry. These arise in a simple limit of the measurement problem, and they show an entanglement transition that can be solved exactly by a mapping to a problem of traveling waves.

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IHES Covid-19 regulations:

- all the participants who will attend the event in person will have to keep their mask on in indoor spaces
and where the social distancing is not possible;
- speakers will be free to wear their mask or not at the moment of their talk if they feel more comfortable
without it;
- Up to 25 persons in the conference room, every participant will be asked to be able to provide a health pass
- Over 25 persons in the conference room, every participant will be asked to provide a health pass which will
be checked at the entrance of the conference room.

==================================================================

Pour être informé des prochains séminaires vous pouvez vous abonner à la liste de diffusion en écrivant un mail à sympa@listes.math.cnrs.fr avec comme sujet: "subscribe quantum_encounters_seminar PRENOM NOM"
(indiquez vos propres prénom et nom) et laissez le corps du message vide.

5th October, 2021 - 10:00 Yoshiko Ogata (Univ. Tokyo)

Classification of Gapped Ground State Phases in Quantum Spin Systems

Recently, classification problems of gapped ground state phases attract a lot of attention in quantum statistical mechanics. We explain about operator algebraic approach to these problems.
 

Participer à la réunion Zoom
https://us02web.zoom.us/j/89796579935?pwd=Ukt3R1VxUFdORHZENHJyN0k3R3Radz09

ID de réunion : 897 9657 9935
Code secret : 459425

Pour être informé des prochains séminaires vous pouvez vous abonner à la liste de diffusion en écrivant un mail à sympa@listes.math.cnrs.fr avec comme sujet: "subscribe quantum_encounters_seminar PRENOM NOM"
(indiquez vos propres prénom et nom) et laissez le corps du message vide.

1st June, 2021 - 16:00 Peter C.W. Holdsworth (Ecole Normale Supérieure de Lyon)

Emergence: from spins to charges

Emergence is a major buzz word of our times. My working definition, which gives plenty of room for manoeuvre is: the appearance of many body phenomena of higher symmetry than that of the Hamiltonian and degrees of freedom at the microscopic level.

In this colloquium I will discuss a topical example - the frustrated magnetic material, spin ice. Here, to an excellent approximation a classical field theoretic description with continuous U(1) symmetry emerges from Ising like degrees of freedom. The associated quasi-particles appear to be freely moving magnetic charge - magnetic monopoles - and the system behaves as a magnetic Coulomb fluid in the grand canonical ensemble with intrinsic topological properties. With the addition of quantum fluctuations the emergent magneto-statics develops further into a complete analogue of quantum electrodynamics. I will aliment this discussion with experimental results from a wide range of systems.


https://us02web.zoom.us/j/85270888147?pwd=aXdOVXBTSmNEU00vVFE2bXhqdE5vdz09

ID de réunion : 852 7088 8147
Code secret : 276852

27th April, 2021 - 17:00 Anton Kapustin (Caltech)

Noncommutative Geometry of Quantum Lattice Models and the Higher Berry Phase

Recently methods of quantum statistical mechanics have been fruitfully applied to the study of phases of quantum lattice systems at zero temperature. For example, a rigorous definition of a Short-Range Entangled phase of matter has been given and a classification of such phases in one spatial dimension has been achieved. I will discuss some of these developments, focusing on the topology and geometry of the space of Short-Range Entangled states. According to a conjecture of A. Kitaev, these spaces form a loop spectrum in the sense of homotopy theory. This conjecture implies that to any family of Short-Range entangled states in one dimension one can associate a gerbe on the parameter space. I will show how to construct such a gerbe. Thе curvature of this gerbe is a closed 3-form with quantized periods and can be regarded as a higher-dimensional generalization of the curvature of the Berry connection.


https://us02web.zoom.us/j/81778962715?pwd=QnpNS2ErSnBCTWRYUHphd1VMMysyZz09

ID de réunion : 817 7896 2715
Code secret : 800452