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DTSTART;TZID=America/New_York:20231204T140000
DTEND;TZID=America/New_York:20231204T150000
DTSTAMP:20260430T040129
CREATED:20240226T112024Z
LAST-MODIFIED:20240226T112046Z
UID:10002869-1701698400-1701702000@cmsa.fas.harvard.edu
SUMMARY:The Fractional Quantum Hall Effect at ν=5/2: Past\, Recent\, and Future
DESCRIPTION:Topological Quantum Matter Seminar \nSpeaker:  Ken K. W. Ma (Northeastern University) \nTitle: The Fractional Quantum Hall Effect at ν=5/2: Past\, Recent\, and Future \nAbstract: The discovery of fractional quantum Hall (FQH) states started a new chapter in modern physics. Nowadays\, more than 70 FQH states at different filling factors have been observed. Among them\, the FQH state at the filling factor ν=5/2 in GaAs (or the 5/2 state) remains one of the most special and attractive states. Since its discovery in 1987\, different possible topological orders have been proposed to describe the 5/2 state. Some of them can host an exotic type of particles\, known as non-Abelian anyons. Recent experiments have provided more insights into the understanding of the 5/2 state\, but its underlying nature is still under debate. \nIn this talk\, I will review the basics of the 5/2 state [1] and a more unified theoretical description of different possible topological orders of the 5/2 state that we have recently proposed [2]. I will also review the groundbreaking thermal Hall conductance experiment [3] and the follow-up quantum Hall interface experiments by the Weizmann Institute group [4\, 5]\, and discuss what possible lessons that we can learn from the experimental results. Lastly\, I will talk about some possible directions and related topics for future investigations. \nReferences: \n[1] K. K. W. Ma\, M. R. Peterson\, V. W. Scarola\, and K. Yang\, “Fractional quantum Hall effect at the filling factor ν = 5/2” in Encyclopedia of Condensed Matter Physics (Second Edition)\, edited by T. Chakraborty\, Academic Press (2024).\n[2] K. K. W. Ma and D. E. Feldman\, “The sixteenfold way and the quantum Hall effect at half-integer filling factors”\, Phys. Rev. B 100\, 035302 (2019).\n[3] M. Banerjee\, M. Heiblum\, V. Umansky\, D. E. Feldman\, Y. Oreg\, and A. Stern\, “Observation of half-integer thermal Hall conductance”\, Nature (London) 559\, 205 (2018).\n[4] B. Dutta\, W. Yang\, R. Melcer\, H. K. Kundu\, M. Heiblum\, V. Umansky\, Y. Oreg\, A. Stern\, D. Mross\, “Distinguishing between non-Abelian topological orders in a quantum Hall system”\, Science 375\, 193 (2021).\n[5] B. Dutta\, V. Umansky\, M. Banerjee\, and M. Heiblum\, “Isolated ballistic non-Abelian interface channel”\, Science 377\, 1198 (2022). \n 
URL:https://cmsa.fas.harvard.edu/event/tqms_12423/
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-12.04.23.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20231201T163000
DTEND;TZID=America/New_York:20231201T173000
DTSTAMP:20260430T040129
CREATED:20240222T101228Z
LAST-MODIFIED:20240222T103010Z
UID:10002799-1701448200-1701451800@cmsa.fas.harvard.edu
SUMMARY:Quantum information: the interplay of mathematics and physics
DESCRIPTION:Topological Quantum Matter Seminar \nSpeaker: Kaifeng Bu (Harvard University) \nTitle: Quantum information: the interplay of mathematics and physics \nAbstract: I will provide an introduction to quantum information\, which points to a new connection with experiment on the one hand\, and a potential new area of mathematical analysis on the other. I will introduce two recent results about the application of this new area in quantum information: (1) a novel Quantum Central Limit Theorem (QCLT)\, and (2) a physically realizable protocol for testing and measuring quantum advantage. This talk is based on a collaboration with Arthur Jaffe\, and Weichen Gu (PNAS120(25)2023\, arXiv: 2302.08423\, arXiv:2306.09292). \n 
URL:https://cmsa.fas.harvard.edu/event/tqms_12123/
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-12.01.23.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20231129T170000
DTEND;TZID=America/New_York:20231129T180000
DTSTAMP:20260430T040129
CREATED:20240221T100735Z
LAST-MODIFIED:20240221T100813Z
UID:10002773-1701277200-1701280800@cmsa.fas.harvard.edu
SUMMARY:Anomalous Landau level analog in solids: search and implications for 2d heterostructures
DESCRIPTION:Topological Quantum Matter Seminar \nSpeaker: Valentin Crepel\, Flatiron Institute \nTitle: Anomalous Landau level analog in solids: search and implications for 2d heterostructures \nAbstract: The recent experimental observation of fractional Chen insulators — the analog of fractional quantum Hall states realized in absence of any applied magnetic field — calls for more detailed theoretical investigations of the special conditions allowing their emergence. To gain some insight on this problem\, we can rely on (i) our extensive knowledge of the physical properties of Landau levels\, and (ii) the identification of their exact solid state analogs. \nIn this talk\, (i-ii) will be discussed in the case of ”anomalous Landau levels”\, which are protected by an index theorem\, and whose most famous representative are the zero-th Landau levels of graphene. In particular\, I will derive the generic form of Bloch wave functions realizing the analog of such anomalous Landau levels\, and argue that twisted transition metal dichalcogenides homobilayers (in which fractional Chern insulators were first discovered) almost realize such anomalous bands.
URL:https://cmsa.fas.harvard.edu/event/tqms_112923/
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-11.29.23.docx-1_Page_1.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20231120T140000
DTEND;TZID=America/New_York:20231120T150000
DTSTAMP:20260430T040129
CREATED:20240222T103744Z
LAST-MODIFIED:20240222T105558Z
UID:10002800-1700488800-1700492400@cmsa.fas.harvard.edu
SUMMARY:Emergent composite-fermion Luttinger liquid at the half-filled Landau level
DESCRIPTION:Topological Quantum Matter Seminar \nSpeaker: Prashant Kumar\, University of Chicago \nTitle: Emergent composite-fermion Luttinger liquid at the half-filled Landau level \nAbstract: The half-filled Landau level is a fascinating point in the phase diagram of quantum Hall effect that has been hypothesized to be a non-Fermi liquid where composite-fermions (CFs) emerge at low energies. In this talk\, I will present our numerical calculations of the excitation spectrum of this state on infinite cylinders. By comparing our results with a quasi-1D theory of CFs\, we show compelling quantitative evidence for the emergence of a Luttinger liquid of composite-fermions at low energies\, distinct from that of electrons. I will comment on some future directions.
URL:https://cmsa.fas.harvard.edu/event/tqms_112023/
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-11.20.23-1.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20231115T103000
DTEND;TZID=America/New_York:20231115T113000
DTSTAMP:20260430T040129
CREATED:20240222T105852Z
LAST-MODIFIED:20240222T105913Z
UID:10002803-1700044200-1700047800@cmsa.fas.harvard.edu
SUMMARY:A bulk gap in the presence of edge states for a truncated Haldane pseudopotential
DESCRIPTION:Topological Quantum Matter Seminar \nSpeaker: Amanda Young\, UIUC \nTitle: A bulk gap in the presence of edge states for a truncated Haldane pseudopotential \nAbstract: Haldane pseudopotentials were first introduced as Hamiltonian models for the fractional quantum Hall effect\, and it has been long expected that they should exhibit the characteristic properties of this exotic phase of matter\, including a spectral gap above the ground state energy. We will discuss recent work that verified this gap conjecture for a truncated version of the 1/3-filled Haldane pseudopotential in the cylinder geometry. Numerical evidence suggested that for open boundary conditions the gap of the truncated model closes as the cylinder radius converges to zero and that this closure is due to the presence of edge modes; in contrast\, for periodic boundary conditions\, the gap remains robustly order one in the same radius limit. The standard scheme for applying spectral gap estimating techniques to the model with periodic boundary conditions\, though\, produces a lower bound on the bulk gap that still reflects the energy of the edge modes. To obtain an estimate on the bulk gap that reflects its true behavior\, a new gap estimating strategy was developed. By customizing the spectral gap method to key invariant subspaces of the Hamiltonian\, we are able to successfully avoid the edge states and produce a more accurate lower bound on the bulk gap. In this talk\, we discuss this invariant subspace strategy for proving bulk gaps in the presence of edge states. This is based off joint work with S. Warzel. \n  \n 
URL:https://cmsa.fas.harvard.edu/event/tqms_111523/
LOCATION:Virtual
CATEGORIES:Topological Quantum Matter Seminar
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Topological-Seminar-11.15.23.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20231109T163000
DTEND;TZID=America/New_York:20231109T173000
DTSTAMP:20260430T040129
CREATED:20240222T110254Z
LAST-MODIFIED:20240222T110327Z
UID:10002805-1699547400-1699551000@cmsa.fas.harvard.edu
SUMMARY:Uniqueness of Landau levels and their analogs with higher Chern numbers
DESCRIPTION:Topological Quantum Matter Seminar \nSpeaker: Bruno Mera\, Instituto Superior Tecnico \nTitle: Uniqueness of Landau levels and their analogs with higher Chern numbers \nAbstract: Lowest Landau level wavefunctions are eigenstates of the Hamiltonian of a charged par- ticle in two dimensions under a uniform magnetic field. They are known to be holomorphic both in real and momentum spaces\, and also exhibit uniform\, translationally invariant\, geometrical properties in momentum space. In this talk\, using the Stone-von Neumann the- orem\, we show that lowest Landau level wavefunctions are indeed the only possible states with unit Chern number satisfying these conditions. We also prove the uniqueness of their direct analogs with higher Chern numbers and provide their expressions. \nRef: Bruno Mera and Tomoki Ozawa. Uniqueness of Landau levels and their analogs with higher Chern numbers. arXiv:2304.00866\, 2023. arXiv:2304.00866. \n 
URL:https://cmsa.fas.harvard.edu/event/tqms_11823/
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-11.09.23.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20231102T163000
DTEND;TZID=America/New_York:20231102T180000
DTSTAMP:20260430T040129
CREATED:20240222T112133Z
LAST-MODIFIED:20240222T112133Z
UID:10002808-1698942600-1698948000@cmsa.fas.harvard.edu
SUMMARY:Landscape of quantum phases in quantum materials
DESCRIPTION:Joint Quantum Matter in Mathematics and Physics and Topological Quantum Matter Seminar \nSpeaker: Liujun Zou (Perimeter Institute) \nTitle: Landscape of quantum phases in quantum materials\n\nAbstract: A central goal of condensed matter physics is to understand which quantum phases of matter can emerge in a quantum material. For this purpose\, one should be able to not only describe the quantum phases using some effective field theories\, but also capture the important microscopic information of the material via mathematical formulation. In this talk\, I will present a framework to classify quantum phases in quantum materials\, where the microscopic information of a material is encoded in its quantum anomaly. I will talk about the application of this framework to classify various exotic quantum phases of matter in different lattice systems. Using our framework\, we have obtained many results unexpected from the previous literature.
URL:https://cmsa.fas.harvard.edu/event/tqms_102523/
LOCATION:CMSA Room G10\, CMSA\, 20 Garden Street\, Cambridge\, MA\, 02138\, United States
CATEGORIES:Quantum Matter,Topological Quantum Matter Seminar
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Special-Joint-QMMP-Topological-QM-11.02.23.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20231101T103000
DTEND;TZID=America/New_York:20231101T113000
DTSTAMP:20260430T040129
CREATED:20240222T111415Z
LAST-MODIFIED:20240222T111747Z
UID:10002806-1698834600-1698838200@cmsa.fas.harvard.edu
SUMMARY:Unveiling Correlated Topological Insulators through Fermionic Tensor Network States
DESCRIPTION:Topological Quantum Matter Seminar \nSpeaker: Shenghan Jiang\, Kavli Institute for Theoretical Sciences UCAS \nTitle: Unveiling Correlated Topological Insulators through Fermionic Tensor Network States \nAbstract: The study of topological band insulators has revealed fascinating phases characterized by band topology indices\, harboring extraordinary boundary modes protected by anomalous symmetry actions. In strongly correlated systems\, it has been established that topological insulator phases persist as stable phases. However\, due to the inability to express the ground states of such systems as Slater determinants\, the formulation of generic variational wavefunctions for numerical simulations is highly desirable.\nIn this talk\, we tackle this challenge by developing a comprehensive framework with fermionic tensor network states. Starting from simple assumptions\, we write down tensor equations\, construct edge theories and extract quantum anomaly data for topological insulators. By exhaustively exploring all possible sets of equations\, we achieve a systematic classification of topological insulator phases. Imposing the solutions of a given set of equations onto local tensors\, we obtain generic variational wavefunctions for corresponding topological insulator phases. Our methodology provides a crucial first step towards simulating topological insulators in strongly correlated systems. \n 
URL:https://cmsa.fas.harvard.edu/event/tqms_11123/
LOCATION:Virtual
CATEGORIES:Topological Quantum Matter Seminar
ATTACH;FMTTYPE=image/png:https://cmsa.fas.harvard.edu/media/CMSA-Topological-Seminar-11.01.23.docx-1.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20231018T123000
DTEND;TZID=America/New_York:20231018T133000
DTSTAMP:20260430T040129
CREATED:20240223T113304Z
LAST-MODIFIED:20240223T113334Z
UID:10002866-1697632200-1697635800@cmsa.fas.harvard.edu
SUMMARY:Composite fermions and the fractional quantum anomalous Hall effect
DESCRIPTION:Topological Quantum Matter Seminar \nSpeaker: Hart Goldman\, University of Chicago \nTitle: Composite fermions and the fractional quantum anomalous Hall effect \nAbstract: Recent experiments have revealed evidence for fractional quantum anomalous Hall (FQAH) states at zero magnetic field in a growing number of moire materials. In this talk\, I will argue that a composite fermion description\, already a unifying framework for the phenomenology of 2d electron gases at high magnetic fields\, provides a similarly powerful perspective in this new zero-field context. In particular\, a central prediction of the composite fermion framework is a non-Fermi liquid metal of composite fermions at even-denominator fillings. To this end\, I will present exact diagonalization evidence for such composite Fermi liquid states at zero magnetic field in twisted MoTe2 bilayers\, at fillings n = 1/2 and n = 3/4. Dubbing these states anomalous composite Fermi liquids (ACFLs)\, I will argue that they play a central organizing role in the FQAH phase diagram. I will also develop a long wavelength theory for this ACFL state\, which offers concrete experimental predictions that I will discuss in relation to current measurements. For example\, upon doping the composite Fermi sea\, one obtains a Jain sequence of FQAH states consistent with those observed experimentally\, as well as a new type of commensurability oscillations originating from the superlattice potential intrinsic to the system. Finally\, I will discuss opportunities for new physics not possible in quantum Hall systems at finite magnetic field.
URL:https://cmsa.fas.harvard.edu/event/tqms_101823/
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-10.18.23.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20231016T140000
DTEND;TZID=America/New_York:20231016T150000
DTSTAMP:20260430T040129
CREATED:20240222T090812Z
LAST-MODIFIED:20240222T090812Z
UID:10002793-1697464800-1697468400@cmsa.fas.harvard.edu
SUMMARY:Breaking ergodicity: quantum scars and regular eigenstates
DESCRIPTION:Topological Quantum Matter Seminar \nSpeaker: Ceren Dag\, Harvard \nTitle: Breaking ergodicity: quantum scars and regular eigenstates \nAbstract: Quantum many-body scars (QMBS) consist of a few low-entropy eigenstates in an otherwise chaotic many-body spectrum and can weakly break ergodicity resulting in robust oscillatory dynamics. The notion of QMBS follows the original single-particle scars introduced within the context of quantum billiards\, where scarring manifests in the form of a quantum eigenstate concentrating around an underlying classical unstable periodic orbit (UPO). A direct connection between these notions remains an outstanding problem. Here\, we study a many-body spinor condensate that\, owing to its collective interactions\, is amenable to the diagnostics of scars. We characterize the system’s rich dynamics\, spectrum\, and phase space\, consisting of both regular and chaotic states. The former are low in entropy\, violate the Eigenstate Thermalization Hypothesis (ETH)\, and can be traced back to integrable effective Hamiltonians\, whereas most of the latter are scarred by the underlying semiclassical UPOs\, while satisfying ETH. We outline an experimental proposal to probe our theory in trapped spin-1 Bose-Einstein condensates. If time permits\, I will also mention our latest efforts in introducing spatial dimension to this model with a true semiclassical limit\, and how quantum scars persist to exist in a many-body system. Reference: arXiv 2306.10411\, in peer review.
URL:https://cmsa.fas.harvard.edu/event/tqms_101623/
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-10.16.23.docx-1.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20231004T103000
DTEND;TZID=America/New_York:20231004T113000
DTSTAMP:20260430T040129
CREATED:20240221T111722Z
LAST-MODIFIED:20240221T111849Z
UID:10002779-1696415400-1696419000@cmsa.fas.harvard.edu
SUMMARY:Dipolar and modulated symmetry protected topological phases
DESCRIPTION:<strong>Topological Quantum Matter Seminar</strong> \n<strong>Speaker:</strong> Ho Tat Lam\, MIT \n<strong>Title:</strong> Dipolar and modulated symmetry protected topological phases \n<strong>Abstract:</strong> Modulated symmetries are symmetries whose symmetry generators exhibit spatial modulations. We will discuss one-dimensional symmetry protected topological (SPT) phases protected by modulated symmetries. We will present a simple recipe for constructing modulated SPT models by generalizing the concept of decorated domain walls. We will then focus on the simplest modulated SPT protected by dipolar symmetries\, classify them using matrix product states and construct their response field theories using twisted finite tensor gauge theories.
URL:https://cmsa.fas.harvard.edu/event/tqms_10423/
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-10.04.23.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20230920T103000
DTEND;TZID=America/New_York:20230920T113000
DTSTAMP:20260430T040129
CREATED:20240223T104903Z
LAST-MODIFIED:20240223T104903Z
UID:10002851-1695205800-1695209400@cmsa.fas.harvard.edu
SUMMARY:Exact Results in Flat Band Hubbard Models
DESCRIPTION:Topological Quantum Matter Seminar \nSpeaker: Jonah Herzog-Arbeitman\, Princeton University \nTitle: Exact Results in Flat Band Hubbard Models \nAbstract: Flat bands\, like those in the kagome lattice or twisted bilayer graphene\, are a natural setting for studying strongly coupled physics since the interaction strength is the only energy scale in the problem. They can exhibit unconventional behavior in the multi-orbital case: the mean-field theory of flat band attractive Hubbard models shows the possibility of superconductivity even though the Fermi velocity of the bands is strictly zero. However\, it is not necessary to resort to this approximation. We demonstrate that the groundstates and low-energy excitations of a large class of attractive Hubbard models are exactly solvable\, offering a rare\, microscopic view of their physics. The solution reveals the importance of quantum geometry in escaping (some of) BCS phenomenology within a tractable and nontrivial strong coupling theory.
URL:https://cmsa.fas.harvard.edu/event/tqms_92023/
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-09.20.23.docx-1.png
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20230913T103000
DTEND;TZID=America/New_York:20230913T113000
DTSTAMP:20260430T040129
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:20260430T040129
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:20260430T040129
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:20260430T040129
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:20260430T040129
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:20260430T040129
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:20260430T040129
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:20260430T040129
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:20260430T040129
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:20260430T040129
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:20260430T040129
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:20260430T040129
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:20260430T040129
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:20260430T040129
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:20260430T040129
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:20260430T040129
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:20260430T040129
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:20260430T040129
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
END:VCALENDAR