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BEGIN:VEVENT
DTSTART;TZID=America/New_York:20230913T103000
DTEND;TZID=America/New_York:20230913T113000
DTSTAMP:20260430T160042
CREATED:20230904T061048Z
LAST-MODIFIED:20240223T113738Z
UID:10001122-1694601000-1694604600@cmsa.fas.harvard.edu
SUMMARY:Phase transitions out of quantum Hall states in moire TMD bilayers
DESCRIPTION:Topological Quantum Matter Seminar \nSpeaker: Xueyang Song (MIT) \nTitle: Phase transitions out of quantum Hall states in moire TMD bilayers \nAbstract: Motivated by the recent experimental breakthroughs in observing Fractional Quantum Anomalous Hall (FQAH) states in moir\’e Transition Metal Dichalcogenide (TMD) bilayers\, we propose and study various unconventional phase transitions between quantum Hall phases and Fermi liquids or charge ordered phases upon tuning the bandwidth.  At filling -2/3\, we describe a direct transition between the FQAH state and a Charge Density Wave (CDW) insulator. The critical theory resembles that of the familiar deconfined quantum critical point (DQCP) but with an additional Chern-Simons term. At filling -1/2\, we study the possibility of a continuous transition between the composite Fermi liquid (CFL) and the Fermi liquid (FL) building on and refining previous work by  Barkeshli and McGreevy.   Crucially we show that translation symmetry alone is enough to enable a second order CFL-FL transition. We argue that there must be critical CDW fluctuations though neither phase has long range CDW order.  A striking signature is a universal jump of resistivities at the critical point. With disorder\, we argue that the CDW order gets pinned and the CFL-FL evolution happens through an intermediate electrically insulating phase with mobile neutral fermions. A clean analog of this insulating phase with long range CDW order and a neutral fermi surface can potentially also exist.  We also present a critical theory for the CFL to FL transition at filling -3/4.  Our work opens up a new avenue to realize deconfined criticality and fractionalized phases beyond familiar Landau level physics in the moire Chern band system.
URL:https://cmsa.fas.harvard.edu/event/tqms_91323/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Topological Quantum Matter Seminar
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Topological-Seminar-09.12.23.docx-2.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20230912T160000
DTEND;TZID=America/New_York:20230912T170000
DTSTAMP:20260430T160042
CREATED:20240223T104300Z
LAST-MODIFIED:20240223T104300Z
UID:10002849-1694534400-1694538000@cmsa.fas.harvard.edu
SUMMARY:Homotopy classes of loops of Clifford unitaries
DESCRIPTION:Topological Quantum Matter Seminar \nSpeaker: Roman Geiko\, UCLA \nTitle: Homotopy classes of loops of Clifford unitaries \nAbstract: We study Clifford locality-preserving unitaries and stabilizer Hamiltonians by means of Hermitian K-theory. We demonstrate how the notion of algebraic homotopy of modules over Laurent polynomial rings translates into the connectedness of two short-range entangled stabilizer Hamiltonians by a shallow Clifford circuit. We apply this observation to a classification of homotopy classes of loops of Clifford unitaries. The talk is based on a work in collaboration with Yichen Hu.  https://arxiv.org/abs/2306.09903.
URL:https://cmsa.fas.harvard.edu/event/tqms_91223/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Topological Quantum Matter Seminar
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Topological-Seminar-09.12.23-1.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20230503T123000
DTEND;TZID=America/New_York:20230503T133000
DTSTAMP:20260430T160042
CREATED:20230817T183740Z
LAST-MODIFIED:20240216T085646Z
UID:10001284-1683117000-1683120600@cmsa.fas.harvard.edu
SUMMARY:Generative Adversarial Networks (GANs): An Analytical Perspective
DESCRIPTION:Speaker: Xin Guo\, UC Berkeley \nTitle: Generative Adversarial Networks (GANs): An Analytical Perspective \nAbstract: Generative models have attracted intense interests recently. In this talk\, I will discuss one class of generative models\, Generative Adversarial Networks (GANs).  I will first provide a gentle review of the mathematical framework behind GANs. I will then proceed to discuss a few challenges in GANs training from an analytical perspective. I will finally report some recent progress for GANs training in terms of its stability and convergence analysis. \n 
URL:https://cmsa.fas.harvard.edu/event/collquium-5323/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Colloquium
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Colloquium-05.03.2023.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20230426T123000
DTEND;TZID=America/New_York:20230426T133000
DTSTAMP:20260430T160042
CREATED:20230817T183259Z
LAST-MODIFIED:20240122T053311Z
UID:10001283-1682512200-1682515800@cmsa.fas.harvard.edu
SUMMARY:Boundary behavior at classical and quantum phase transitions
DESCRIPTION:Speaker: Max Metlitski (MIT) \nTitle: Boundary behavior at classical and quantum phase transitions \nAbstract: There has been a lot of recent interest in the boundary behavior of materials. This interest is driven in part by the field of topological states of quantum matter\, where exotic protected boundary states are ubiquitous. In this talk\, I’ll ask: what happens at a boundary of a system\, when the bulk goes through a phase transition. While this question was studied in the context of classical statistical mechanics in the 70s and 80s\, basic aspects of the boundary phase diagram for the simplest classical phase transitions have been missed until recently. I’ll describe progress in this field\, as well as some extensions to quantum phase transitions. \n 
URL:https://cmsa.fas.harvard.edu/event/collquium-42623/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Colloquium
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Colloquium-04.26.2023.rev2_.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20230420T133000
DTEND;TZID=America/New_York:20230420T143000
DTSTAMP:20260430T160042
CREATED:20230817T182708Z
LAST-MODIFIED:20240216T085423Z
UID:10001282-1681997400-1682001000@cmsa.fas.harvard.edu
SUMMARY:Black hole collider physics
DESCRIPTION:Speaker: Julio Parra Martinez\, Caltech \nTitle: Black hole collider physics \nAbstract: Despite more than a century since the development of Einstein’s theory\, the general relativistic two-body problem remains unsolved. A precise description of its solution is now essential\, as it is necessary for understanding the strong-gravity dynamics of compact binaries observed at LIGO/VIRGO/KAGRA and in future gravitational wave observatories. In this talk\, I will describe how considering the scattering of black holes and gravitons can shed new light on this problem. I will explain how using modern ideas from collider and particle physics we can calculate scattering observables in classical gravity\, and extract the basic ingredients that describe the bound binary dynamics. Such calculations have produced state-of-art predictions for current and future gravitational wave observatories\, which open the door for further discovery as we enter this new era of precision gravitational physics.
URL:https://cmsa.fas.harvard.edu/event/collquium-42023/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Colloquium
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Colloquium-04.20.2023.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20230403T110000
DTEND;TZID=America/New_York:20230403T120000
DTSTAMP:20260430T160042
CREATED:20230817T181822Z
LAST-MODIFIED:20240122T052041Z
UID:10001280-1680519600-1680523200@cmsa.fas.harvard.edu
SUMMARY:Black hole microstate counting from the gravitational path integral
DESCRIPTION:Colloquium \nSpeaker: Luca Iliesiu\, Stanford \nTitle: Black hole microstate counting from the gravitational path integral \nAbstract: Reproducing the integer count of black hole micro-states from the gravitational path integral is an important problem in quantum gravity. In the first part of the talk\, I will show that\, by using supersymmetric localization\, the gravitational path integral for 1/16-BPS black holes in supergravity can reproduce the index obtained in the string theory construction of such black holes. A more refined argument then shows that not only the black hole index but also the total number of black hole microstates within an energy window above extremality that is polynomially suppressed in the charges also matches this string theory index. In the second part of the talk\, I will present a second perspective on this state count and show how the BPS Hilbert space can be obtained by directly preparing states using the gravitational path integral. While such a preparation naively gives rise to a Hilbert space of BPS states whose dimension is much larger than expected\, I will explain how non-perturbative corrections in the overlap of such states are again responsible for reproducing the correct dimension of the Hilbert space.
URL:https://cmsa.fas.harvard.edu/event/colloquium-4323/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Colloquium
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Colloquium-04.03.2023.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20230329T123000
DTEND;TZID=America/New_York:20230329T133000
DTSTAMP:20260430T160042
CREATED:20230817T181436Z
LAST-MODIFIED:20240216T102824Z
UID:10001279-1680093000-1680096600@cmsa.fas.harvard.edu
SUMMARY:Scattering amplitudes in quantum field theory
DESCRIPTION:Speaker: Ruth Britto (Trinity College Dublin) \nTitle: Scattering amplitudes in quantum field theory \nAbstract: Particle collider experiments require a detailed description of scattering events\, traditionally computed through sums of Feynman diagrams. However\, it is not practical to evaluate Feynman diagrams directly for all significant scattering processes. Moreover\, adding all diagrams reveals many cancellations: scattering amplitudes in theories such as QCD take remarkably simple forms. This simplicity is a clue that the perturbative theory is perhaps best understood without reference to Feynman diagrams. In fact\, it has recently become possible to explain some of this simplicity. I will show how to derive many amplitudes efficiently and elegantly\, and propose taming the remaining complexity with ideas drawn from combinatorics and geometry.
URL:https://cmsa.fas.harvard.edu/event/collquium-32923/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Colloquium
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Colloquium-03.29.2023.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20230322T123000
DTEND;TZID=America/New_York:20230322T133000
DTSTAMP:20260430T160042
CREATED:20230817T181152Z
LAST-MODIFIED:20240216T104444Z
UID:10001278-1679488200-1679491800@cmsa.fas.harvard.edu
SUMMARY:Synchronization in a Kuramoto Mean Field Game
DESCRIPTION:Speaker: Mete Soner (Princeton University) \nTitle: Synchronization in a Kuramoto Mean Field Game \nAbstract:  Originally motivated by systems of chemical and biological oscillators\, the classical Kuramoto model has found an amazing range of applications from neuroscience to Josephson junctions in superconductors\, and has become a  key mathematical model to describe self organization in complex systems. These autonomous oscillators are coupled through a nonlinear interaction term which plays a central role in the long term behavior of the system. While the system is not synchronized when this term is not sufficiently strong\, fascinatingly\, they exhibit an abrupt transition to a full synchronization above a critical value of the interaction parameter.  We explore this system in the mean field formalism.  We treat the system of oscillators as an infinite particle system\, but instead of positing the dynamics of the particles\, we let the individual particles determine endogenously their behaviors by minimizing a cost functional and eventually\, settling in a Nash equilibrium.  The mean field game also exhibits a bifurcation from incoherence to self-organization.  This approach has found interesting applications including circadian rhythms and jet-lag recovery.  This is joint work with Rene Carmona of Princeton and Quentin Cormier of INRIA\, Paris.
URL:https://cmsa.fas.harvard.edu/event/collquium-32223/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Colloquium
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Colloquium-03.22.2023.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20230308T123000
DTEND;TZID=America/New_York:20230308T133000
DTSTAMP:20260430T160042
CREATED:20230817T180824Z
LAST-MODIFIED:20240228T111406Z
UID:10001277-1678278600-1678282200@cmsa.fas.harvard.edu
SUMMARY:Conformal symmetry\, Optimization algorithms and the Critical Phenomena
DESCRIPTION:Speaker: Ning Su\, University of Pisa \nTitle: Conformal symmetry\, Optimization algorithms and the Critical Phenomena \nAbstract: In the phase diagram of many substances\, the critical points have emergent conformal symmetry and are described by conformal field theories. Traditionally\, physical quantities near the critical point can be computed by perturbative field theory method\, where conformal symmetry is not fully utilized. In this talk\, I will explain how conformal symmetry can be used to determine certain physical quantities\, without even knowing the fine details of the microscopic structure. To compute the observables precisely\, one needs to develop powerful numerical techniques. In the last few years\, we have invented many computational tools and algorithms\, and predicted critical exponents of Helium-4 superfluid phase transition and Heisenberg magnet to very high precision.
URL:https://cmsa.fas.harvard.edu/event/collquium-3823/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Colloquium
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/02CMSA-Colloquium-03.08.2023.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20230302T160000
DTEND;TZID=America/New_York:20230302T170000
DTSTAMP:20260430T160042
CREATED:20230817T180503Z
LAST-MODIFIED:20240216T085143Z
UID:10001276-1677772800-1677776400@cmsa.fas.harvard.edu
SUMMARY:The string/black hole transition in anti de Sitter space
DESCRIPTION:Speaker: Erez Urbach\, Weizmann Institute of Science \nTitle: The string/black hole transition in anti de Sitter space \nAbstract: String stars\, or Horowitz-Polchinski solutions\, are string theory saddles with normalizable condensates of thermal-winding strings. In the past\, string stars were offered as a possible description of stringy (Euclidean) black holes in asymptotically flat spacetime\, close to the Hagedorn temperature. I will discuss the thermodynamic properties of string stars in asymptotically (thermal) anti-de Sitter background (including AdS3 with NS-NS flux)\, their possible connection to small black holes in AdS\, and their implications for holography. I will also present new “winding-string gas” saddles for confining holographic backgrounds such as the Witten model\, and their relation to the deconfined phase of 3+1 pure Yang-Mills.
URL:https://cmsa.fas.harvard.edu/event/collquium-3223/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Colloquium
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/02CMSA-Colloquium-03.02.2023.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20230222T123000
DTEND;TZID=America/New_York:20230222T133000
DTSTAMP:20260430T160042
CREATED:20230817T180053Z
LAST-MODIFIED:20240215T111058Z
UID:10001275-1677069000-1677072600@cmsa.fas.harvard.edu
SUMMARY:The Black Hole Information Paradox: A Resolution on the Horizon?
DESCRIPTION:Speaker: Netta Engelhardt (MIT) \nTitle: The Black Hole Information Paradox: A Resolution on the Horizon? \nAbstract: The black hole information paradox — whether information escapes an evaporating black hole or not — remains one of the most longstanding mysteries of theoretical physics. The apparent conflict between validity of semiclassical gravity at low energies and unitarity of quantum mechanics has long been expected to find its resolution in a complete quantum theory of gravity. Recent developments in the holographic dictionary\, and in particular its application to entanglement and complexity\, however\, have shown that a semiclassical analysis of gravitational physics can reproduce a hallmark feature of unitary evolution. I will describe this recent progress and discuss some promising indications of a full resolution of the information paradox.
URL:https://cmsa.fas.harvard.edu/event/collquium-22223/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Colloquium
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/02CMSA-Colloquium-02.22.2023.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20230213T123000
DTEND;TZID=America/New_York:20230213T133000
DTSTAMP:20260430T160042
CREATED:20230817T175704Z
LAST-MODIFIED:20240222T165748Z
UID:10001274-1676291400-1676295000@cmsa.fas.harvard.edu
SUMMARY:Complete Calabi-Yau metrics: Recent progress and open problems
DESCRIPTION:Speaker: Tristan Collins\, MIT \nTitle: Complete Calabi-Yau metrics: Recent progress and open problems \nAbstract: Complete Calabi-Yau metrics are fundamental objects in Kahler geometry arising as singularity models or “bubbles” in degenerations of compact Calabi-Yau manifolds.  The existence of these metrics and their relationship with algebraic geometry are the subjects of several long standing conjectures due to Yau and Tian-Yau. I will describe some recent progress towards the question of existence\, and explain some future directions\, highlighting connections with notions of algebro-geometric stability.
URL:https://cmsa.fas.harvard.edu/event/collquium-21323/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Colloquium
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/02CMSA-Colloquium-02.13.2023-.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20230208T123000
DTEND;TZID=America/New_York:20230208T133000
DTSTAMP:20260430T160042
CREATED:20230817T175326Z
LAST-MODIFIED:20240214T112702Z
UID:10001273-1675859400-1675863000@cmsa.fas.harvard.edu
SUMMARY:From spin glasses to Boolean circuits lower bounds - Algorithmic barriers from the overlap gap property
DESCRIPTION:Speaker: David Gamarnik (MIT) \nTitle: From spin glasses to Boolean circuits lower bounds. Algorithmic barriers from the overlap gap property \nAbstract: Many decision and optimization problems over random structures exhibit an apparent gap between the existentially optimal values and algorithmically achievable values. Examples include the problem of finding a largest independent set in a random graph\, the problem of finding a near ground state in a spin glass model\, the problem of finding a satisfying assignment in a random constraint satisfaction problem\, and many many more. Unfortunately\, at the same time no formal computational hardness results exist which  explains this persistent algorithmic gap. \nIn the talk we will describe a new approach for establishing an algorithmic intractability for these problems called the overlap gap property. Originating in statistical physics theory of spin glasses\, this is a simple to describe property which a) emerges in most models known to exhibit an apparent algorithmic hardness; b) is consistent with the hardness/tractability phase transition for many models analyzed to the day; and\, importantly\, c) allows to mathematically rigorously rule out a large class of algorithms as potential contenders\, specifically the algorithms which exhibit a form of stability/noise insensitivity. \nWe will specifically show how to use this property to obtain stronger (stretched exponential) than the state of the art (quasi-polynomial) lower bounds on the size of constant depth Boolean circuits for solving the two of the aforementioned problems: the problem of finding a large independent set in a sparse random graph\, and the problem of finding a near ground state of a p-spin model. \nJoint work with Aukosh Jagannath and Alex Wein
URL:https://cmsa.fas.harvard.edu/event/collquium-2823/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Colloquium
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/02CMSA-Colloquium-02.08.2023.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20221207T100000
DTEND;TZID=America/New_York:20221207T110000
DTSTAMP:20260430T160042
CREATED:20230705T075744Z
LAST-MODIFIED:20240216T091837Z
UID:10001134-1670407200-1670410800@cmsa.fas.harvard.edu
SUMMARY:Controlling Quantum Matter with Quantum Cavity Fields
DESCRIPTION:Topological Quantum Matter Seminar \nSpeaker: Vasil Rokaj (Harvard) \nTitle: Controlling Quantum Matter with Quantum Cavity Fields \nAbstract: Cavity modification of material properties and phenomena is a novel research field motivated by the advances in strong light-matter interactions [1]. For condensed matter systems it has been demonstrated experimentally that the transport properties of 2D materials can be modified via coupling to vacuum fields [2\,3]. While in polaritonic chemistry it has been shown that ground state chemical properties can be controlled with cavity fields [4]. In the first part of my talk\, I will present how the quantized cavity field can alter the conduction properties of a condensed matter system by focusing on the paradigmatic Sommerfeld model of the free electron gas [5]. The exact analytic solution of the Sommerfeld model in the cavity will be presented as well as its fundamental properties. Then\, in the second part of the talk\, I will focus on a many-particle system of cold ions in a harmonic trap coupled to the cavity field. I will show how this system couples collectively to the cavity and that hybrid states between light and matter\, known as polaritons\, emerge. The formation of polaritons leads to the modification of the properties of the cold ions and enhances the localization of the many-body wave function [6]. Connections to experiments will be discussed as well. \n[1] F. Garcia-Vidal\, C. Ciuti\, T. W. Ebbesen\, Science\, 373\, 178 (2021) \n[2] G. L. Paravicini-Bagliani et al.\, Nat. Phys. 15\, 186-190 (2019) \n[3] F. Appugliese et al.\, Science 375 (6584)\, 1030-1034 (2022) \n[4] T. W. Ebbesen\, Acc. Chem. Res. 49\, 11\, 2403–2412 (2016) \n[5] V. Rokaj\, M. Ruggenthaler\, F. G. Eich\, A. Rubio\, Phys. Rev. Research 4\, 013012 (2022) \n[6] V. Rokaj\, S.I. Mistakidis\, H.R. Sadeghpour\, arXiv:2207.03436 (2022)
URL:https://cmsa.fas.harvard.edu/event/tqms_12722/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Topological Quantum Matter Seminar
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Topological-Seminar-12.07.22.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20221123T090000
DTEND;TZID=America/New_York:20221123T100000
DTSTAMP:20260430T160042
CREATED:20230705T075447Z
LAST-MODIFIED:20240216T092538Z
UID:10001135-1669194000-1669197600@cmsa.fas.harvard.edu
SUMMARY:Continuum field theory of graphene bilayer system
DESCRIPTION:Topological Quantum Matter Seminar \nSpeaker: Jian Kang\, School of Physical Science and Technology\, ShanghaiTech University\, Shanghai\, China \nTitle: Continuum field theory of graphene bilayer system \nAbstract: The Bistritzer-MacDonald (BM) model predicted the existence of the narrow bands in the magic-angle twisted bilayer graphene (MATBG)\, and nowadays is a starting point for most theoretical works. In this talk\, I will briefly review the BM model and then present a continuum field theory [1] for graphene bilayer system allowing any smooth lattice deformation including the small twist angle. With the gradient expansion to the second order\, the continuum theory for MATBG [2] produces the spectrum that almost perfectly matches the spectrum of the microscopic model\, suggesting the validity of this theory. In the presence of the lattice deformation\, the inclusion of the pseudo-vector potential does not destroy but shift the flat band chiral limit to a smaller twist angle. Furthermore\, the continuum theory contains another important interlayer tunneling term that was overlooked in all previous works. This term non-negligibly breaks the particle-hole symmetry of the narrow bands and may be related with the experimentally observed particle-hole asymmetry. \n1. O. Vafek and JK\, arXiv: 2208.05933.\n2. JK and O. Vafek\, arXiv: 2208.05953. \n 
URL:https://cmsa.fas.harvard.edu/event/tqms_112322/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Topological Quantum Matter Seminar
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Topological-Seminar-11.23.22.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20221116T123000
DTEND;TZID=America/New_York:20221116T133000
DTSTAMP:20260430T160042
CREATED:20230817T174642Z
LAST-MODIFIED:20240214T112838Z
UID:10001271-1668601800-1668605400@cmsa.fas.harvard.edu
SUMMARY:Noether’s Learning Dynamics: Role of Symmetry Breaking in Neural Networks
DESCRIPTION:Colloquium \nSpeaker: Hidenori Tanaka (NTT Research at Harvard) \nTitle: Noether’s Learning Dynamics: Role of Symmetry Breaking in Neural Networks \nAbstract: In nature\, symmetry governs regularities\, while symmetry breaking brings texture. In artificial neural networks\, symmetry has been a central design principle\, but the role of symmetry breaking is not well understood. Here\, we develop a Lagrangian formulation to study the geometry of learning dynamics in neural networks and reveal a key mechanism of explicit symmetry breaking behind the efficiency and stability of modern neural networks. Then\, we generalize Noether’s theorem known in physics to describe a unique symmetry breaking mechanism in learning and derive the resulting motion of the Noether charge: Noether’s Learning Dynamics (NLD). Finally\, we apply NLD to neural networks with normalization layers and discuss practical insights. Overall\, through the lens of Lagrangian mechanics\, we have established a theoretical foundation to discover geometric design principles for the learning dynamics of neural networks.
URL:https://cmsa.fas.harvard.edu/event/collquium-111622/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Colloquium
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Colloquium-11.16.22-2.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20221116T100000
DTEND;TZID=America/New_York:20221116T113000
DTSTAMP:20260430T160042
CREATED:20230705T075111Z
LAST-MODIFIED:20240216T092731Z
UID:10001136-1668592800-1668598200@cmsa.fas.harvard.edu
SUMMARY:Vacuum fluctuations in cavities: breakdown of the topological protection in the integer Quantum Hall effect
DESCRIPTION:Topological Quantum Matter Seminar \nSpeaker: Jérôme Faist  (ETH Zurich) \nTitle: Vacuum fluctuations in cavities: breakdown of the topological protection in the integer Quantum Hall effect \nAbstract: When a collection of electronic excitations are strongly coupled to a single mode cavity\, mixed light-matter excitations called polaritons are created. The situation is especially interesting when the strength of the light-matter coupling ΩR is such that the coupling energy becomes close to the one of the bare matter resonance ω0. For this value of parameters\, the system enters the so-called ultra-strong coupling regime\, in which a number of very interesting physical effects were predicted caused by the counter-rotating and diamagnetic terms of the Hamiltonian. \nIn a microcavity\, the strength of the electric field caused by the vacuum fluctuations\, to which the strength of the light-matter coupling ΩR is proportional\, scales inversely with the cavity volume. One very interesting feature of the circuit-based metamaterials is the fact that this volume can be scaled down to deep subwavelength values in all three dimension of space.1 Using metamaterial coupled to two-dimensional electron gases under a strong applied magnetic field\, we have now explored to which extend this volume can be scaled down and reached a regime where the stability of the polariton is limited by diffraction into a continuum of plasmon modes2. \nWe have also used transport to probe the ultra-strong light-matter coupling3\, and show now that the latter can induce a breakdown of the integer quantum Hall effect4. The phenomenon is explained in terms of cavity-assisted hopping\, an anti-resonant process where an electron can scatter from one edge of the sample to the other by “borrowing” a photon from the cavity5. We are also evaluating a proposal suggesting that the value of the quantization voltage can be renormalized by the cavity6. \n  \n\nScalari\, G. et al. Ultrastrong Coupling of the Cyclotron Transition of a 2D Electron Gas to a THz Metamaterial. Science 335\, 1323–1326 (2012).\nRajabali\, S. et al. Polaritonic Nonlocality in Light Matter Interaction. Nat Photon 15\, 690–695 (2021).\nParavicini-Bagliani\, G. L. et al. Magneto-Transport Controlled by Landau Polariton States. Nat. Phys. 15\, 186–190 (2019).\nAppugliese\, F. et al. Breakdown of topological protection by cavity vacuum fields in the integer quantum Hall effect. Science 375\, 1030–1034 (2022).\nCiuti\, C. Cavity-mediated electron hopping in disordered quantum Hall systems. Phys. Rev. B 104\, 155307 (2021).\nRokaj\, V.\, Penz\, M.\, Sentef\, M. A.\, Ruggenthaler\, M. & Rubio\, A. Polaritonic Hofstadter butterfly and cavity control of the quantized Hall conductance. Phys. Rev. B 105\, 205424 (2022).\n\n 
URL:https://cmsa.fas.harvard.edu/event/tqms_111622/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Topological Quantum Matter Seminar
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Topological-Seminar-11.16.22.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20221102T124500
DTEND;TZID=America/New_York:20221102T134500
DTSTAMP:20260430T160042
CREATED:20230817T174336Z
LAST-MODIFIED:20240121T174258Z
UID:10001270-1667393100-1667396700@cmsa.fas.harvard.edu
SUMMARY:Doping and inverting Mott insulators on semiconductor moire superlattices
DESCRIPTION:Speaker: Liang Fu (MIT) \n\n\nTitle: Doping and inverting Mott insulators on semiconductor moire superlattices \nAbstract: Semiconductor bilayer heterostructures provide a remarkable platform for simulating Hubbard models on an emergent lattice defined by moire potential minima. As a hallmark of Hubbard model physics\, the Mott insulator state with local magnetic moments has been observed at half filling of moire band. In this talk\, I will describe new phases of matter that grow out of the canonical 120-degree antiferromagnetic Mott insulator on the triangular lattice. First\, in an intermediate range of magnetic fields\, doping this Mott insulator gives rise to a dilute gas of spin polarons\, which form a pseudogap metal. Second\, the application of an electric field between the two layers can invert the many-body gap of a charge-transfer Mott insulator\, resulting in a continuous phase transition to a quantum anomalous Hall insulator with a chiral spin structure. Experimental results will be discussed and compared with theoretical predictions.
URL:https://cmsa.fas.harvard.edu/event/collquium-11222/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Colloquium
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Colloquium-11.02.22.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20221102T090000
DTEND;TZID=America/New_York:20221102T100000
DTSTAMP:20260430T160042
CREATED:20230705T074816Z
LAST-MODIFIED:20240215T092523Z
UID:10001137-1667379600-1667383200@cmsa.fas.harvard.edu
SUMMARY:Optical axion electrodynamics
DESCRIPTION:Topological Quantum Matter Seminar \nSpeaker: Junyeong Ahn (Harvard) \nTitle: Optical axion electrodynamics \nAbstract: Electromagnetic fields in a magneto-electric medium behave in close analogy to photons coupled to the hypothetical elementary particle\, the axion. This emergent axion electrodynamics is expected to provide novel ways to detect and control material properties with electromagnetic fields. Despite having been studied intensively for over a decade\, its theoretical understanding remains mostly confined to the static limit. Formulating axion electrodynamics at general optical frequencies requires resolving the difficulty of calculating optical magneto-electric coupling in periodic systems and demands a proper generalization of the axion field. In this talk\, I will introduce a theory of optical axion electrodynamics that allows for a simple quantitative analysis. Then\, I will move on to discuss the issue of the Kerr effect in axion antiferromagnets\, refuting the conventional wisdom that the Kerr effect is a measure of the net magnetic moment. Finally\, I will apply our theory to a topological antiferromagnet MnBi2Te4. \nReferences:\n[1] Theory of Optical Axion Electrodynamics\, J. Ahn\, S.Y. Xu\, A.Vishwanath\, arXiv:2205.06843
URL:https://cmsa.fas.harvard.edu/event/tqms_1122/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Topological Quantum Matter Seminar
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Topological-Seminar-11.2.22.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20221026T123000
DTEND;TZID=America/New_York:20221026T133000
DTSTAMP:20260430T160042
CREATED:20230817T174027Z
LAST-MODIFIED:20240121T174027Z
UID:10001269-1666787400-1666791000@cmsa.fas.harvard.edu
SUMMARY:Clique listing algorithms
DESCRIPTION:Speaker: Virginia Vassilevska Williams (MIT) \nTitle: Clique listing algorithms \nAbstract: A k-clique in a graph G is a subgraph of G on k vertices in which every pair of vertices is linked by an edge. Cliques are a natural notion of social network cohesiveness with a long history. \nA fundamental question\, with many applications\, is “How fast can one list all k-cliques in a given graph?”. \nEven just detecting whether an n-vertex graph contains a k-Clique has long been known to be NP-complete when k can depend on n (and hence no efficient algorithm is likely to exist for it). If k is a small constant\, such as 3 or 4 (independent of n)\, even the brute-force algorithm runs in polynomial time\, O(n^k)\, and can list all k-cliques in the graph; though O(n^k) time is far from practical. As the number of k-cliques in an n-vertex graph can be Omega(n^k)\, the brute-force algorithm is in some sense optimal\, but only if there are Omega(n^k) k-cliques. In this talk we will show how to list k-cliques faster when the input graph has few k-cliques\, with running times depending on the number of vertices n\, the number of edges m\, the number of k-cliques T and more. We will focus on the case when k=3\, but we will note some extensions. \n(Based on joint work with Andreas Bjorklund\, Rasmus Pagh\, Uri Zwick\, Mina Dalirrooyfard\, Surya Mathialagan and Yinzhan Xu)
URL:https://cmsa.fas.harvard.edu/event/collquium_102722/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Colloquium
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Colloquium-10.26.22.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20221019T160000
DTEND;TZID=America/New_York:20221019T173000
DTSTAMP:20260430T160042
CREATED:20230705T073706Z
LAST-MODIFIED:20240229T103702Z
UID:10001139-1666195200-1666200600@cmsa.fas.harvard.edu
SUMMARY:Symmetric Mass Generation
DESCRIPTION:Topological Quantum Matter Seminar \nSpeaker: Yizhuang You\, UC San Diego \nTitle: Symmetric Mass Generation\n\nAbstract: Symmetric mass generation (SMG) is a novel mechanism for massless fermions to acquire a mass via a strong-coupling non-perturbative interaction effect. In contrast to the conventional Higgs mechanism for fermion mass generation\, the SMG mechanism does not condense any fermion bilinear coupling and preserves the full symmetry. It is connected to a broad range of topics\, including anomaly cancellation\, topological phase classification\, and chiral fermion regularization. In this talk\, I will introduce SMG through toy models\, and review the current understanding of the SMG transition. I will also mention recent numerical efforts to investigate the SMG phenomenon. I will conclude the talk with remarks on future directions.
URL:https://cmsa.fas.harvard.edu/event/tqms_101922/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Topological Quantum Matter Seminar
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Topological-Seminar-10.19.22.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20221019T123000
DTEND;TZID=America/New_York:20221019T133000
DTSTAMP:20260430T160042
CREATED:20230817T173735Z
LAST-MODIFIED:20240214T113414Z
UID:10001268-1666182600-1666186200@cmsa.fas.harvard.edu
SUMMARY:The Mobility Edge of Lévy Matrices
DESCRIPTION:Colloquium \nSpeaker: Patrick Lopatto (Brown) \nTitle: The Mobility Edge of Lévy Matrices \nAbstract: Lévy matrices are symmetric random matrices whose entry distributions lie in the domain of attraction of an alpha-stable law; such distributions have infinite variance when alpha is less than 2. Due to the ubiquity of heavy-tailed randomness\, these models have been broadly applied in physics\, finance\, and statistics. When the entries have infinite mean\, Lévy matrices are predicted to exhibit a phase transition separating a region of delocalized eigenvectors from one with localized eigenvectors. We will discuss the physical context for this conjecture\, and describe a result establishing it for values of alpha close to zero and one. This is joint work with Amol Aggarwal and Charles Bordenave.
URL:https://cmsa.fas.harvard.edu/event/collquium-101922/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Colloquium
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Colloquium-10.19.22.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20221012T123000
DTEND;TZID=America/New_York:20221012T133000
DTSTAMP:20260430T160042
CREATED:20230817T173346Z
LAST-MODIFIED:20240222T165414Z
UID:10001267-1665577800-1665581400@cmsa.fas.harvard.edu
SUMMARY:Complete disorder is impossible: Some topics in Ramsey theory
DESCRIPTION:Colloquium \nSpeaker: James Cummings\,Carnegie Mellon University \nTitle: Complete disorder is impossible: Some topics in Ramsey theory \nAbstract: The classical infinite Ramsey theorem states that if we colour pairs of natural numbers using two colours\, there is an infinite set all of whose pairs get the same colour. This is the beginning of a rich theory\, which touches on many areas of mathematics including graph theory\, set theory and dynamics. I will give an overview of Ramsey theory\, emphasizing the diverse ideas which are at play in this area.
URL:https://cmsa.fas.harvard.edu/event/collquium-101222/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Colloquium
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Colloquium-10.12.22-1.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20221012T090000
DTEND;TZID=America/New_York:20221012T100000
DTSTAMP:20260430T160042
CREATED:20230705T072724Z
LAST-MODIFIED:20240229T103813Z
UID:10001140-1665565200-1665568800@cmsa.fas.harvard.edu
SUMMARY:Engineering topological phases with a superlattice potential
DESCRIPTION:Topological Quantum Matter Seminar \nSpeaker: Jennifer Cano (Stony Brook and Flatiron Institute) \nTitle: Engineering topological phases with a superlattice potential\n\nAbstract: We propose an externally imposed superlattice potential as a platform for manipulating topological phases\, which has both advantages and disadvantages compared to a moire superlattice. In the first example\, we apply the superlattice potential to the 2D surface of a 3D topological insulator. The superlattice potential creates tunable van Hove singularities\, which\, when combined with strong spin-orbit coupling and Coulomb repulsion give rise to a topological meron lattice spin texture. Thus\, the superlattice potential provides a new route to the long sought-after goal of realizing spontaneous magnetic order on the surface of a 3D TI. In the second example\, we show that a superlattice potential applied to Bernal-stacked bilayer graphene can generate flat Chern bands\, similar to in twisted bilayer graphene\, whose bandwidth can be as small as a few meV. The superlattice potential offers flexibility in both lattice size and geometry\, making it a promising alternative to achieve designer flat bands without a moire heterostructure.
URL:https://cmsa.fas.harvard.edu/event/tqms_101222/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Topological Quantum Matter Seminar
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Topological-Seminar-10.12.22.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20221005T160000
DTEND;TZID=America/New_York:20221005T170000
DTSTAMP:20260430T160042
CREATED:20230817T173038Z
LAST-MODIFIED:20240229T110447Z
UID:10001266-1664985600-1664989200@cmsa.fas.harvard.edu
SUMMARY:Quantum statistical mechanics of charged black holes and strange metals
DESCRIPTION:Colloquium \nPlease note this colloquium will be held at a special time:  4:00-5:00 pm. \nSpeaker: Subir Sachdev (Harvard) \nTitle: Quantum statistical mechanics of charged black holes and strange metals\n\nAbstract: The Sachdev-Ye-Kitaev model was introduced as a toy model of interacting fermions without any particle-like excitations. I will describe how this toy model yields the universal low energy quantum theory of generic charged black holes in asymptotically 3+1 dimensional Minkowski space. I will also discuss how extensions of the SYK model yield a realistic theory of the strange metal phase of correlated electron systems.\n\n\nSlides: cmsa22
URL:https://cmsa.fas.harvard.edu/event/colloquium_10522/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Colloquium
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Colloquium-10.05.22-1.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20220928T123000
DTEND;TZID=America/New_York:20220928T133000
DTSTAMP:20260430T160042
CREATED:20230817T172722Z
LAST-MODIFIED:20240229T110654Z
UID:10001265-1664368200-1664371800@cmsa.fas.harvard.edu
SUMMARY:The Tree Property and uncountable cardinals
DESCRIPTION:Colloquium \nSpeaker: Dima Sinapova (Rutgers University) \nTitle: The Tree Property and uncountable cardinals \nAbstract: In the late 19th century Cantor discovered that there are different levels of infinity. More precisely he showed that there is no bijection between the natural numbers and the real numbers\, meaning that the reals are uncountable. He then went on to discover a whole hierarchy of infinite cardinal numbers. It is natural to ask if finitary and countably infinite combinatorial objects have uncountable analogues. It turns out that the answer is yes. \nWe will focus on one such key combinatorial property\, the tree property. A classical result from graph theory (König’s infinity lemma) shows the existence of this property for countable trees. We will discuss what happens in the case of uncountable trees.\n\n 
URL:https://cmsa.fas.harvard.edu/event/collquium-title-tba-2-2/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Colloquium
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Colloquium-09.28.22.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20220928T090000
DTEND;TZID=America/New_York:20220928T100000
DTSTAMP:20260430T160042
CREATED:20230705T072111Z
LAST-MODIFIED:20240216T111812Z
UID:10001141-1664355600-1664359200@cmsa.fas.harvard.edu
SUMMARY:Extracting the quantum Hall conductance from a single bulk wavefunction from the modular flow
DESCRIPTION:Topological Quantum Matter Seminar \nSpeaker: Ruihua Fan\, Harvard University \nTitle: Extracting the quantum Hall conductance from a single bulk wavefunction from the modular flow\n\nAbstract: One question in the study of topological phases is to identify the topological data from the ground state wavefunction without accessing the Hamiltonian. Since local measurement is not enough\, entanglement becomes an indispensable tool. Here\, we use modular Hamiltonian (entanglement Hamiltonian) and modular flow to rephrase previous studies on topological entanglement entropy and motivate a natural generalization\, which we call the entanglement linear response. We will show how it embraces a previous work by Kim&Shi et al on the chiral central charge\, and furthermore\, inspires a new formula for the quantum Hall conductance.\n\nReferences: https://arxiv.org/abs/2206.02823\, https://arxiv.org/abs/2208.11710
URL:https://cmsa.fas.harvard.edu/event/tqm92822/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Topological Quantum Matter Seminar
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Topological-Seminar-09.28.22.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20220921T123000
DTEND;TZID=America/New_York:20220921T133000
DTSTAMP:20260430T160042
CREATED:20240214T114047Z
LAST-MODIFIED:20240502T145616Z
UID:10002705-1663763400-1663767000@cmsa.fas.harvard.edu
SUMMARY:Moduli spaces of graphs
DESCRIPTION:Colloquium\n\nSpeaker: Melody Chan\, Brown\n\nTitle: Moduli spaces of graphs\n\nAbstract: A metric graph is a graph—a finite network of vertices and edges—together with a prescription of a positive real length on each edge. I’ll use the term “moduli space of graphs” to refer to certain combinatorial spaces—think simplicial complexes—that furnish parameter spaces for metric graphs. There are different flavors of spaces depending on some additional choices of decorations on the graphs\, but roughly\, each cell parametrizes all possible metrizations of a fixed combinatorial graph. Many flavors of these moduli spaces have been in circulation for a while\, starting with the work of Culler-Vogtmann in the 1980s on Outer Space. They have also recently played an important role in some recent advances using tropical geometry to study the topology of moduli spaces of curves and other related spaces. These advances give me an excuse to give what I hope will be an accessible introduction to moduli spaces of graphs and their connections with geometry.
URL:https://cmsa.fas.harvard.edu/event/collquium-92122/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Colloquium
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Colloquium-09.21.22.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20220921T090000
DTEND;TZID=America/New_York:20220921T100000
DTSTAMP:20260430T160042
CREATED:20230705T064901Z
LAST-MODIFIED:20240229T110819Z
UID:10001142-1663750800-1663754400@cmsa.fas.harvard.edu
SUMMARY:Geometric test for topological states of matter
DESCRIPTION:Topological Quantum Matter Seminar\nSpeaker: Semyon Klevtsov\, University of Strasbourg \nTitle: Geometric test for topological states of matter \nAbstract: We generalize the flux insertion argument due to Laughlin\, Niu-Thouless-Tao-Wu\, and Avron-Seiler-Zograf to the case of fractional quantum Hall states on a higher-genus surface. We propose this setting as a test to characterise the robustness\, or topologicity\, of the quantum state of matter and apply our test to the Laughlin states. Laughlin states form a vector bundle\, the Laughlin bundle\, over the Jacobian – the space of Aharonov-Bohm fluxes through the holes of the surface. The rank of the Laughlin bundle is the \ndegeneracy of Laughlin states or\, in presence of quasiholes\, the dimension of the corresponding full many-body Hilbert space; its slope\, which is the first Chern class divided by the rank\, is the Hall conductance. We compute the rank and all the Chern classes of Laughlin bundles for any genus and any number of quasiholes\, settling\, in particular\, the Wen-Niu conjecture. Then we show that Laughlin bundles with non-localized quasiholes are not projectively flat and that the Hall current is precisely quantized only for the states with localized quasiholes. Hence our test distinguishes these states from the full many-body Hilbert space. Based on joint work with Dimitri Zvonkine (CNRS\, University of Paris-Versaille). \n 
URL:https://cmsa.fas.harvard.edu/event/geometric-test-for-topological-states-of-matter/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Topological Quantum Matter Seminar
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Topological-Seminar-09.21.22.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20220914T120000
DTEND;TZID=America/New_York:20220914T130000
DTSTAMP:20260430T160042
CREATED:20240214T114614Z
LAST-MODIFIED:20240229T110925Z
UID:10002707-1663156800-1663160400@cmsa.fas.harvard.edu
SUMMARY:Strategyproof-Exposing Mechanisms Descriptions
DESCRIPTION:Colloquium \nSpeaker: Yannai Gonczarowski (Harvard)\n\nTitle: Strategyproof-Exposing Mechanisms Descriptions \nAbstract: One of the crowning achievements of the field of Mechanism Design has been the design and usage of the so-called “Deferred Acceptance” matching algorithm. Designed in 1962 and awarded the Nobel Prize in 2012\, this algorithm has been used around the world in settings ranging from matching students to schools to matching medical doctors to residencies. A hallmark of this algorithm is that unlike many other matching algorithms\, it is “strategy-proof”: participants can never gain by misreporting their preferences (say\, over schools) to the algorithm. Alas\, this property is far from apparent from the algorithm description. Its mathematical proof is so delicate and complex\, that (for example) school districts in which it is implemented do not even attempt to explain to students and parents why this property holds\, but rather resort to an appeal to authority: Nobel laureates have proven this property\, so one should listen to them. Unsurprisingly perhaps\, there is a growing body of evidence that participants in Deferred Acceptance attempt (unsuccessfully) to “game it\,” which results in a suboptimal match for themselves and for others. \nBy developing a novel framework of algorithm description simplicity—grounded at the intersection between Economics and Computer Science—we present a novel\, starkly different\, yet equivalent\, description for the Deferred Acceptance algorithm\, which\, in a precise sense\, makes its strategyproofness far more apparent. Our description does have a downside\, though: some other of its most fundamental properties—for instance\, that no school exceeds its capacity—are far less apparent than from all traditional descriptions of the algorithm. Using the theoretical framework that we develop\, we mathematically address the question of whether and to what extent this downside is unavoidable\, providing a possible explanation for why our description of the algorithm has eluded discovery for over half a century. Indeed\, it seems that in the design of all traditional descriptions of the algorithm\, it was taken for granted that properties such as no capacity getting exceeded should be apparent. Our description emphasizes the property that is important for participants to correctly interact with the algorithm\, at the expense of properties that are mostly of interest to policy makers\, and thus has the potential of vastly improving access to opportunity for many populations. Our theory provides a principled way of recasting algorithm descriptions in a way that makes certain properties of interest easier to explain and grasp\, which we also support with behavioral experiments in the lab. \nJoint work with Ori Heffetz and Clayton Thomas.
URL:https://cmsa.fas.harvard.edu/event/collquium-title-tba/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Colloquium
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Colloquium-09.14.22-1.png
END:VEVENT
END:VCALENDAR